[Index]
Monthly Global Tropical Cyclone Summary August 1999
[Summaries and Track Data] [Prepared by Gary Padgett]
| Tropical
Cyclones
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Monthly Global Tropical Cyclone Summary August 1999 [Summaries and Track Data] [Prepared by Gary Padgett] |
MONTHLY GLOBAL TROPICAL CYCLONE SUMMARY
AUGUST, 1999
(For general comments about the nature of these summaries, as well as
information on how to download the tabular cyclone track files, see
the Author's Note at the end of this summary.)
***********************************************************************
!!!!! IMPORTANT NOTE BY AUTHOR !!!!!
At the outset of writing these summaries, I determined that I would
not introduce any tropical cyclone nomenclature, names, or numbering
systems that were not assigned by any tropical cyclone warning center.
This month I am making a departure from this policy in a limited
fashion. During August there were several weather systems in the
Northwest Pacific basin which, in the opinion of some very
knowledgeable and trained meteorologists (and also in the author's
opinion), were very likely tropical storms but for which no tropical
cyclone warning agency issued any warnings. Since these systems
deserve further study, and since they were clustered both temporally
and spatially; in order to reduce confusion in any possible discussions
which this summary may engender, I have decided to assign unofficial
names to three of them which have no other name or number.
To label these cyclones I have decided to use the letters of the
Greek alphabet. The letters of the Greek alphabet are widely known
through their frequent use as symbols in various scientific
disciplines (e.g., mathematics, physics, astronomy) and should lead
to no cross-cultural problems (which can happen, for instance, when
a common English name may sound similar to a "bad" word in another
language). In the future, whenever some weather system occurs which:
(1) in the opinion of some reliable professional meteorologist(s),
was quite possibly a tropical cyclone (or else a hybrid with strong
tropical-like features); (2) likely contained 1-min MSW of at least
34 kts; and (3) was not officially assigned a name or number by any
TCWC--then I shall report on it in a monthly summary and designate it
with the next available letter of the Greek alphabet. I do not
anticipate this happening very often, and with 24 letters in
the Greek alphabet, it should be quite a few years before it would be
necessary to start over. (Incidentally, for those who are not aware
of it, the names of the Greek letters are the backup names for the
Atlantic and Eastern North Pacific basins should the regular list of
names be exhausted in any given year.)
***********************************************************************
!!!!! NEW ACRONYMS !!!!!
About a year ago I developed a glossary of frequently-used acronyms
and abbreviations which are commonly and widely used in tropical
cyclone-related discussions. I have included the glossary in two or
three monthly summaries which were shorter in length. This month's
summary is anything but short, so I don't want to make it longer by
appending the glossary at the end. But there is one very common
acronym which is widely used in tropical cyclone discussions which I
am going to start employing: LLCC - which stands for low-level
circulation center. I am tired of having to type out that rather
lengthy term so many times. Another one which I have started using
in this summary is CP - central pressure.
***********************************************************************
AUGUST HIGHLIGHTS
--> Tropical Atlantic comes alive with a bang--first time since prior
to 1944 with two Category 4 hurricanes before 1 September
--> Catetory 4 hurricane strikes sparsely populated south Texas
--> Large Category 2 hurricane brushes southeast Atlantic coast
--> Long-lived hurricane travels from Eastern Pacific, through the
Central Pacific, and across the Dateline into the Western Pacific
--> Large, sprawling typhoon affects Philippines and southern China
--> Spectacular outbreak of high-latitude midget tropical cyclones in
Western North Pacific
***********************************************************************
ACTIVITY BY BASINS
ATLANTIC (ATL) - North Atlantic Ocean, Caribbean Sea, Gulf of Mexico
Activity for August: 1 tropical storm
3 hurricanes
NOTE: Much of the information presented below was obtained from the
TPC/NHC discussion bulletins issued with every regular advisory. All
references to sustained winds imply a 1-min averaging period unless
otherwise noted. A special thanks to John Wallace, a tropical cyclone
enthusiast and college student from San Antonio, for providing me with
a log which he had kept of all Atlantic/Northeast Pacific tropical
waves that proved to be very valuable in helping to trace the pre-
depression history of some of the cyclones. John also sent me much
of the rainfall, surface observations, and damage reports on Hurricanes
Bret and Dennis.
Atlantic Activity for August
----------------------------
The pattern of tropical cyclone activity in the Atlantic basin
during August, 1999, was similar to that during August, 1998. In both
years the tropical Atlantic basin was very quiet through most of the
first three weeks of the month, then things became quite active with
four named storms forming before month's end. Also in both years,
the portions of the basin seeing cyclogenesis were similar: the
tropical Atlantic between the Antilles and Cape Verde Islands and
the western Gulf of Mexico. And North Carolina and Texas were the
two U. S. states affected by tropical cyclones in both years.
One aspect in which the two years differed, however, was in the
number and severity of major hurricanes. August, 1998, produced one
major (or intense) hurricane--Hurricane Bonnie--which just managed to
reach Category 3 on the Saffir/Simpson scale whereas in August of this
year, two hurricanes became severe Category 4 hurricanes. Bret was
a small, intense hurricane in the extreme western Gulf of Mexico which
made landfall in a very sparsely populated county in south Texas while
Cindy was a large, severe Cape Verde hurricane which reached its peak
intensity as it recurved northeastward into the North Atlantic several
hundred miles east of Bermuda.
According to the Best Track file for the Atlantic basin, 1999 is
the first year since the beginning of aerial reconnaissance in 1944
in which two Category 4 hurricanes occurred before the beginning
of September. The only other year which possibly may have achieved
this distinction was 1958. Hurricane Cleo in mid-August appears to
have been a solid Category 4, even allowing for the supposed upward
bias in MSW values prior to 1970. But Hurricane Daisy, just off the
U. S. East Coast in late August (taking the Best Track MSW estimates
at face value), was only a Category 3 with a peak MSW of 110 kts--yet
the lowest measured central pressure was 935 mb--rather low for a
MSW of only 110 kts. Dunn and Miller, in the summary of the 1958
hurricane season in their classic book _Atlantic Hurricanes_, report
the highest winds (presumably sustained) in Daisy as being in excess of
150 mph (130 kts), which admittedly seems to be a little on the high
side, although the peak MSW and minimum central pressure for Hurricane
Georges in September, 1998, were 135 kts and 937 mb. Perhaps when the
on-going revision of the Atlantic hurricane database is completed,
Daisy's MSW will have been revised upwards a bit.
Hurricane Bret (TC #3)
18 - 25 August
-----------------------
Bret was a small but vicious hurricane which popped up in the Bay
of Campeche, moved northward and northwestward and struck the coast
of Texas about halfway between Brownsville and Corpus Christi as an
intense hurricane. (Whether or not Bret should be regarded as a
Category 3 or 4 hurricane at landfall will be discussed later.)
A tropical wave left the coast of Africa around 3 Aug and made its way
across the tropical Atlantic and Caribbean Sea. The wave apparently
interacted with a mid-level trough in the Northwestern Caribbean
around mid-month and the resulting area of disturbed weather moved
across the Yucatan Peninsula on 16-17 Aug. After moving out into the
Bay of Campeche the disturbance began to show increasing convective
organization. A USAF Reserves Hurricane Hunter flight on the
afternoon of 18 Aug found that a surface circulation had formed, so
advisories were initiated on Tropical Depression #3 at 2100 UTC. The
broad, poorly-organized center was located around 275 nm east-southeast
of Tampico, Mexico. (The parent tropical wave continued on across
Central America into the Eastern North Pacific where it eventually
developed into TD-11E on 23 Aug.)
The convective pattern was rather chaotic at first and the system
meandered around in the Bay of Campeche for about 48 hours. During the
morning of the 19th convection began to increase with a 180 nm-wide
band covering the LLCC. An early afternoon reconnaissance flight found
a central pressure of 1006 mb and an inner wind maximum forming with
39-kt flight level winds only 7 nm from the center; therefore, the
depression was upgraded to Tropical Storm Bret at 2100 UTC. Bret was
located roughly 225 nm east-southeast of Tampico. (A morning
reconnaissance flight had found 35-45 kt winds 90 nm from the center
just offshore of Veracruz, but this is an area notorious for wind
funneling effects.)
Dr. Jack Beven of TPC/NHC has raised the interesting question of
just what caused Bret's "inner spinner" to form. A Hurricane Hunter
flight had found a very broad center with the radius of maximum winds
on the order of 50-80 nm on the first three fixes. However, the final
fix (near 1700 UTC on the 19th) showed the development of the inner
wind maximum with a radius of maximum winds of 7 nm coincident with
the formation of the aforementioned convective band near the center.
Examination of the aircraft data suggests that this wind maximum
formed in place and did not contract in from a greater distance as
is often the case.
The next aircraft into the storm found a 10 nm-wide eye--a somewhat
unusual feature in a tropical storm with a central pressure of 1000 mb
and 56-kt winds at 450 m flight level. Once having formed, this
very tight core remained with Bret throughout its life. The radius
of maximum winds in subsequent aircraft fixes was never more than 22 nm
and usually less than 15 nm. A reconnaissance flight early on 20 Aug
found a peak flight-level wind of 60 kts only 4 nm from the center.
Tropical Storm Bret continued to steadily increase in intensity on
20 Aug as it moved slowly northward parallel to the Mexican Gulf Coast.
By 21/0000 UTC the central pressure had dropped to 981 mb and a GPS
dropwindsonde in the north eyewall reported surface winds of 62 kts;
therefore, Bret was upgraded to a hurricane at 0300 UTC at a position
about 175 nm east of Tampico. The central pressure in Bret dropped
from 981 mb at 0000 UTC down to 976 mb at 1800 UTC. A GPS
dropwindsonde reported a maximum wind of 112 kts at 900 mb in the
eyewall so the official MSW at 2100 UTC was reported as 90 kts.
Between 1800 UTC and 0000 UTC, 22 Aug, Bret underwent a period of
very rapid intensification. A reconnaissance flight during the early
evening of 21 Aug found maximum flight level winds of 110 kts, but
GPS dropwindsondes revealed far, far stronger winds at lower levels.
Winds of 149 kts were measured at 888 mb. A mean boundary layer wind
of 141 kts was measured and 155 kts was found at 90 m above the
surface. One dropwindsonde was reporting 135 kts shortly before falling
into the ocean. Bret's CP dropped from 976 mb at 21/1703 UTC to
958 mb at 21/2115 UTC--a drop of 18 mb in four hours, which translates
to a drop of 4.5 mb per hour.
Bret was upgraded from a Category 1 hurricane to a Category 4
hurricane with 115-kt MSW in a space of 12 hours. The hurricane
was centered about 150 nm east-southeast of Brownsville, Texas,
at this time. To further complicate the picture, as least from
a forecasting scenario, Bret's predicted turn to the northwest had not
occurred and the hurricane temporarily accelerated a bit on a straight
northward course, being influenced by an upper-level ridge over the
Yucatan Peninsula and a cold LOW over Texas.
Severe Hurricane Bret was at its estimated peak intensity of 120 kts
from 0600 UTC on 22 Aug until the western edge of the eyewall made
landfall on Padre Island around 1700-1800 UTC. Hurricane force winds
only extended out 30 nm from the center and gales covered a zone less
than 150 nm wide. The hurricane began to turn more to the northwest
(as had been consistently forecast) very early on 22 Aug and by the
time of landfall was moving just north of due west. The small, intense
hurricane made landfall in Kenedy County, which, according to the
_1998 World Almanac_, had in 1996 a resident population of 438 (humans)
while press reports indicated the county was home to over 55,000 head
of cattle. The landfall point of Bret was almost exactly halfway
between Brownsville and Corpus Christi, being about 60 nm from either.
At 22/1900 UTC Buoy 42020 off the Texas coast reported an 8-min avg
wind of 58 kts with a peak gust of 73 kts. Lowest pressure was 989.2
mb and the peak wave height was 8.2 m. (The relationship of the buoy
to Bret's center at this time is unknown to the author.) At 2200 UTC,
just after the center of the hurricane had made landfall, the CMAN
station at Port Aransas (near Corpus Christi) reported a peak 2-min
avg wind of 41 kts with a peak gust of 50 kts.
The hurricane was downgraded to a tropical storm at 23/1200 UTC
and to a dissipating depression at 2300 UTC. Bret drifted across
Kenedy County into Brooks County where the last NHC advisory was
written at 1800 UTC. The remnant depression drifted northwestward
to a point north of Laredo and then turned back to the west, crossing
the Rio Grande and dissipating in northern Coahuila State, Mexico.
A branch of the Brownsville Forecast Office located in Kenedy
County reported estimated winds of 80-85 kts. The lowest measured
central pressure in Bret was 944 mb at 22/1200 UTC, and the pressure
was 946 mb when the western edge of the eyewall reached Padre Island.
During the early morning hours of 23 Aug a wind gust of 70 kts was
reported in Falfurrias, Texas. Doppler radar rainfall estimates
indicated that about 760 mm of rain fell on Kenedy County and up to
250 mm on both Corpus Christi and Laredo. Rainfall amounts of 125 mm
or more were common over much of southern Texas. A storm report
from the Hydrometeorological Prediction Center stated that NESDIS
satellite estimates indicated that 200 mm of rain may have fallen
in north-central Mexico, just southwest of Maverick County, Texas,
between 1745 and 2345 UTC on 24 Aug. By early evening of the 24th
the Rio Grande at Laredo was at 2.57 m (flood stage being 2.44 m)
and the Aransas River near Skidmore was almost at flood stage of
3.96 m.
There were four deaths near Laredo, Texas, related to a collision
between a truck and tractor on some slick roads. Bret's storm surge
cut 12 new inlets through the long, narrow Padre Island. There were
at least four confirmed tornadoes spawned by the hurricane--perhaps
more.
An interesting question to ponder is: Should Bret be regarded as
a Category 3 or Category 4 hurricane on the Saffir/Simpson scale at
landfall? Patrick Hoareau, a great tropical cyclone enthusiast from
Rennes, France, sent me several excellent radar images of Bret as it
approached the coast. At 22/2300 UTC the western eyewall was directly
over Padre Island and the convection in that portion of the storm
was more intense than it had been a couple of hours earlier. The
2100 UTC NHC advisory reported the MSW at 120 kts, but in an
intermediate bulletin at 2300 UTC, the MSW had been lowered to
110 kts. Given the increase in the intensity of the convection in
the western eyewall between 2100 and 2300 UTC, it seems quite plausible
that the 120-kt winds were still present when the western eyewall
moved over Padre Island, which, in the author's opinion, would qualify
Bret as a landfalling Category 4 hurricane.
In any case, overall damage from Bret was relative minor due to the
storm's small size and the fact that it made landfall in a virtually
unpopulated stretch of coast. If this hurricane, at the same
intensity, had made a direct strike on a large city, damage would
likely have been several billion dollars. Or to look at it from
another perspective--Bret was comparable in size to the famous Labor
Day Storm which devastated the Florida Keys in 1935. If a storm
like that one with its 892-mb central pressure and possibly 175-kt
winds had struck Kenedy County, Texas, the overall damage would likely
not have been a whole lot greater than that caused by Bret.
Hurricane Cindy (TC #4)
19 - 31 August
------------------------
A very strong, well-organized tropical wave moved off the African
coast on 18 Aug and immediately began to show signs of further
development. Advisories were initiated on Tropical Depression #4
at 0000 UTC on 19 Aug when the center was located about 300 nm east-
southeast of the Cape Verde Islands. Ship VRVK6, located about
150 nm south of the depression's center, reported a pressure of 1011 mb
and southwest winds of 33 kts at 19/0000 UTC. There were some very
cold cloud tops to -80 C south and west of the center, and satellite
intensity estimates by the morning of 19 Aug had reached 35 kts from
TAFB but were only 25 kts from SAB and AFGWC. There was a 45-60 nm
spread in the satellite-based center fixes. The depression was under
easterly shear of about 25-30 kts at the time.
After some initial relocation of the center, the depression
commenced on a westerly track at around 13 kts. During the evening of
the 19th the little available data on the depression showed strongly
mixed signals. Satellite and microwave imagery showed the system to be
adversely affected by northeasterly shear with no convection close to
the presumed center. On the other hand, ship 4QUM reported southwest
winds of 45 kts at 1800 UTC and 37 kts at 20/0000 UTC--the second
report being about 150 nm south-southeast of the center. The NHC
specialist on duty, James Franklin, wrote that he interpreted those
winds as reflecting the large-scale flow of the ITCZ, perhaps enhanced
by the depression. By the afternoon of the 20th, the depression was
still sheared but the center had become more involved with the
convection, so the system was upgraded to Tropical Storm Cindy at
2100 UTC with the center located about 175 nm southwest of the Cape
Verdes.
Visible imagery during the morning of 21 Aug showed a respectful-
looking banding pattern, and Dvorak estimates nudged upwards during
the day. At 22/0300 UTC Cindy was upgraded to a 65-kt hurricane
about 475 nm west-southwest of the Cape Verdes. The easterly
shear, however, remained strong and Cindy's forward motion slowed
considerably, keeping the storm in a shearing environment. As a
result, Cindy was downgraded back to a tropical storm at 1800 UTC on
22 Aug. Over the next few days, Cindy's center was at times exposed
east of the deep convection, but the storm had a large cyclonic
envelope and was able to hold its own fairly well. The estimated MSW
never dropped below 50 kts and remained at that value from 23/0000 UTC
through 24/1200 UTC. By the morning of 25 Aug Cindy had changed little
in intensity but the cloud pattern had become more organized. The
vertical shear had decreased considerably and the underlying waters
were becoming warmer. Cindy was reclassified as a 65-kt hurricane at
0300 UTC on 26 Aug. At 1800 UTC ship ELUU6 reported winds of 60 kts
about 90 nm from the center; hence, the MSW was upped to 75 kts at
2100 UTC.
Hurricane Cindy had been moving on a general west-northwesterly
track for several days, but by the time it regained hurricane intensity
the storm was beginning to move northwestward in a broad recurve which
would take it well east of Bermuda. Visible images on the morning
of 27 Aug showed a ragged eye and the official MSW was increased to
90 kts. By 28/0000 UTC Dvorak numbers from TAFB and SAB had reached
T6.0, so Cindy's MSW was increased further to 100 kts, making it the
second major hurricane of the season. Six hours later the hurricane
displayed a 35 nm-wide eye embedded in a CDO with cloud tops of -65
to -75 C. The MSW was increased further to 115 kts, making Cindy the
second Category 4 hurricane within a week. Cindy's winds reached
their peak of 120 kts at 1200 UTC on the 28th and maintained that
intensity for 18 hours. The lowest CP in Cindy, based upon satellite
estimates only, was 944 mb. By 29/0600 UTC the hurricane was beginning
to weaken a bit as the MSW decreased to 110 kts. Cindy was at this
time 325 nm due east of Bermuda and was beginning to move to the north-
northeast.
At the time of peak intensity Hurricane Cindy was a very large
storm. Hurricane force winds covered a zone about 135 nm in diameter
while tropical storm force (gale force) winds swept an area almost
500 nm in diameter. After recurvature Cindy began to weaken fairly
quickly as SSTs decreased and vertical shear increased. Winds were
down to 80 kts by 1200 UTC on 30 Aug, and Cindy was downgraded to
a 60-kt tropical storm at 31/0900 UTC. At 1200 UTC Cindy was located
about 425 nm southeast of Cape Race, Newfoundland, and the MSW was
down to 45 kts. By midday visible satellite images revealed that
Cindy was rapidly being absorbed by a much stronger extratropical
cyclone approaching from the northwest.
One final note--Cindy was absorbed into a large, vigorous extra-
tropical LOW, but the hurricane itself had been an absorber. About
the time that Cindy reached peak intensity, the remnants of tiny
Tropical Storm Emily were absorbed into Cindy's much larger and
stronger circulation.
Hurricane Dennis (TC #5)
24 August - 7 September
-------------------------
Hurricane Dennis apparently formed from the northern extent of a
tropical wave which emerged off the African coast around 15 Aug.
(The southern extent of the wave was responsible for the formation
of Tropical Storm Emily.) The wave moved across the Atlantic for
several days without much associated convection. Thunderstorms
began to increase on 21 Aug when the system was located a few hundred
miles east of the Leeward Islands. The convection became even better
organized on the 22nd and a Hurricane Hunter plane investigated the
area on 23 Aug. No surface circulation could be located by the
reconnaissance flight, but by later in the day satellite imagery
indicated that the convection had become more concentrated and a ship
with call sign WZJF reported southeast winds of 30 kts about 90 nm
northeast of the center. Advisories were begun on Tropical
Depression #5 at 24/0000 UTC when the center was estimated to be
about 175 nm northwest of San Juan, Puerto Rico.
During the morning of 24 Aug there were mixed signals regarding
the depression's intensity. Satellite intensity estimates from TAFB
and SAB were 35 kts and 30 kts, respectively, and ship WZJF reported
35-kt winds. However, the wind direction at the ship remained
unchanged over six hours, suggesting that the circulation was poorly
defined or elongated, and this was supported by an observation from
ship 3FOB5, located west of the estimated center, of east winds of
only 5 kts. A reconnaissance flight during the morning found only a
very broad, ill-defined center with strong gusty winds well to the
east of the possible center. However, another flight in early
afternoon found that the system had strengthened some and a special
intermediate advisory, upgrading the depression to Tropical Storm
Dennis, was issued at 1800 UTC with the MSW estimated at 40 kts.
The storm's poorly-defined center was located about 400 nm northwest
of San Juan and all the tropical storm force winds were confined to
the eastern and northern quadrants.
For the next three days Dennis moved on a slow northwesterly course
parallel to the Bahama Islands. During the 25th the center became
more involved with the deep convection and the MSW had increased to
55 kts by 25/1800 UTC. Outflow was well-defined in all quadrants
although there were some low clouds moving away from the circulation
in the northwest quadrant. An early evening flight by the Hurricane
Hunters found FLW of 67 kts and a CP of 998 mb--down 10 mb in 24
hours. However, the next flight early on 26 Aug found FLW of 80 kts
and pressure down to 995 mb, so Dennis was upgraded to a 65-kt
hurricane at 0900 UTC located about 400 nm east-southeast of Miami.
Most of the models were indicating that Dennis would intensify rather
quickly into a strong Category 2 or Category 3 hurricane, but this
did not materialize. An upper-level anticyclone over central Cuba
restricted outflow somewhat over the southwest quadrant, and the
inner core of Dennis never got well-established. The storm's MSW
reached 70 kts at 26/1200 UTC and remained pegged there for a couple
of days although the CP dropped down to 975 mb during this time.
During the night of 27-28 Aug Hurricane Dennis turned to more of a
north-northwesterly heading which carried it just east of Great Abaco
Island in the Bahamas. (Less than three weeks later a much, much
stronger Hurricane Floyd would follow a very similar path through the
same area.) By 28/0000 UTC ham radio operators at Little Harbor
on Great Abaco reported wind gusts to 58 kts. Radar images relayed
from the NOAA P-3 reconnaissance aircraft showed a fully-formed eye
35-40 nm in diameter. The center was embedded within very cold cloud
tops of -75 C and the CP was down to 980 mb. During the evening
Man-of-War Cay reported gusts to 68 kts. The center of Dennis passed
very close to Green Turtle Cay on the morning of the 28th where ham
radio operators reported that wind gusts reached 100 kts with some
storm surge flooding. There were reports of significant damage to
boats and crops in the Abacos where winds were still gusting to 60 kts
at 1500 UTC.
Around 0600 UTC on 29 Aug the hurricane passed about 30 nm east of
NOAA buoy 41010 (28.9N, 78.5W). The buoy reported a peak 8-min avg
wind of 54 kts at 0500 UTC; a peak gust of 72 kts at 0800 UTC; a
minimum pressure of 980.2 mb at 0800 UTC; and a peak wave height of
7.6 m at 2100 UTC on 28 Aug. (Thanks to John Wallace for collecting
and sending me this information.)
Dennis began to strengthen on 28 Aug, reaching its estimated peak
intensity of 90 kts with a CP of 969 mb by 1800 UTC. At this time the
hurricane was centered less than 200 nm off the central Florida coast
and about 325 nm south-southeast of Charleston, South Carolina. A GPS
dropwindsonde on the morning of the 29th measured a peak wind of 109
kts. The large-scale circulation pattern favored intensification and
ocean temperatures were quite warm, but the lack of a well-defined
inner core prevented Dennis from reaching its full potential intensity-
wise. On the afternoon of the 29th the storm sported a ragged banding-
type eye 45-50 nm in diameter. The hurricane by this time was moving
due northward off the Georgia coast, and by 0000 UTC on 30 Aug had
begun to turn to the north-northeast. Dennis' CP fell to its lowest
value in the storm's history at 30/0000 UTC when a reconnaissance
flight measured 963 mb.
On 30 Aug the hurricane turned to a northeasterly heading which
was parallel to and about 100 nm off the Carolina coasts. Dennis'
forward motion accelerated to about 18 kts for a few hours as it
sped by Cape Hatteras. Sustained winds of 44 kts were reported from
Oak Island near Cape Fear as Dennis raced by. The automated station
at Frying Pan Shoals (33.5N, 77.5W) reported sustained winds of
hurricane force from 0800 UTC through 1300 UTC as the storm passed
off the North Carolina coast, reaching a maximum of 80 kts with a
peak gust to 93 kts at 1000 UTC. Dennis' CP remained below 970 mb
throughout the 30th and a USAF reconnaissance flight during the
afternoon found 98-kt winds in the southeast quadrant at flight level.
GPS dropwindsonde data over several days showed that Dennis was not
particularly effective at bringing high winds down to the surface.
(In contrast Hurricane Bret was very effective at doing this, which
just goes to underscore the extreme individuality of hurricanes.)
Dennis passed about 120 nm north of NOAA buoy 41002 (32.3N, 75.2W)
around 1600 UTC on the 30th. At 1200 UTC that buoy was reporting 8-min
avg winds of 41 kts (which continued through 1600 UTC), a peak gust
to 58 kts, and a peak wave height of 11.3 m. The minimum pressure of
997.6 mb was reported at 1100 UTC. The by-now weakening hurricane
was located about 30 nm north of NOAA buoy 41001 (34.7N, 72.6W) at
31/1200 UTC. However, most of the extremes from this buoy were
reported near 30/2200 UTC when the center was still more than 50 nm
to the west-northwest. The 8-min avg wind was 47 kts, peak gust
was 60 kts, and the peak wave height was 10.3 m at 2200 UTC while the
minimum pressure of 976 mb was reported at 31/0400 UTC. As Dennis
swept by the North Carolina coast on 30 Aug it was a rather large
hurricane: the diameter of hurricane force winds was around 130 nm
and the area experiencing gales was approximately 300 nm wide. (Thanks
to John Wallace and Eric Blake for sending the buoy information to me.)
Dennis' forward motion slowed considerably and the storm came to
a crashing halt approximately 100 nm east of Cape Hatteras by early
on 31 Aug and remained quasi-stationary in the same general spot for
nearly three days before beginning a slow drift to the south. During
this time the convection associated with Dennis all but disappeared
completely, due initially to vertical shear and the intrusion of cool,
drier air. The hurricane was downgraded to a 60-kt tropical storm
at 0000 UTC on 1 Sep. The NHC Discussion Bulletins on 31 Aug and
1 Sep consistently remarked that Dennis looked more like an extra-
tropical LOW than a tropical cyclone--there was little convection and
the strongest winds were not close to the center. Only the sporadic
re-development of heavy showers near the center gave any hint of
tropical characteristics. At this stage Dennis barely met Dvorak
classification criteria.
However, the mid-latitude westerlies shifted to the north, reducing
the shear, and Dennis remained near the Gulf Stream, which of course
is a vast oceanic heat source. Visible satellite images on the
morning of 2 Sep showed a well-organized system with denser clouds
near and over the center, although the tops were not very cold. During
the afternoon a vessel with call sign ELFV3 reported west-southwest
winds of 45 kts about 50 nm southwest of the center. During the
evening hours there was some limited deep convection noted east of the
center with tops of about -45 C. Dennis at this stage still showed
some subtropical characteristics as it appeared to be embedded within
a larger-scale cyclonic circulation.
Tropical Storm Dennis drifted generally southward to a point about
150 nm southeast of Cape Hatteras by 03/1800 UTC. With a high-pressure
ridge building to the northeast, the storm commenced on a slow, steady
northwesterly track toward the North Carolina coast. The MSW remained
near 50 kts for about three days during this time with the CP running
from about 985-990 mb. On the morning of the 4th WSR-88D data from
Morehead City, North Carolina, indicated that Dennis was becoming
better organized. Reflectivity data showed 65-kt winds aloft, and an
earlier reconnaissance flight had found FLW of 66 kts. A GPS drop-
windsonde reported surface winds of 63 kts and a CP of 984 mb as Dennis
was making landfall on the North Carolina coast around 2100 UTC just
northeast of Cape Lookout and a short distance east of the Morehead
City/Beaufort area. An automated weather station at Cape Hatteras
reported a peak gust of 53 kts about the time of landfall.
Dennis continued moving northwestward across North Carolina as it
weakened, being downgraded to a depression at 05/0600 UTC. By the
evening of the 5th winds were down to 10-15 kts at lower elevations
near the center of Dennis while winds in excess of 25 kts were still
being reported at higher elevations of southwestern Virginia and
western North Carolina and also in coastal waters off North Carolina
and Virginia. The weakening system passed near Raleigh-Durham,
North Carolina and entered Virginia near Danville. It then turned
more to the north, passing between Roanoke and Lynchburg, across
northeastern West Virginia, extreme western Maryland (passing near
Cumberland), western Pennsylvania (passing near Johnstown), and on
into western New York. The final position supplied by the Hydro-
meteorological Prediction center placed the weak LOW over extreme
southern Lake Ontario, just northwest of Rochester, New York.
By Sunday evening, 5 Sep, up to 150 mm of rain had fallen over
north-central North Carolina, with totals of 125 to 200 mm common
over upslope areas in the Blue Ridge Mountains. A storm total of
214 mm was reported at Mills Creek Dam in Augusta County, Virginia;
and 209 mm at North Mountain in Rockbridge County, also in Virginia.
There was significant overwash near Hatteras, and some homes near
Nags Head (that were built right on the water) got washed away by
sand erosion and waves. In the Outer Banks there were some power
outages, minor flooding, loss of some shingles and siding, etc.;
however, overall damage from Dennis was minor. (The information
in this paragraph was gleaned from a posting to the WX-TALK discussion
list by Mike Mogil from Rockville, Maryland.)
Tropical Storm Emily (TC #6)
24 - 28 August
-----------------------------
Tropical Storm Emily apparently formed from the southern extent of
a tropical wave which emerged off the African coast around 15 Aug.
(The northern extent of the wave was responsible for the formation
of Hurricane Dennis.) The wave moved across the Atlantic for
several days without much associated convection. Thunderstorms
began to increase on 21 Aug when the system was located a thousand
miles or so east of the Windward Islands. The system was mentioned
in the six-hourly Tropical Weather Outlooks issued by TPC/NHC over the
next few days as being somewhat organized and having some potential
for development. By the morning of 24 Aug satellite images revealed
that the disturbance was quite well-organized with indications of a
circulation although there was some unfavorable shear in the area.
A Hurricane Hunter reconnaissance flight was dispatched to
investigate the area around midday. Surprisingly, they found a very
small, tightly-wound tropical storm with a CP of 1004 mb and MSW of
55 kts! The new storm was named Emily, and this was only about two
hours or less after Dennis had been named. Tropical Storm Emily was
a small storm with gales extending out from the center about 50 nm.
Winds of 50 kts were confined to the northeastern quadrant within about
25 nm of the center. At 24/1800 UTC Emily was centered about 340 nm
east-southeast of Barbados. Emily was moving very slowly northward,
apparently due to the larger Tropical Storm Cindy to the east which was
disrupting the tradewind flow.
During the night of 24-25 Aug northerly shear increased and Emily's
center became exposed with only limited deep convection south of the
center. The MSW was decreased to 40 kts on the 25/0300 UTC advisory.
The next afternoon a reconnaissance flight found FLW of 45 kts and a
CP of 1005 mb, so it appeared that Emily was holding its own against
the shear. A French buoy with ID 41101, near 14.6N, 56.2W, reported
a pressure of 1009.2 mb and east-southeast winds of 27 kts (apparently
at 26/0600 UTC). This observation helped to verify the synoptic
position and motion. Tropical Storm Emily moved slowly northwestward
to a position about 260 nm east of Guadeloupe by 1800 UTC on the 26th.
A mid-level circulation had been left behind 150 nm to the south of the
LLCC, so Emily was downgraded to a tropical depression at 2100 UTC.
At this juncture Emily began to move at a slightly faster pace on
an almost due northward track. During the night of 26-27 Aug
convection began to increase around Emily's LLCC and outflow over the
system began to improve somewhat. A reconnaissance flight early on
27 Aug made two center fixes that were almost 50 nm apart. The second
fix had the better defined center with a CP of 1009 mb and a FLW of
44 kts to the east of the center. Since the fixes were so far apart
and the convection was still poorly organized, Emily remained
classified as a depression for the time being.
A flight into the depression in the early afternoon of the 27th
found a CP of 1007 mb and a peak FLW of 49 kts at 450 m; therefore,
Emily was upgraded once more to a tropical storm. Emily was a very
tiny storm--tropical storm force winds extended out only 10 nm to the
northeast and 25 nm to the southeast. Six hours later the gale radii
in these quadrants had increased to 25 nm and 30 nm, respectively.
The circulation of Emily would have just about fit inside the eye of
Hurricane Cindy.
Emily's revival as a tropical storm was short-lived--on the morning
of 28 Aug the distance between tiny Emily and the very large, severe
Hurricane Cindy continued to decrease. The 0900 UTC Discussion
Bulletin indicated that low-level cloud elements to the west of Emily
were moving from west to east around the large circulation of Cindy,
indicating that Emily was getting entrained into Cindy. The final
advisory at 1500 UTC indicated that Emily was dissipating as it was
being absorbed into Hurricane Cindy. The 28/1200 UTC position
placed the center of the dissipating Emily about 375 nm south-southeast
of Cindy's center.
It is interesting to note that for the first two days of Emily's
life, even with the much larger Cindy approaching from the east and
the outflow from Dennis impinging on the storm from the northwest,
and in spite of the persisent shear over the small storm, some of the
numerical models were still predicting that Emily would reach hurricane
force. This possibility was reflected in official TPC/NHC forecasts
through 26/1500 UTC, which was the advisory before Emily was first
downgraded to a depression.
***********************************************************************
NORTHEAST PACIFIC (NEP) - North Pacific Ocean East of Longitude 180
Activity for August: 2 tropical depressions
1 tropical storm
2 hurricanes
NOTE: Much of the information presented below was obtained from the
TPC/NHC discussion bulletins issued with every regular advisory (CPHC
for locations west of 140W.) All references to sustained winds imply
a 1-min averaging period unless otherwise noted.
A special thanks to John Wallace, a tropical cyclone enthusiast
and college student from San Antonio, for providing me with a log
which he had kept of all Atlantic/Northeast Pacific tropical waves
that proved to be very valuable in helping to trace the pre-depression
history of the cyclones.
Northeast Pacific Activity for August
-------------------------------------
Two hurricanes came to life in Northeastern Pacific waters during
August, both of which moved across 140W into the Honolulu AOR (Central
North Pacific). One of these, Hurricane Dora, continued on across
the Central Pacific and crossed the International Dateline into the
Northwest Pacific Basin. Dora was the first tropical cyclone to
originate east of 140W and emerge into the Western Pacific since
Hurricane John of Aug-Sep, 1994. In addition to Hurricanes Dora
and Eugene, Tropical Storm Fernanda and two tropical depressions
formed during the month. Both of the non-developing depressions were
quite short-lived. The first, TD-09E, formed on 13 Aug about 650 nm
southwest of Cabo San Lucas, Mexico. This depression likely formed
from a tropical wave which had first been identified in the tropical
Atlantic east of the Lesser Antilles on 1 Aug and had crossed Central
America into the Northeast Pacific basin on 9 Aug. The depression had
a broad center which remained exposed east of an area of deep
convection. The system moved slowly west-northwestward and had
dissipated by 15/0000 UTC less than 200 nm from its point of origin.
TD-11E formed around 1800 UTC on 23 Aug about 125 nm southwest of
Cabo San Lucas. This depression formed from a tropical wave which
left the African coast on 3 Aug and entered the Northeast Pacific on
16 Aug. (This was the same wave which appeared to be instrumental
in the formation of Hurricane Bret in the Gulf of Mexico.) Even as
the first advisory was issued on this system it was nearing cooler
SSTs and convection was on the wane. The depression initially moved
northwestward, then was relocated to the southwest at 24/1200 UTC.
The final advisory at 1800 UTC placed the dissipating center about
200 nm west-southwest of Cabo San Lucas.
Hurricane Dora (TC-07E / TS 9911)
6 - 23 August
----------------------------------
A tropical wave left the coast of western Africa on 23 Jul and made
its way westward across the tropical Atlantic. The main wave axis
had entered the eastern Caribbean by 29 Jul and had slipped across
Central America and into the Eastern Pacific by 4 Aug. Late on 5 Aug
the cloud pattern associated with the disturbance had become better
organized with a curved band forming to the west of the deepest
convection. Also, a ship (KSBG) reported a 30-kt easterly wind from
near the center and a pressure of 1003 mb. Advisories were initiated
on Tropical Depression 07E at 06/0000 UTC with the center estimated
to be about 300 nm south-southwest of Acapulco.
The depression maintained deep convection with good banding during
the day and at 1800 UTC was upgraded to Tropical Storm Dora about
350 nm south-southwest of Manzanillo. Dora continued to steadily
increase in strength throughout 7 Aug and at 0600 UTC on 8 Aug was
upgraded to a 65-kt hurricane based on the presence of a symmetric
and cold CDO and a Dvorak T-number of 4.5 from TAFB. Dora at this
time was centered approximately 450 nm south-southwest of Cabo San
Lucas. The MSW had increased to 90 kts by 09/0000 UTC and SSM/I
imagery at 0100 UTC depicted an eye that was open on the west side.
Throughout the day Dora continued to display a small, well-defined
eye 5-10 nm in diameter, and the MSW had increased to 100 kts by
10/0000 UTC, making Dora a Category 3 hurricane on the Saffir/Simpson
scale. The eye of Hurricane Dora was located about 675 nm southwest
of Cabo San Lucas at this time.
Guided by a strong and persistent subtropical ridge, the constancy
of Dora's westward track for thousands of miles across the tropical
Pacific was amazing. From the point at which Dora was upgraded to
a tropical storm on 6 Aug to the point at which it crossed the Dateline
on the 19th, the storm moved westward almost 4500 nm while gaining only
about 250 nm in latitude. Dora became a Category 4 hurricane at
1800 UTC on 10 Aug and maintained that status through 1200 UTC on
the 13th, reaching a peak MSW of 120 kts and a minimum CP of 943 mb
at 0000 UTC on 12 Aug when Dvorak estimates from SAB, TAFB, and AFGWC
reached 127 kts, 115 kts, and 115 kts, respectively. During the
period when Dora was at peak intensity, the diameter of the eye was
about 20 nm and surrounded by a ring of solid convection with cloud
tops generally between -65 and -75 C.
By the afternoon of 13 Aug some stable trade wind clouds were
being advected into the hurricane from the north and west and the
cloud tops were warming. Dvorak numbers were decreasing and Dora's
MSW was lowered to 110 kts at 1800 UTC. Hurricane Dora crossed
140W and entered the CPHC's area of responsibility around 14/0600 UTC
at a point about 800 nm east-southeast of Hilo, Hawaii. Dora was
moving into cooler waters and the MSW had dropped to 85 kts by this
time. The peak winds were further reduced to 70 kts six hours later.
However, on 15 Aug Dora appeared to re-intensify. The Hurricane
Hunters flew some reconnaissance missions into Dora since it posed
a potential threat to Hawaii. CPHC increased the MSW to 90 kts
at 15/0600 UTC and to 100 kts 24 hours later as Dora passed about
200 nm south of South Point on the Big Island. During a reconnaissance
mission on the 16th a GPS dropwindsonde measured a maximum wind of
120 kts at the 982-mb level with a mean boundary layer wind of 114 kts.
Satellite imagery at this time was certainly supportive of a Category
3 hurricane on the Saffir/Simpson scale. Although quite intense as
it passed south of Hawaii, the only effects of the storm were enhanced
trades and some high surf along the Big Island's southeast-facing
shores.
After passing Hawaii Dora began to steadily weaken as its forward
motion accelerated. The hurricane passed about 65 nm south of tiny
Johnston Island around 0600 UTC on 18 Aug as a minimal hurricane. The
strongest winds measured on the island were 35 to 40 kts in gusts with
a minimum pressure of 1011 mb. By comparison, Hurricane John passed
nearer the island on 25 Aug 1994 as a more intense storm and produced
serious damage that was then estimated at $15 million. After passing
by Johnston Island Dora continued to sail westward toward the Dateline.
The storm crossed into the Northwest Pacific basin around 2100 UTC
on 19 Aug at a point about 650 nm west of Johnston Island. The first
warning from JTWC downgraded Dora to a 60-kt tropical storm.
Shortly before reaching the 180th meridian Dora was beginning to
turn to a slightly more west-northwesterly track. After crossing the
Dateline the weakening storm turned more to the northwest. As Dora
traversed the easternmost reaches of the Northwest Pacific basin, it
began to run into increasing southwesterly shear. SSM/I data from
around 21/0000 UTC indicated only an isolated area of convection
about 40 nm northeast of the LLCC. The MSW was dropped to 40 kts
at 0600 UTC based on satellite intensity estimates of 25 kts and
45 kts. JTWC downgraded Dora to a tropical depression at 22/0000 UTC
when the system was centered about 300 nm northeast of Wake Island.
The LLCC was fully exposed about 75 nm west of the deepest convection.
(JMA was of course issuing warnings on Dora and maintained the system
as a tropical storm through 23/0000 UTC.) Convection persisted for
a couple of days in spite of the shear due to an area of diffluence
over the region. The final position of Dora, supplied by JMA,
located the dissipating system about 475 nm north-northeast of Wake
Island at 1200 UTC on 23 Aug.
The factors responsible for the rather sudden and unexpected
re-intensification of Dora to major hurricane status over marginally
warm waters are somewhat obscure. Rich Henning, a Major in the
USAF Reserves and a meteorologist with the 46th Weather Squadron at
Eglin AFB--and a member of the 53rd Weather Reconnaissance Squadron
(The Hurricane Hunters)--suggests that Dora's resurgence was an
example of the effects of mesoscale and convective scale inner core
processes when they are undisturbed by significant shear. Early
on 16 Aug there was a burst of strong convection in the northeast
quadrant just as the eyewall drop mentioned above was released.
Drops into the eye the previous day and night reported eye surface
air temperatures around 24 C, but the lack of any shear had allowed
the eyewall to consolidate very nicely. However, Dr. Mark Lander
of the Water and Environmental Research Institute at the University
of Guam questions whether or not Dora actually re-intensified all
that significantly. Mark's contention is that the hurricane had not
previously weakened as much as the operational advisories had
reported.
Hurricane Eugene (TC-08E)
6 - 15 August
--------------------------
Hurricane Eugene formed from the tropical wave which left the coast
of Africa just before the one from which Dora developed. The pre-Eugene
system started its march across the Atlantic around 19 Jul, was near
the Lesser Antilles on 25 Jul, and had entered the Northeast Pacific
basin by 25 Jul--about four days ahead of the pre-Dora wave. First
visible satellite imagery on 6 Aug indicated that an area of disturbed
weather about 900 nm southwest of Cabo San Lucas had rapidly increased
in organization and that a tropical depression had formed. By afternoon
the small system had remained well-organized and was developing some
good banding features; therefore, it was upgraded to Tropical Storm
Eugene at 1800 UTC. Eugene seemed to intensify in discrete steps.
After reaching tropical storm intensity, the MSW held at 35 kts for
about 24 hours. At 07/1800 UTC a cloud band was observed wrapping
around the center giving the appearance of the formation of a banding-
type eye; therefore, the MSW was increased from 35 to 50 kts.
The intensity inched upward to 55 kts on the next advisory but
remained there until 08/1800 UTC when a small eye 10 nm in diameter
had made an appearance in satellite images. At that point Eugene
was upgraded to a 70-kt hurricane at a location about 1150 nm
southwest of Cabo San Lucas. By 1200 UTC on 9 Aug Eugene had
reached its estimated peak intensity of 95 kts which it maintained
for 24 hours. The estimated minimum CP was 965 mb. These peak
measurements were based on Dvorak estimates of a strong T5.0 from
both TAFB and SAB. Eugene weakened some on the 10th as the eye
disappeared and the CDO slipped southeastward from the storm's center.
The MSW was decreased to 85 kts at 1200 UTC but held there for a
day or so. During the early evening hours the CDO had become
somewhat irregular but had cold cloud tops to -80 C.
Eugene had moved on a course slightly north of due west from its
formation until it reached hurricane intensity. From that point the
storm moved west-southwestward for a day or so, then took off on an
almost due-west track which carried it into the Central North Pacific
sub-region. Eugene entered Honolulu's AOR around 1200 UTC on 11 Aug
at a point about 1000 nm east-southeast of Hilo. Eugene by this time
had weakened some with the MSW estimated at 75 kts. The hurricane's
intensity remained in a steady state at 75 kts for about 36 hours
before more rapid weakening set in.
The Hurricane Hunters made a flight into the storm on 12 Aug and
found the center farther south than anticipated based on the previous
track. Shearing had increased slightly and the LLCC was exposed. The
MSW was decreased to 65 kts at 1800 UTC, and Eugene was downgraded to
a 55-kt tropical storm six hours later at a position about 575 nm
east-southeast of Hilo. Shearing over the storm continued and Eugene
slowly weakened as it moved westward. The system was downgraded to a
tropical depression at 15/0000 UTC when it was centered about 350 nm
south-southwest of South Point on the Big Island. From this point the
depression drifted west-southwestward for another 24 hours. SSTs were
still warm (27 C) and vertical shear decreased significantly in this
region but Eugene was not able to regenerate. By 1800 UTC on 15 Aug
only isolated convective cells remained south of the LLCC, so CPHC
issued the last advisory at that time with the weak center located
about 550 nm east-southeast of Johnston Island.
Tropical Storm Fernanda (TC-10E)
17 - 22 August
---------------------------------
A tropical wave left the coast of western Africa on 1 Aug but
apparently weakened on 4 Aug near 39W. Another wave was introduced
near 52W on 5 Aug in the Tropical Weather Discussions issued four times
daily by TPC/NHC. (John Wallace, who supplied me with tropical wave
information, thought that this second wave was possibly a
re-development of the earlier wave.) The new (or else rejuvenated)
wave reached the eastern Caribbean on 7 Aug and had emerged into the
Eastern Pacific by 11 Aug. The disturbance continued to move to the
west and by 17 Aug was located several hundred miles south-southwest
of Baja California. A scatterometer overpass at 0600 UTC showed a
well-defined circulation with 25-kt winds about 75 nm from the center.
With abundant convection over the western semicircle and Dvorak
intensity estimates up to 30 kts from SAB, the first advisory on
Tropical Depression 10E was issued at 1500 UTC. The poorly-organized
center was estimated to be about 650 nm south-southwest of Cabo San
Lucas.
The new depression was undergoing significant easterly shear which
kept the strong convection pushed to the west of the LLCC. Dvorak
intensity estimates from TAFB, SAB, and AFGWC had all reached 35 kts
by 18/0000 UTC but the system was still carried as a tropical
depression due to the poorly-defined center. By 0600 UTC multi-channel
satellite imagery indicated that the LLCC was just on the northeastern
edge of the deep convection so the depression was upgraded to
Tropical Storm Fernanda on the 0900 UTC advisory. The system
had been moving on a west-northwesterly track, and this motion
continued through 1200 UTC on 19 Aug when Fernanda reached a point
about 750 nm west-southwest of Cabo San Lucas. The storm reached its
peak intensity of 55 kts and minimum estimated CP of 994 mb at this
time. Fernanda remained in a shearing environment but the shear had
decreased enough that some convection had wrapped around the eastern
side of the center.
After reaching peak intensity Fernanda began moving on a slow
west-southwesterly track and slowly weakened. The easterly shear
continued to affect the cyclone and SSTs were slowly cooling off.
Fernanda had weakened to a minimal 35-kt tropical storm by 1200 UTC
on the 20th, and would likely have been downgraded to a depression
at 1800 UTC had it not been for some SSM/I wind speed estimates of
30 kts which suggested that higher winds might be found underneath
the convective area. However, by the 21/0300 UTC advisory Fernanda
had become a tight swirl of low clouds with a cluster of deep
convection to the southwest of the LLCC; hence, it was downgraded to
a tropical depression at that time. Occasional bursts of convection
continued for a day or so but the system continued to slowly spin
down. The final advisory placed the dissipating center almost
1300 nm west-southwest of Cabo San Lucas at 1200 UTC on 22 Aug.
***********************************************************************
NORTHWEST PACIFIC (NWP) - North Pacific Ocean West of Longitude 180
Activity for August: 3 tropical depressions (a)
7 tropical storms (b)
4 typhoons (c)
(a) - it is unknown to the author if JMA classified any of these
systems as depressions; one of these was possibly a midget
tropical storm but was not treated as such by JTWC
(b) - one of these was a visitor from east of the Dateline; three
of these were not warned on as tropical cyclones by any
TCWCs in the Northwest Pacific basin
(c) - three of these were considered typhoons only by JTWC, the
other being Typhoon Olga which was active at the beginning
of the month and was briefly classified as a minimal typhoon
by JMA
NOTE: Most of the information on each cyclone's history presented in
the narrative will be based upon JTWC's advisories, and references to
winds should be understood as a 1-min avg MSW unless otherwise noted.
However, in the accompanying tracking document I have made comparisons
of coordinates with JMA (Japan) and the Philippines (PAGASA) when their
positions differed from JTWC's by usually 40-50 nm or more. Also, as
announced earlier in a separate posting, a column of 10-min avg MSW
is included--the values being obtained from either PAGASA's or JMA's
advisories. A special thanks to Michael V. Padua, owner of the
Typhoon '99 webpage, for sending me the PAGASA and JMA tracks.
In the title line for each storm I plan to reference all the cyclone
names/numbers I have available: JTWC's depression number and name (if
any), JMA's tropical storm numeric designator, and PAGASA's name for
systems forming in or passing through their area of responsibility.
Northwest Pacific Activity for August
-------------------------------------
"It was a month of interesting discrepancies between the various
warning centers in matters of tropical cyclone classification." This
statement opened the discussion of Northwest Pacific tropical activity
in the July summary. But the discrepancies alluded to in the July
summary were nothing compared to what happened in August. The monsoon
trough extended to fairly high subtropical latitudes during the month,
and there was an outbreak of several small, midget tropical cyclones.
Three of these were not warned on by either JMA or JTWC, while amongst
the other three, there were very significant differences in intensity
estimates.
For information on the three unnamed/unnumbered systems, I am
indebted to Dr. Mark Lander of the University of Guam for providing
me with much information and some satellite imagery of these systems.
Dr. Lander is a professor at the Water and Environmental Institute of
the Pacific at the University of Guam, and for 10 years worked under
a grant from the Office of Naval Research to conduct research on
tropical cyclones. Dr. Lander worked closely during those years with
forecasters and satellite analysts at JTWC. A very special thanks to
Mark for cluing me in on these systems and sending me all the
information.
There were three non-developing tropical depressions during August,
but one of these, TD-18W, is covered separately below since Mark Lander
felt it was quite likely a tropical storm. Tropical Depression 14W
emerged from the cloud band of a monsoon gyre southeast of Japan. An
exposed LLCC was noted as early as 7 Aug to the south of Iwo Jima.
The weak system drifted northward and by 1800 UTC on 8 Aug had become
sufficiently organized that JTWC initiated depression warnings. The
center was located about 130 nm north-northwest of Iwo Jima at that
time. The depression drifted generally north-northwestward throughout
its life, steered by a ridge to the northeast. A TUTT located to the
west and an anticyclone to the east created southerly shear which
prevented the depression from strengthening significantly. Mark
Lander sent me an observation from Chichijima at 09/0000 UTC of
southerly winds of 25 kts with a peak gust to 41 kts. This was after
the depression had made its closest approach to the island. On the
basis of this observation and the system's satellite signature, Mark
was of the opinion that this depression could have been a 35-45 kt
tropical storm at the time. The system weakened as it approached
Japan and was only a weak depression when it limped ashore southeast
of Osaka.
Tropical Depression 15W may have been traceable to a disturbance
noted east of Luzon on 8 Aug which drifted slowly northward. This
disturbance appeared to weaken on the 14th northeast of Okinawa.
JTWC initiated warnings on the system at 0600 UTC on 16 Aug when an
exposed LLCC was located less than 100 nm west of southern Kyushu.
The depression was possibly a hybrid-type system, having both tropical
and extratropical features. The system drifted east-northeastwards
towards southwestern Kyushu, then turned northward across the island
and dissipated in the Korea Strait to the north. The depression was in
a vertical shear environment which prevented it from strengthening--
MSW were never estimated higher than 25 kts. A TUTT over southeast
Korea created diffluence which enhanced convection, but the persistent
westerly shear displaced most of the convection to the east, resulting
in the heaviest rains falling on Shikoku and southern Honshu.
Typhoon Olga/Ising (TC-11W / TY 9907)
28 July - 3 August
--------------------------------------
Typhoon Olga was active into the early days of August, but since the
storm began in July, it was covered in the Global Tropical Cyclone
Summary for July, 1999. Please refer to that summary for details on
Olga/Ising.
Tropical Storm Paul (TC-12W / TS 9908)
3 - 8 August
---------------------------------------
A broad LLCC developed southwest of Guam on 1 Aug with convection
in all quadrants but in an environment of fairly strong vertical
shear. The LOW moved north-northwestward and by the next day was
located west of the Marianas. The system had the appearance of a
typical monsoon depression with strongest winds on the periphery of
the circulation. JTWC issued a Formation Alert on 2 Aug and another
one on the 3rd. At this stage the system was still poorly organized
and exhibited multiple small LLCCs within the larger circulation.
JTWC initiated tropical depression warnings at 1200 UTC on 3 Aug when
the broad center was estimated to be about 325 nm southwest of Iwo
Jima.
On 4 Aug the depression was still poorly organized with several
smaller LLCCs, but there were ship reports of winds to 30 kts. Early
on the 4th the center appeared to re-organize farther to the north as
two cloud bands developed north of the system. JMA upgraded the
depression to TS 9908 at 04/1200 UTC when the center was located
approximately 335 nm west of Iwo Jima. Based upon ship reports of
35-kt winds, JTWC upgraded the system to Tropical Storm Paul at
1800 UTC. Paul basically tracked northwestward along the periphery
of a large monsoon gyre that was centered to the southeast of Okinawa.
Paul was initially undergoing shear and the LLCC was exposed, but on
the 5th some deep convection wrapped more tightly around the center.
Based on this and some ship reports of winds to 45 kts, JTWC estimated
the peak intensity of Paul to have been 50 kts at 0600 UTC while the
storm was centered about 250 nm east of Okinawa. (JMA's 10-min MSW
estimate at this time was 45 kts, which is in close agreement.)
However, later on 5 Aug Paul began to weaken as apparently some
dry air entrainment caused the deepest convection to collapse. JTWC
downgraded the storm back to a depression at 06/0000 UTC. Paul
merged with the gyre east of Okinawa and began to resemble a large
monsoon depression once more with strongest winds along the periphery.
The depression continued to move on a northwesterly track which
carried it across southern Kyushu and into the Yellow Sea. Convection
increased some as it crossed Kyushu but was mostly sheared to the
north. (JMA maintained Paul as a tropical storm until it had crossed
Kyushu.) The center of the depression passed just south of Cheju
Island at 07/0800 UTC where Mosulpo reported a minimum pressure of
992 mb and a MSW of 25 kts (presumably a 10-min avg). The final JTWC
warning at 0000 UTC on 8 Aug placed the weakening center in the Yellow
Sea just off the coast of China. The remnants were forecast to move
inland and dissipate near Qingdao.
Tropical Storm Rachel (TC-13W / TS 9909)
5 - 9 August
-----------------------------------------
An area of convection formed in the South China Sea on 4 Aug and
synoptic observations indicated that a broad circulation might be
forming in the monsoon trough. JMA classified the system as a tropical
depression at 1800 UTC on 5 Aug when it was centered about 100 nm
south-southeast of Hong Kong. On 6 Aug the convection began to
rapidly increase in organization and the upper-level environment was
favorable for further intensification; therefore, JTWC initiated
warnings at 0600 UTC. Six hours later, based primarily upon remote
wind measurements from scatterometer and microwave data, the system
was upgraded to Tropical Storm Rachel about 250 nm east of Hong Kong.
The storm was sheared somewhat as it was steered to the east-northeast
by a mid-level ridge to the southeast. At 07/0000 UTC the satellite
current intensity estimate was only 25 kts, but there were ship
reports of winds to 35 kts along the periphery.
Rachel moved rather quickly in an east-northeasterly direction and
crossed Taiwan between 0600 and 1200 UTC on 7 Aug. The storm weakened
as it crossed the Chungyang Mountains and JTWC temporarily dropped
warnings. JMA, which had not yet classified the system as a tropical
storm, continued issuing depression bulletins. After crossing Taiwan,
Rachel continued moving rapidly east-northeastward and began to
recover from the effects of moving over the island. JTWC
re-instated warnings on the system at 1800 UTC on 7 Aug when the
system was centered about 150 nm east of Taipei while JMA upgraded
Rachel to TS 9909 at this time. Based on synoptic ship reports of
winds to 35 kts JTWC upgraded Rachel back to tropical storm intensity
at 08/0600 UTC when the storm was just northwest of Okinawa. However,
the storm was sheared with the deepest convection located about 100 nm
north-northeast of the center of circulation.
Rachel turned to the north around 08/1200 UTC and later to the
north-northwest as a subtropical ridge built over Japan. The storm
also began to weaken and was downgraded to a depression at 1800 UTC.
Some sporadic convection continued, but by 09/1800 UTC scatterometer
and microwave data indicated no winds greater than 20 kts and virtually
no convection, so the final warning was issued at that time with the
dissipating center between Kyushu and Shanghai.
!!!!! YET ANOTHER SPECIAL NOTE !!!!!
--------------------------------------
I am now about to embark on something I've not done since I began
writing these monthly summaries--cover systems for which warnings were
not issued operationally by an established TCWC, but rather for which
the information came from an unofficial, private (but very
knowledgeable and well-qualified) source. I've already explained my
use of the Greek alphabet names and revealed the source of my
information on these storm systems. The purpose of this note is to
underscore the fact that by reporting on these systems, I am not
trying to criticize nor "point a finger" at any TCWCs. I know
full well how subjective and often difficult tropical cyclone
intensity estimation from satellite imagery is, and I also understand
the operational pressures and commitments which forecasters are often
under. My hope is that by reporting on these interesting systems,
some qualified person(s), whether at one of the warning centers or
elsewhere, will undertake to study them more closely and perhaps come
up with some results which will enable forecasters in the future to
have a better handle on the character and intensity of such storm
systems.
For starters, it is my opinion (and also of some other qualified
meteorologists) that part of the problem is that the Dvorak method
of satellite intensity analysis does not handle such midget storm
systems well. A recent case in point---on 24 Aug a Hurricane Hunter
reconnaissance flight investigated a tropical wave east of the
Windward Islands. There was a small area of convection with some hints
of a possible circulation, but nothing that really suggested the
presence of a tropical depression or storm. Yet the Hunters found
a very small, well-developed tropical storm (which was named Emily)
only 10 kts shy of hurricane force!
Another example: at one point in dealing with the early stages of
Typhoon Tanya, forecasters had simultaneous Dvorak intensity estimates
of 25 kts and 65 kts! In general, JMA followed the more conservative
estimates while JTWC tended to follow the higher values or else "split
the middle." Mark Lander's MSW estimates tend to be the most liberal.
In addition to sending me information on the systems I have dubbed
"Alpha", "Beta", and "Gamma", Mark provided me with information on and
his version of Best Track files for Tanya, TD-18W, and Virgil.
One final point--part of the reluctance of some warning centers to
issue warnings on such systems is that their formation at higher
latitudes may raise the question of whether or not they really should
be considered tropical cyclones. I have rarely attempted any
editorializing in these summaries, but to put in my "two cents worth":
I personally feel that, if a given disturbance has the basic
requirements of a complete surface circulation and a 1-min MSW of
34 kts or higher, and (1) regardless of its mode of formation lies
over fairly warm subtropical waters (even if not quite 26 deg C),
(2) is essentially non-frontal in character nor has any obvious
source of significant baroclinic energy, and (3) has organized
central (or near-central) convection which has persisted for about
18 to 24 hours or more; then the system is most likely being
primarily driven by the latent heat of condensation in a manner
similar to cyclones in the deep tropics and should be considered
a tropical cyclone. And if a given system does not completely meet
all three criteria, but comes rather close to doing so, then I am
in favor of somewhat liberally "stretching" the definition of
a tropical storm in the interest of enhancing public concern and
also in the interest of helping to maintain consistency in historical
statistics of tropical cyclones. In pre-satellite days, no doubt
there were occasionally some hybrid-type storm systems in the
subtropics of most all oceanic basins which were treated as tropical
cyclones and included in the seasonal statistics. End editorial!
The narrative presented below on the three unnamed/unnumbered
systems which I have identified with Greek letters is taken in toto
from information provided by Dr. Mark Landers. Information on
Typhoons Tanya and Virgil and TC-18W is based upon Mark's notes
plus information gleaned from the Remarks section of JTWC warnings.
Tropical Cyclone "Alpha"
8 - 11 August
------------------------
This midget system possibly originated from a small, persistent area
of disturbed weather in the subtropics as early as 5 Aug located near
27N, 163E. From 5 to 7 Aug this area of disturbed weather (associated
with a TUTT) drifted slowly northward and by the morning of 8 Aug
had developed a well-defined LLCC associated with a consolidated area
of deep convection. At 08/0000 UTC the circulation was centered
about 750 nm north-northwest of Wake Island--far to the east of Japan.
A small shield of persistent deep convection moved over the LLCC by
the evening of the 8th and pushed intensity estimates to around 35 kts.
The intensity hovered near minimal tropical storm intensity for about
12-18 hours as the system drifted generally eastward.
After this time the small cyclone moved eastward and weakened as
the LLCC became exposed on the west side. The system remained poorly
organized for over a day until a fast re-intensification commenced
on the night of 10 Aug. "Alpha" began to accelerate rapidly to the
northeast and reached an estimated peak intensity of 55 kts around
0000 UTC on 11 Aug as it crossed the International Dateline at a
point about 625 nm north-northwest of Midway Island. A visible
satellite image taken about this time appears to reveal (in the
opinion of the author and of some others with whom I have shared the
image) a quite well-organized tropical storm. After peaking "Alpha"
raced northeastward into the mid-latitudes of the North Pacific and
weakened several hundred miles south of the southwestern Aleutian
Islands.
Tropical Cyclone "Beta"
9 - 11 August
-----------------------
The second midget tropical system of the Western Pacific subtropics
was quite short-lived. It formed along an east-west cloud band with
TD-14W to its west and "Alpha" to its east. At 0000 UTC on 9 Aug a
LLCC was evident and was located almost 900 nm east-southeast of Tokyo
and an equivalent distance east-northeast of Iwo Jima. The midget
system moved southeastward as central convection increased. By
10/0000 UTC Mark Lander estimates that minimal tropical storm intensity
was achieved. "Beta" reached an estimated peak MSW of about 45 kts
by 1200 UTC on the 10th. Early on 11 Aug (local time) the central
convection collapsed, leaving behind a well-defined LLCC. This center
began to move off to the northeast and during the night of the 11th
the remnant vortex of "Beta", located nearly 500 nm north-northwest of
Wake Island, was swept into the primary cloud band of a rapidly
deepening and very large subtropical cyclone located to its northwest.
Tropical Cyclone "Gamma"
11 - 17 August
------------------------
This system formed about 200 nm east-northeast of Okinawa in a
monsoon trough displaced to an unusually high-latitude. It was
preceded in this area by three other tropical cyclones--Tropical Storm
Paul, Tropical Storm Rachel, and TD-14W. After TD-14W crossed Japan
and died in the Sea of Japan and the remnants of Rachel were near
Shanghai, the monsoon trough still stretched eastward from central
China to the waters south of Japan. The tropical disturbance which
became "Gamma" formed at the eastern end of the trough and moved to
the northeast, then more to the east, for the next two days. During
the night of 13 Aug the small system slowed, turned to the north,
and intensified. By the local morning hours of 14 Aug the well-
defined and partially exposed LLCC could be seen located about 60 nm
south-southeast of the mouth of Tokyo Bay. "Gamma" then turned to
the northwest and made landfall just to the southwest of Tokyo at
approximately 0500 UTC on 14 Aug. The system weakened and crossed
Japan, emerging into the Sea of Japan on 15 Aug, where it meandered
about for 36 hours in a possible interaction with TD-15W to its
south-southwest. After 16/0600 UTC the weakened vortex accelerated
to the northeast, losing its identity on the 17th west of northern
Honshu.
Mark Lander had access to some synoptic reports in the Tokyo area
during the passage of this system: Tokyo Heliport (RJTI), Tokyo
International Airport (RJTT), Tateyama (RJTE), and Yokota AB (RJTY).
As the center passed nearby each of these stations on its north-
northwest trek, the minimum sea-level pressure fell to 998 mb and
the peak sustained 10-min wind (on-the-hour reports) reached 24 kts
and 25 kts at RJTT and RJTI, respectively. These stations had an
abrupt shift of the wind from east to southeast at the time of the
minimum pressure and the peak sustained wind. Very heavy rain was
experienced with "Gamma", the highest being 168 mm in 24 hours at
Yokota AB. The highest winds and deepest convection were on the
north and east side of the LLCC. Ship observations from the offshore
waters to the east indicated east to east-southeast 10-min avg winds
of 30 kts. By using the most commonly used conversion factor, this
would translate into a 1-min avg MSW of 34 kts, which would suggest
that "Gammma" likely briefly reached minimal tropical storm intensity
from a 1-min avg perspective.
Typhoon Sam/Luding (TC-16W / STS 9910)
18 - 23 August
---------------------------------------
While the subtropics were literally crawling with midget tropical
cyclones, Typhoon Sam was a large, sprawling tropical cyclone which
began in the monsoon trough in the deep tropics. An area of
convection was noted on 11 Aug far to the southeast of Guam, apparently
associated with an easterly wave. A Significant Tropical Weather
Outlook (STWO) issued by JTWC on 12 Aug relocated the disturbance
farther west, to the south of Guam. Isolated pockets of convection
were flaring up along the periphery of a broad LLCC with diffluence
aloft. The system remained quasi-stationary for two or three days,
and a STWO on the 14th indicated that the LLCC was very weak and it
appeared to be more like a wave in the tropical easterlies. On 16 Aug
the system resumed moving westward and on the 17th JTWC issued a
Formation Alert. Most of the convection was east of the LLCC and the
system displayed monsoon depression characteristics with most of the
stronger winds along the periphery.
By 0000 UTC on 18 Aug the disturbance was sufficiently organized
that PAGASA initiated advisories, naming the system Luding when it
was approximately 300 nm east of Catanduanes Island in the central
Philippines. JTWC began writing warnings six hours later. The
depression at this time had a very large circulation with sporadic
regions of deep convection. Tropical Depression Luding began moving
on a northwesterly course, steered by a subtropical ridge to its
northeast. PAGASA upgraded Luding to a tropical storm at 1800 UTC
on 18 Aug when the storm was centered about 200 nm east-northeast of
the aforementioned Catanduanes Island. JTWC followed suit six hours
later, naming the system Tropical Storm Sam. The storm had been
moving on a northwesterly track, but after 19/1200 UTC it turned to
more of a west-northwesterly course, apparently due to a ridge building
to the north.
Tropical Storm Luding reached the extreme northeastern coast of the
island of Luzon during the late morning (local time) of 20 Aug with
the MSW estimated at 50 kts (both by JTWC and PAGASA). The storm
passed about 40 nm south of Cabo Engano, crossed the extreme northern
end of Luzon, and exited the island just north of Laoag around 1200
UTC. Convection had become well-organized around the LLCC and the
storm maintained its intensity (or even increased a little) even
though its inflow was interrupted somewhat by a mountain range in
northern Luzon. After leaving Luzon behind, Sam/Luding continued
to increase in intensity as it drew a bead on Hong Kong. JTWC briefly
upgraded the storm to a minimal typhoon at 21/0600 UTC but downgraded
it back to a 60-kt tropical storm on the next warning.
The weakening was only temporary, however, and the storm moved under
an upper-level anticyclone. Deep convection was present in the
southeast quadrant and also west of the center. JTWC reclassified
Sam as a typhoon at 22/0000 UTC when the center was only about 50 nm
southeast of Hong Kong. Typhoon Sam reached its peak intensity of
75 kts shortly before making landfall near Hong Kong. The typhoon
crossed right over the city and continued northwestward into southern
China. At 1800 UTC the cyclone, now a weakening tropical storm with
40-kt MSW, was located very near the city of Canton. By 0600 UTC
on 23 Aug the system was dissipating inland in southern China.
In northern Luzon one fatality was attributed to the storm--a
woman drowned while crossing a swollen river on her way home to
fetch clothes for her sick child. Nine were reported injured and
over 4000 people were temporarily displaced by floodwaters. Most
of the major roads to and from Baguio City were closed due to the
rains and also to landslides. (Thanks to Michael V. Padua for
sending me this information.)
Unfortunately, at the time Typhoon Sam was making landfall near Hong
Kong, I was "up to my ears" just retrieving and downloading real-time
warnings and advisories for about six simultaneous tropical cyclones,
so I missed locating any reports of damage or casualties on the web.
Patrick Hoareau, of Rennes, France, sent me a little bit of
information. One person died while attempting to windsurf, and one
other person died due to an airliner crash at Hong Kong which was
related to the typhoon. Cheung Chau, just outside the harbor, reported
a peak wind gust of 84 kts. (Thanks to Patrick for sending me this
information.)
If anyone has any more information on the effects of Typhoon Sam,
please e-mail it to me and I will include it in a future summary.
Typhoon Tanya (TC-19W / STS 9912)
18 - 25 August
----------------------------------
Typhoon Tanya appeared to be a TUTT cell-related development which
occurred near the southwestern end of an old washed-out frontal
boundary which trailed across the subtropical ridge axis. On 18 Aug
there was a LLCC associated with a small area of convection related
to the TUTT cell. The system was then located east of the Dateline
approximately 200 nm north of Midway Island. In Mark Lander's
opinion, the system had already reached tropical storm intensity
by 18/1800 UTC. The small system initially moved southwestward
and then swung to a westward course, crossing the Dateline about
150 nm northwest of Midway around 0300 UTC on 19 Aug. Environmental
sea level pressures in the area where the storm developed were around
1023 mb--somewhat akin to that of similar tropical storms which have
formed in the Atlantic subtropics. During Tanya's initial tropical
storm stage, the diameter of the area encompassed by tropical storm
force winds was less than 75 nm.
JTWC issued the first depression warning at 1800 UTC on 19 Aug
shortly after the system had entered the Northwest Pacific basin.
(Mark's MSW estimate at this time was 60 kts.) Six hours later the
system was upgraded to Tropical Storm Tanya with 35-kt winds, while
Mark's track has Tanya reaching typhoon intensity at this time.
Dvorak current intensity estimates at this time available to JTWC
were 25 kts and 55 kts, and at 20/0600 UTC were 25 kts and 65 kts!
JTWC upgraded Tanya to a typhoon at 21/0000 UTC when a 15 nm-wide
eye became visible in satellite imagery. Dr. Lander cited a study
by Ray Zehr which indicates that based upon Atlantic reconnaissance
investigations, when an eye first becomes visible in a storm with
a CDO-type pattern of convection, the average intensity (1-min MSW)
is usually around 80 kts.
With an anticyclone in place over the storm and steered by a
subtropical ridge to the north, Tanya continued to intensify. JTWC's
peak intensity for Tanya was 70 kts from 0000 to 0600 UTC on 22 Aug,
while Mark Lander's peak was 90 kts from 0000 to 1200 UTC on 21 Aug.
At Tanya's peak intensity, the diameter of the area experiencing
tropical storm-force winds was less than 150 nm, and the maximum radius
of 50-kt winds was only 20 nm. The typhoon was located about 750 nm
north of Wake Island at the time it reached its peak intensity. By
22/1200 UTC the convection was less organized and Tanya was beginning
to weaken. The storm, which had been moving west-northwestward, began
to turn more to the northwest after 1800 UTC on 22 Aug. Vertical shear
associated with an upper-level anticyclone over Japan increased and
Tanya continued to weaken. By 23/0000 UTC the LLCC was completely
exposed with the nearest deep convection about 20 nm to the south.
The weakening storm recurved to the north and northeast on 23 Aug,
passing almost 900 nm east of Japan. Rather suprisingly, JTWC's
intensity estimate at this time was significantly higher than Mark's.
However, JTWC quickly weakened the storm, downgrading it to a tropical
depression at 23/1800 UTC and writing the last warning at 24/0000 UTC,
whereas Mark's track maintains Tanya as a minimal tropical storm all
the way to latitude 43.0N, 172.5E at 0000 UTC on 25 Aug, by which time
the storm had likely become extratropical. This position placed the
center over 1000 nm northwest of Midway Island. (Note: The peak
10-min MSW assigned to Tanya by JMA was 50 kts on 21 and 22 Aug.)
Tropical Cyclone (TC-18W)
19 - 25 August
--------------------------
As Tropical Storm Tanya was gaining strength near the International
Dateline, another small-scale tropical system was beginning to take
shape about 750 nm east of Iwo Jima. This system was treated as
a 25-kt tropical depression (one warning gave the MSW as 30 kts) by
JTWC, but Mark Lander estimates the system to have been an unnamed
tropical storm with a peak MSW of 45 kts. (I have no information
from JMA on this system.) From its beginning far to the east of Iwo
Jima on 19 Aug, the system moved slowly in a general northwesterly
direction. A SWTO issued by JTWC at 19/2300 UTC mentioned an area
of convection with a LLCC evident beneath the convection; yet no
warnings were initiated on the system.
Dr. Lander believes the system reached tropical storm intensity
at around 1800 UTC on 20 Aug when it was located about 650 nm
northeast of Iwo Jima. Mark sent me a high-resolution visible picture
of the system at about that time which reveals a small area of deep
convection with considerable banding, and the LLCC appears to be under
or very near the deepest convection. As noted in a JTWC warning,
the convection does seem to be elongated somewhat due to some vertical
shearing. JTWC initiated depression warnings at 21/1800 UTC--at the
same time Mark Lander's track shows the cyclone to be at its peak
intensity of 45 kts. The system was being steered west-northwestward
at this time by a low-level ridge to its north. The modest
intensification was enhanced by the storm's being located in a region
of diffluence generated by a TUTT to the southwest and an upper-level
anticyclone to the northeast.
By 0000 UTC on 23 Aug the center had become fully exposed with the
convection sheared off to the west. The system was at this time
located about 300 nm east-southeast of Tokyo and began to move slowly
to the north. Vertical shear had increased and the system quickly
weakened. JTWC dropped warnings at 24/0000 UTC, but Mark's track
carries the residual depression for another 24 hours to a position
about 600 nm east-northeast of Tokyo.
Hurricane Dora (TC-07E / TS 9911)
6 - 23 August
----------------------------------
Hurricane Dora was an unusally long-lived and far-travelled North
Pacific tropical cyclone which trekked from near 100W south of
Acapulco, Mexico, all the way across the Northeast Pacific basin
and entered the Northwest Pacific basin. Along the way Dora became
a rather severe hurricane with MSW estimated at 120 kts, but by the
time the storm crossed the International Dateline into the Northwest
Pacific basin on 20 Aug, winds were down to 60 kts and the cyclone
continued to weaken and dissipate over the next three days. The
full history of Hurricane Dora, including the short time it was
west of the Dateline, is described in the section of this summary
covering the Northeast Pacific basin.
Typhoon Virgil (TC-19W / TS 9913)
22 August - 3 September
----------------------------------
Typhoon Virgil was the final midget tropical cyclone to form in
subtropical latitudes of the Northwest Pacific during the month of
August, and perhaps became the most intense. The storm originated
from an isolated area of disturbed weather on 22 Aug located about
225 nm northwest of Iwo Jima. The system drifted south-southeastward
and on 23 Aug acquired a well-defined partially exposed LLCC and a
small area of persistent deep convection. According to Mark
Lander's track, the system had reached minimal tropical storm
intensity by 23/0600 UTC about 150 nm northwest of Iwo Jima. At
about the same time JTWC issued a Formation Alert on the system.
JTWC began issuing depression warnings at 0000 UTC on 24 Aug.
The depression/tropical storm began to move generally in an
east-northeastward direction which it followed for most of its life.
The system appeared to be sheared from the east on 24 Aug, but on
the morning of 25 Aug (local time) it acquired a CDO as the shear
lessened. Around 0530 UTC (afternoon locally) a small eye became
visible in the CDO. At 25/0600 UTC JTWC upgraded the system
from TD-19W directly to Typhoon Virgil with 65-kt MSW. (Mark's
intensity estimate is similar at this time--70 kts, while JMA
was still calling the system a depression. JMA's 10-min MSW was
never higher than 45 kts for Virgil.) JTWC's peak MSW of 75 kts
for Virgil was reached at 1800 UTC on the 25th. Around the time
Virgil reached typhoon intensity the radius of 35-kt winds was
estimated to be 40 nm and radius of 50-kt winds only 20 nm. At
peak intensity Virgil displayed a 14-nm wide eye with a maximum
radius of gale-force winds of 70 nm. At the time that Virgil was
upgraded to typhoon intensity it was located only about 100 nm
northeast of Iwo Jima.
Mark Lander's analysis assigns a peak intensity of 100 kts to
Virgil from 1200 UTC through 1800 UTC on 25 Aug. According to Mark,
manual Dvorak intensity estimates for Virgil reached T5.5 (102 kts),
and a solid T5.0-T5.5 with a peak of T5.8 from 1030 UTC to 1730 UTC
on 25 Aug using Ray Zehr's objective digital Dvorak method. Virgil's
eye persisted through the night, and then on 26 Aug the LLCC became
partially exposed on the north side of the deep convection due to
northeasterly shear from an anticyclone to the north. By 1800 UTC
the MSW was down to 50 kts and the LLCC was about 70 nm north of the
deepest convection. Dvorak current intensity estimates received
by JTWC at 27/0000 UTC were 35 kts and 65 kts, so JTWC maintained
the MSW at 50 kts. (Actually, Mark Lander's estimate at this time was
lower--down to 45 kts.)
JTWC downgraded Virgil to a tropical depression at 27/1800 UTC
when it was located roughly 375 nm northeast of Iwo Jima, and issued
the final warning at 29/0000 UTC. Mark Lander's track keeps Virgil
as a minimal tropical storm through 1200 UTC on the 29th, and then
tracks the very weak residual depression on a slow eastward to east-
northeastward course through 0600 UTC on 3 Sep when it was located
almost 1500 nm to the east of Japan. The depression moved very
slowly during its decaying phase as it was steered by a low-level
ridge to the east-southeast but running counter to a low-level ridge
to the north.
Tropical Storm Wendy/Mameng (TC-20W / TS 9914)
30 August - 4 September
-----------------------------------------------
A STWO issued by JTWC on 29 Aug mentioned an area of convection
located northwest of Yap that was associated with a rapidly moving
easterly wave. A broad LLCC had formed by the next day and the
upper-level environment was favorable for strengthening. PAGASA
initiated warnings at 1800 UTC when the broad center was located
about 400 nm northwest of Palau, naming the system Mameng. Mameng
was a classic example of a monsoon depression with two large, primary
clusters of deep convection and with the strongest winds located on
the periphery of the circulation. (According to some e-mail from
Mark Landers, one of these clusters of convection passed over Palau,
dropping almost 100 mm of rain in less than 6 hours.) JTWC issued
a Formation Alert at 31/0230 UTC. Animated visible imagery suggested
that a separate LLCC had started to develop in association with one
of the two main clusters of convection within the broader circulation
of Mameng.
The first depression warning was issued by JTWC at 0000 UTC on
1 Sep, placing the center about 175 nm east-southeast of Catanduanes
Island. On 1 and 2 Sep there was considerable divergence between
JTWC's and PAGASA's positions with the latter agency's positions
being somewhat to the north and west of JTWC's. Since the convection
at this stage was somewhat "chunky" in nature, it is likely that the
two TCWC's were focusing on different clusters of convection. PAGASA
upgraded Mameng to a tropical storm at 1800 UTC on 1 Sep when it was
located about 100 nm east of Catanduanes Island. Tropical Storm
Mameng was in many ways a clone of Typhoon Sam/Luding two weeks
earlier, both in its area and mode of formation and in its subsequent
track. The storm began to track generally in a northwestward
direction which carried it just off the northeastern tip of Luzon
(Cabo Engano) and through the Babuyan Islands. At 02/0000 UTC JTWC
relocated the center almost 200 nm farther to the north, due apparently
to a consolidation of convection around another LLCC within the
larger circulation. This northward relocation brought JTWC's
coordinates much more in line with PAGASA's, and after 02/0600 UTC
the two TCWC's were in close agreement on the location of the center.
JTWC upgraded the system to Tropical Storm Wendy at 02/1800 UTC
based upon satellite imagery and synoptic reports of winds to 35 kts.
Wendy/Mameng was located in the Luzon Strait between Taiwan and Luzon
at this time. An anticyclone east of the storm interacting with a
TUTT to the northwest had caused the convection to become rather
elongated. Tropical Storm Wendy continued moving to the northwest
toward the coast of China, making landfall around 2130 UTC on 3 Sep
about 140 nm east-northeast of Hong Kong. As Wendy approached the
Chinese coast, a CDO feature developed near the LLCC and the storm
intensified slightly. The highest MSW reported were 40 kts by JTWC
(1-min avg) and 45 kts by PAGASA (10-min avg). The final warning
from JTWC indicated that winds may have been briefly higher just
prior to landfall.
I have received no reports of damage or casualties resulting from
Tropical Storm Wendy/Mameng. If anyone has any information on the
effects of the storm, please e-mail it to me and I will include
it in a future summary.
***********************************************************************
NORTH INDIAN OCEAN (NIO) - Bay of Bengal and Arabian Sea
Activity for August: 1 tropical depression **
** - classified as a depression by IMD only
North Indian Ocean Activity for August
--------------------------------------
As in July, there were no tropical cyclones of tropical storm or
hurricane intensity in the North Indian Ocean, nor were warnings
issued on any tropical depressions by JTWC. There was one system
that was carried as a tropical depression by the IMD. This system
formed on 6 Aug in the northern extremities of the Bay of Bengal
about 85 nm south of Calcutta. The weak depression subsequently
moved west-northwestward and inland into Orissa state on the 7th.
By 1200 UTC on 8 Aug the weakening LOW was located about 80 km
south of Pendra. The MSW was likely no greater than 25 kts.
***********************************************************************
SOUTH INDIAN OCEAN (SIO) - South Indian Ocean West of Longitude 90E
Activity for August: No tropical cyclones
***********************************************************************
AUSTRALIAN REGION (AUG) - From Longitude 90E Eastward to Longitude 160E
Activity for August: No tropical cyclones
***********************************************************************
SOUTHWEST PACIFIC (SWP) - South Pacific Ocean East of Longitude 160E
Activity for August: No tropical cyclones
***********************************************************************
EXTRA FEATURE
In order to shorten the amount of typing in preparing the narrative
material, I have been in the habit of freely using abbreviations and
acronyms. I have tried to define most of these with the first usage
in a given summary, but I may have missed one now and then. Most of
these are probably understood by a majority of readers but perhaps a
few aren't clear to some. To remedy this I developed a Glossary of
Abbreviations and Acronyms which I included in the July, 1998 summary.
I will include this glossary from time to time, primarily in "lean"
months without a lot of tropical cyclone activity to cover. But if
anyone missed receiving it and wishes to obtain a copy, send me an
e-mail privately and I'll forward them a copy.
***********************************************************************
AUTHOR'S NOTE: This summary should be considered a very preliminary
overview of the tropical cyclones that occur in each month. The cyclone
tracks (provided separately) will generally be based upon operational
warnings issued by the various tropical cyclone warning centers. The
information contained therein may differ somewhat from the tracking and
intensity information obtained from a "best-track" file which is based
on a detailed post-seasonal analysis of all available data. Information
on where to find official "best-track" files from the various warning
centers will be passed along from time to time.
The track files are not being sent via e-mail. They can be retrieved
in the following manner:
(a) FTP to: hrd-type42.nhc.noaa.gov [140.90.176.206]
(b) Login as: anonymous
(c) For a password use your e-mail address
(d) Go to "data" subdirectory (Type: cd data)
(e) Set file type to ASCII (Type: ascii)
(f) Transfer file (Type: get remote_file_name local_file_name )
(The files will be named with an obvious nomenclature--using
August as an example: aug99.tracks)
(g) To exit FTP, type: quit
Both the summaries and the track files are standard text files
created in DOS editor. Download to disk and use a viewer such as
Notepad or DOS editor to view the files.
The first summary in this series covered the month of October,
1997. If anyone wishes to retrieve any of the previous summaries,
they may be downloaded from the aforementioned FTP site at HRD. The
summary files are catalogued with the nomenclature: aug99.sum, for
example.
Back issues can also be obtained from the following websites
(courtesy of Michael Bath, Michael V. Padua, Tom Berg, Michael
Pitt, and Rich Henning):
http://australiansevereweather.com/cyclones/>
http://www.geocities.com/capecanaveral/6825>
http://www.hcane.com/tropcl.html>
http://www.qisfl.net/home/hurricanemike>
http://www.met.fsu.edu/gsc/Docs/Grads/henning/cyclones/>
Another website where much information about tropical cyclones may
be found is the website for the UK Meteorological Office. Their site
contains a lot of statistical information about tropical cyclones
globally on a monthly basis. The URL is:
http://www.met-office.gov.uk/sec2/sec2cyclone/sec2cyclone.html>
TROPICAL CYCLONE REPORTS AVAILABLE
I have discovered that JTWC now has available on its new website
the complete Annual Tropical Cyclone Report for 1997 (1996-1997 season
for the Southern Hemisphere). Also, tracks only for the 1998 tropical
cyclones are currently available.
The URL is: http://199.10.200.33/jtwc.html>
Also, TPC/NHC has available on its webpage nice "technicolor"
tracking charts for the 1998 Atlantic and Eastern North Pacific
tropical cyclones; also, preliminary storm reports for all the 1998
Atlantic and Eastern North Pacific cyclones are now available.
The URL is: http://www.nhc.noaa.gov/1998.html>
Prepared by: Gary Padgett
E-mail: [email protected]
Phone: 334-222-5327 (nights & weekends) / 850-882-2594 (weekdays)
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Document: summ9908.htm
Updated: 4th January 2007 |
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