MONTHLY GLOBAL TROPICAL CYCLONE SUMMARY
SEPTEMBER, 2006
First Installment
SPECIAL NOTE: The September summary is being issued in three
installments. This first installment covers the Atlantic basin, while
the second will cover the Northeast Pacific and North Indian Ocean
basins. The third and final installment will cover the Northwest
Pacific basin.
(The third installment of the August summary, covering the Northwest
Pacific basin, has not yet been issued. Kevin Boyle is writing the
reports for the five NWP storms, and I have received four of them from
Kevin at the present time. Hopefully, the final installment of the
August summary will be issued within a few days.)
NOTE FROM THE AUTHOR: Almost two years ago I began including links to
track graphics prepared by John Diebolt of Tucson, Arizona, and archived
on his tropical cyclone website. A few months back John experienced a
disk crash which resulted in a error. He had to request assistance from
the programmer who had written the map-generation software, but so far
has not been able to get the problem solved. As a convenience to users,
I've also recently been including links to the individual tabular tracks,
prepared by myself, which John had archived on his website. Now, due to
family concerns, John has not had time to place the tracks for recent
cyclones on the website. I have checked the websites listed at the end
of the summaries and found that the entire September track file has been
archived on two of them. The links are:
http://australiasevereweather.com/cyclones/2007/trak0609.htm
http://www.typhoon2000.ph/garyp_mgtcs/sep06tks.txt
(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.)
*************************************************************************
SEPTEMBER HIGHLIGHTS
--> Atlantic more active--four hurricanes form but all recurve well out
in Atlantic--Bermuda, Newfoundland, Azores, Spain, Ireland and
Great Britain experience some effects
--> Major hurricane strikes mainland Mexico
--> Intense typhoon slashes through central Philippines while another
strikes Ryukyus
--> Strong Arabian Sea tropical storm forms just off western Indian
coast
*************************************************************************
ACTIVITY BY BASINS
ATLANTIC (ATL) - North Atlantic Ocean, Caribbean Sea, Gulf of Mexico
Activity for September: 2 hurricanes
2 major hurricanes
Sources of Information
----------------------
Most of the information presented below was obtained from the
various tropical cyclone products issued by the Tropical Prediction
Center/National Hurricane Center (TPC/NHC) in Miami, Florida:
discussions, public advisories, forecast/advisories, tropical weather
outlooks, special tropical disturbance statements, etc. Some
additional information may have been gleaned from the monthly
summaries prepared by the hurricane specialists and available on
TPC/NHC's website. All references to sustained winds imply a
1-minute averaging period unless otherwise noted.
Atlantic Tropical Activity for September
----------------------------------------
Following are some statistics for the Atlantic basin during the month
of September:
September Average
Parameter 2006 1950 - 2005
--------------------------------------------------------
Named Storms (NS) 4 3.5
Hurricanes (H) 4 2.5
Intense Hurricanes (IH) 2 1.3
Named Storm Days (NSD) 30.75 23.0
Hurricane Days (HD) 18.25 12.9
Intense Hurricane Days (IHD) 3.00 3.4
The month of September, 2006, was above average in all but the IHD
parameter, and that was near normal. The overall net tropical cyclone
activity (utilizing all six parameters) was about 132% of the average
level normally seen in September, whereas in August overall tropical
cyclone activity was only about half of normal. The month was only
slightly less active than September, 2005, whose overall tropical
cyclone activity level was about 147% of the average. The Northeast
Pacific basin, which had been extremely active in August, was well
below normal in September.
As the month opened, Tropical Storm Ernesto was approaching the coast
of North Carolina just shy of hurricane intensity. Ernesto made land-
fall in North Carolina early in the morning of 1 September and continued
northward through the Mid-Atlantic states as it weakened over the next
couple of days. The complete report on Hurricane Ernesto may be found
in the August summary.
Four hurricanes formed in the tropical Atlantic during the month of
September, and all recurved northeastward well east of the United States.
Hurricane Florence came the farthest west, passing just west of Bermuda
near its peak intensity of 80 kts. Florence then headed toward the
island of Newfoundland, and though it became extratropical, remained at
hurricane intensity and passed just southeast of the island. Hurricanes
Gordon and Helene both became Category 3 hurricanes but passed northward
several hundred miles east of Bermuda. Helene did not affect any land
areas, but Gordon accomplished the rare feat of passing through the
Azores Islands as a hurricane, although none of the islands experienced
sustained hurricane-force winds. However, the hurricane's extratropical
stage brought very strong winds to the Iberian Peninsula and the British
Isles, causing widespread damage. Finally, at the end of the month
Hurricane Isaac recurved a few hundred miles east of Bermuda and later
brushed Newfoundland's Avalon Peninsula while still a strong tropical
storm.
Reports on all the September hurricanes follow. The link to the
Wikipedia page for the 2006 Atlantic hurricane season is as follows:
http://en.wikipedia.org/wiki/2006_Atlantic_hurricane_season
Detailed reports for Florence and Gordon are available and the links to
those appear in their respective summaries below. However, no detailed
Wikipedia reports are available for Helene and Isaac, but brief reports
with a satellite image and track chart may be found at the above link.
HURRICANE FLORENCE
(TC-06)
3 - 15 September
--------------------------------------
A. Introduction and Storm Origins
---------------------------------
Hurricane Florence was the first Cape Verde hurricane of the 2006
Atlantic tropical cyclone season. The storm was notable in its early
tropical storm stage for its extremely large size. At one point on
8 September gales covered an area 600 nm in diameter--with the MSW at
only 45 kts. Florence eventually consolidated its forces some as it
reached hurricane intensity, but still remained a rather large tropical
cyclone. The storm passed just west of Bermuda on 11 September, later
transforming into a severe extratropical cyclone and bringing winds of
hurricane force to southeastern Newfoundland. Florence was the only
one of the four September hurricanes to pass west of Bermuda--the
remaining three recurved well east of the British colony.
A tropical wave moved off the African coast during the final week of
August and by late on the 28th was located about 525 nm southwest of
the Cape Verde Islands. By the 29th the wave was producing a
concentrated area of convection which was showing some signs of
organization. However, by the next day the shower activity had become
disorganized and no imminent development was anticipated. By the after-
noon of the 31st organization had improved once more and slow development
was considered possible as the system moved slowly westward.
The morning of 1 September found the wave located about 750 nm west-
southwest of the Cape Verdes with little change in organization. Shower
activity increased once more on 2 September with the system now located
about 1050 nm east of the Windward Islands. A second area of disturbed
weather was located to the east and the presence of this disturbance
was thought to be having a slight inhibiting effect on the westernmost
system. The wave located to the east had emerged from the African coast
on 29 August and by 2 September was showing distinct signs of increased
organization. By 3 September it was becoming apparent that the eastern-
most wave was developing, and the first wave, located then only about
350 nm to the west, was forecast to weaken and merge with the second
wave.
Advisories were initiated on Tropical Depression 06 at 2100 UTC on
3 September with the center located approximately 1325 nm east of the
Leeward Islands and moving northwestward at 12 kts. SAB and TAFB were
assigning Dvorak classifications of T2.0, and buoy 41026 had reported
32-kt winds late in the morning, so the initial intensity was estimated
at 30 kts. The circulation was broad and the center was difficult to
pinpoint, and also it was considered likely that multiple swirls were
rotating around a common center. The depression continued moving slowly
west-northwestward over the next couple of days. Banding features were
impressive, but the LLCC was broad with several possible centers. An
upper-level trough to the northwest was inducing some southwesterly
shear over the cyclone, and in conjunction with some drier air,
development proceeded rather slowly.
B. Synoptic History
-------------------
The depression was upgraded to Tropical Storm Florence at 1500 UTC
on 5 September, nearly two days after being classified as TD-06. The
upgrade was based primarily on QuikScat and SSM/I data which showed
large patches of 30-35 kt uncontaminated winds in the northwest quadrant.
Florence at this time was located around 800 nm east of the Leeward
Islands and was moving westward at 10 kts. Gales extended out over
100 nm in the northeast quadrant and 90 nm to the northwest. Once
named, the large, sloppily-organized tropical storm intensified very
slowly. Winds were upped to 40 kts at 05/1800 UTC and to 45 kts at
06/1500 UTC, but remained pegged there for almost three days as Florence
continued on a west-northwesterly track. The cloud pattern was at times
shapeless with very asymmetric convection, and locating the LLCC was a
continuing difficult job. Due to the large size of the storm and given
the forecast track, the Bermuda Weather Service issued a hurricane watch
for the island at 2100 UTC on 8 September with the center located 550 nm
to the south-southeast.
Florence began to display some signs of strengthening on the 8th. An
Air Force hurricane hunter aircraft reached the storm around 0400 UTC
on 9 September and found a CP of 993 mb with a maximum 850-mb FLW of
61 kts about 50 nm northeast of the center, so the MSW was upped to
55 kts at 09/0600 UTC. Satellite intensity estimates had increased to
65 kts from SAB and AFWA and 55 kts from TAFB. However, it was to be
24 hours longer before Florence finally became the season's second
hurricane. During the 9th satellite intensity estimates continued to
rise, but other considerations led to Florence being held at tropical
storm intensity, and later in the day the storm exhibited a slight
weakening trend. The MSW was upped to 60 kts at 1800 UTC, but decreased
slightly to 55 kts three hours later.
Florence was finally upgraded to hurricane status at 0600 UTC on
10 September while located about 315 nm south of Bermuda and moving
north-northwestward at 11 kts. A reconnaissance aircraft had found
peak FLWs of 84 kts and a CP of 976 mb with a pressure drop of 17 mb
in a 12-hour period. Florence continued to strengthen, reaching its
peak intensity of 80 kts at 1800 UTC on the 10th while centered about
200 nm south-southwest of Bermuda. This was based on a peak FLW of
96 kts measured by a reconnaissance plane, but using a slightly lower
reduction factor than the normal 90% used for reducing 700-mb FLWs to
the surface. Dropsonde data suggested that a reduction factor of 80
to 85% would be more appropriate in this case. Florence turned north-
ward on the 11th and made its closest approach to Bermuda around
11/1200 UTC, the center passing about 50 nm to the west of the colony.
The estimated MSW had dropped briefly to 70 kts, but was upped back
to 80 kts three hours later. An elevated observing site in Bermuda
reported a peak wind gust of 97 kts. Hurricane force winds extended
outward 60 nm from the center in the eastern quadrants and 30 nm to
the west. Florence's estimated minimum CP of 972 mb occurred around
11/1800 UTC.
The hurricane had shifted to a north-northeastward heading as it was
brushing by Bermuda, and the track became increasingly northeasterly as
Florence accelerated into the North Atlantic ahead of a mid-level
tropospheric trough over the Canadian Maritimes. The storm also began
to weaken significantly on the 12th and winds had dropped to minimal
hurricane intensity by 0600 UTC. Satellite imagery indicated that
Florence was in the early to mid stages of extratropical transition.
Dry middle and upper-level air wrapping around the southern periphery
had eroded most of the inner-core deep convection. TPC/NHC issued
their final advisory on Florence at 12/2100 UTC with the center located
about 525 nm southwest of Cape Race, Newfoundland, moving northeastward
at 18 kts. The only convection remaining near the center was a band
of showers and thunderstorms located about 100 nm northwest of the
center.
Florence transformed into a powerful extratropical cyclone of
hurricane intensity and actually strengthened to 70 kts while near south-
eastern Newfoundland. This re-intensification was due to interaction
with the upper-level trough located over the Maritimes. The center of
post-tropical Florence passed just south of Cape Race with a CP
estimated around 963 mb. Sustained hurricane-force winds were
recorded on Sagona Island in Fortune Bay with gusts well above hurricane
intensity felt on the Burin Peninsula and along the Newfoundland coast
from Fortune Bay to Burgeo, including St. Pierre and Miquelon. After
passing by Newfoundland the remnants of Florence accelerated eastward
across the North Atlantic, slowly weakening. By 1800 UTC on 15 September
the former hurricane had weakened into a 35-kt gale roughly halfway
between Newfoundland and Ireland. The ex-Florence LOW was eventually
forced northward toward Iceland while a weak front associated with the
storm's remnants made its way toward Great Britain.
C. Meteorological Observations
------------------------------
As noted above, an elevated site in Bermuda (elevation unknown)
reported a peak wind gust of 97 kts.
In Newfoundland, the following peak wind gusts and storm total
rainfalls were reported (data courtesy of Chris Fogarty--thanks to
Chris for sending it along):
Sagona Island 88 kts
St. Lawrence 72 kts 47 mm
St. Pierre 69 kts
Bonavista 59 kts 33 mm
Cape Race 58 kts 25 mm
Grates Cove 58 kts
Pool's Island 58 kts
St. John's 55 kts 48 mm **
Twillingate 54 kts 22 mm
Burgeo 53 kts 14 mm
Argentia 49 kts 35 mm
Gander 42 kts 56 mm
Terra Nova Park 58 mm
Cape Pine * 41 mm
* - private weather station on the southern Avalon Peninsula
** - 48 mm fell at St. John's airport and 52 mm in Mount Pearl
D. Damage and Casualties
------------------------
In Bermuda strong winds blew down trees and power lines, leaving over
25,000 homes and businesses without power at the height of the storm.
A few houses experienced window and roof damage, and a few injuries
were reported. Overall damage, however, was relatively light.
During the course of its life Florence produced strong swells and
dangerous surf conditions, including rip currents, in the northern Lesser
Antilles, Virgin Islands, Puerto Rico, Hispaniola, Bahamas, along the
U. S. Eastern Seaboard, and in the Canadian Maritimes. However, no
fatalities are known to have resulted from Hurricane Florence.
In Newfoundland some damage was reported in the form of blown down
trees and damage to shingles and siding of homes. Also, some boats
and roadways were damaged in and around the Burin Peninsula from heavy
surf conditions. Power outages were also reported in the St. John's
area.
More information can be found in the online Wikipedia report, from
which some of the above information was obtained. The link is:
http://en.wikipedia.org/wiki/Hurricane_Florence_%282006%29
(Report written by Gary Padgett)
HURRICANE GORDON
(TC-07)
11 - 21 September
-------------------------------------
A. Introduction and Storm Origins
---------------------------------
Hurricane Gordon became the first intense hurricane of the 2006
Atlantic hurricane season, reaching Category 3 status on the Saffir/
Simpson scale on 13 September while several hundred miles southeast
of Bermuda. While an impressive Category 3 hurricane, the most
remarkable aspect of Gordon's life was a re-intensification to
Category 2 status near latitude 40N after weakening to a minimal
hurricane three days earlier. Also unusual was the storm's passage
through the central Azores while still a tropical cyclone of hurricane
intensity. Finally, in its extratropical stages the cyclone brought
very strong winds to Spain, the United Kingdom and Ireland.
A tropical wave left the coast of Africa on 1 September and began
marching westward across the tropical Atlantic. By the morning of
4 September the system was located several hundred miles west of the
Cape Verde Islands and was showing signs of organization. The system
looked even better the next day, but by the 6th environmental conditions
had become less favorable. The primary inhibiting factor was the
proximity of the disturbance to large Tropical Storm Florence. The
system continued to move west-northwestward for the next few days and
managed to produce disorganized showers and thunderstorms. As the
separation between this system and Florence slowly increased, conditions
gradually improved and by the afternoon of 10 September thunderstorm
activity had become more concentrated near the circulation center about
475 nm east-northeast of the Leeward Islands.
The first advisory was issued on Tropical Depression 07 at 0300 UTC
on 11 September, the center being located about 455 nm east-northeast
of the Leeward Islands and moving westward at 5 kts. The initial MSW
was set to 25 kts, based on 0000 UTC Dvorak estimates of T1.5 and a
report of WSW 27-kt winds from ship MSJX8 just south of the center.
In contrast to Florence, which was a huge tropical cyclone in area,
TD-07 was a small cyclone with the entire circulation about 300 nm
wide and with deep convection mostly confined to the southern semicircle.
The depression gradually turned to a northwesterly track as its
organization slowly improved. A reconnaissance mission into the system
around midday on the 11th found peak FLWs of 47 kts, corresponding to
38 kts at the surface. Based on this the depression was upgraded to
Tropical Storm Gordon at 11/2100 UTC with the intensity estimated at
40 kts. Gordon was then located about 375 nm northeast of the Leewards,
moving to the northwest at 8 kts. The storm was quite small: gales
extended outward 40 nm from the center to the northeast and 25 nm to
the southeast with no gale-force winds found in the western quadrants.
B. Synoptic History
-------------------
The strengthening Gordon soon turned northward toward a break in
the subtropical ridge left in the wake of Florence. And even though
the earlier storm was moving out of the way, an upper-level LOW
located about 1000 nm to the east-northeast of Gordon was forecast
by the global models to move westward and break down the remaining
subtropical ridge, allowing the storm to recurve. Gordon was upgraded
to a hurricane at 0300 UTC on 13 September while located almost 500 nm
north-northeast of the Leeward Islands and moving north at 8 kts. The
upgrade was based on the appearance of an intermittent eye feature and
Dvorak T-numbers of 4.0 and 4.5 from all agencies. By 1500 UTC Gordon
had developed a well-defined eye and the MSW was increased to 80 kts.
Although not yet forecast to become a major hurricane, Gordon's
satellite signature continued to improve and at 14/0300 UTC the cyclone
was upgraded to a Category 3 hurricane with winds of 105 kts and an
estimated CP of 955 mb, which proved to be Gordon's peak lifetime
intensity. Subjective T-numbers had reached 5.5 while the AODT had
been giving 3-hour estimates of T6.0. Gordon had become embedded in
mid-latitude westerly flow and was moving north-northeastward at 11 kts
from a position about 485 nm east-southeast of Bermuda. Hurricane-force
winds were confined to a zone within about 25 nm of the center while
gales reached outward over 100 nm in all quadrants.
Gordon held on to its 105-kt peak for about 24 hours, then slowly
began to weaken as it began to move over gradually cooling waters. By
the 15th the low-level ridge to the east of Gordon had weakened and the
hurricane was left in an area with very weak steering currents.
Consequently, Gordon remained quasi-stationary for over 24 hours as
mid-level ridging to the north and east of the hurricane strengthened.
The MSW had dropped to 65 kts by 0900 UTC on 16 September, and continued
weakening with extratropical transition within 72 hours was forecast.
Gordon, however, had other plans. The storm continued to display a
ragged eye in infrared imagery for the next day and maintained hurricane
intensity. At 1500 UTC on the 17th satellite imagery depicted an eye,
deep convection and impressive outflow. TAFB and SAB were both reporting
Dvorak estimates of 77 kts with AFWA's being 65 kts, so the MSW was upped
to 70 kts. The storm by this time was moving north-northeastward at
9 kts and the track became increasingly eastward at a faster pace as the
day wore on.
Gordon's winds were upped further to 80 kts at 1500 UTC on the 18th.
The storm was located almost 1000 nm west of the Azores, moving north-
eastward at 17 kts, and all the models were anticipating a due eastward
track as the hurricane was steered by the zonal westerlies north of the
subtropical ridge. At 18/2100 UTC a tropical storm watch was issued
for the Azores as the official forecast carried the storm over or very
near the islands during the next 24 to 36 hours. Hurricane Gordon
reached a secondary peak intensity of 90 kts at 19/0900 UTC while
located about 550 nm west of the island of Terceira in the Azores,
scooting eastward at 24 kts. The eye was remaining distinct and cloud-
free and intensity estimates ranged from 77 to 90 kts. Gordon was
a healthy Category 2 hurricane bearing down on the Azores--something
not seen very often.
As the cyclone approached the Azores it began to slowly weaken but
was still sporting 75-kt winds as it moved rapidly through the central
part of the island chain. At 20/0000 UTC Gordon was located about
210 nm west of Sao Miguel, moving eastward at 29 kts. Six hours later
the center was 75 nm south-southeast of Terceira and the MSW had dropped
to 65 kts. AMSR-E microwave data indicated that the westerlies had
finally begun to affect the vertical structure of Gordon with the
center displaced about 60 nm west of the limited deep convection. The
cyclone was downgraded to a 50-kt tropical storm at 20/1200 UTC but still
had the tropical characteristics of a warm core and deep convection near
the center. However, a strong cold front was approaching the core and
extratropical transition was just around the corner. TPC/NHC issued its
final advisory on Gordon at 20/2100 UTC since the cold front had
interacted with the cyclone's circulation. The storm's center was then
located about 340 nm west of Lisbon, Portugal, racing eastward at 30 kts.
Following extratropical transition, the storm tracked northwestward
toward the Iberian Peninsula, turning northward on 21 September towards
Great Britain and Ireland. By the 22nd it had become completely
absorbed into a large LOW to the west of Ireland.
C. Meteorological Observations
------------------------------
As Gordon sped through the Azores, a 10-min avg sustained wind of
49 kts with a peak gust of 71 kts was reported at Santa Maria airport
in the eastern Azores.
In Spain, as the extratropical stage of Gordon paid a visit to the
Iberian Peninsula, gusts up to 89 kts were reported at Fisterra. Also,
Castro Vicaludo-Oia reported gusts to 91 kts. Some other peak wind
gust reports: Cabo Vilan - 82 kts; Ferrol - 64 kts; Alvedro - 60 kts;
Ancares - 55 kts; Oiz - 59 kts. Near Madrid winds to 57 kts were felt
at Punto Navacerrada. Heavy rains fell across Spain with 65.5 mm
recorded at Canfranc. A pressure of 989.7 hPa was measured at Coruna
(unknown if this was adjusted to MSL). Also, waves up to 7 meters were
reported.
In the UK, a gust of 70 kts was recorded in southwest Great Britain
late on the 21st. Also, gusts to 65 kts affected Northern Ireland
during the night of 21-22 September. High winds were also felt in
Scotland and Ireland from the extratropical remnants of Gordon.
D. Damage and Casualties
------------------------
In the Azores damage was relatively light with only reports of toppled
trees and power lines. A few communities lost electrical power,
particularly on Santa Maria Island.
The strong winds experienced over much of Spain blew down trees,
traffic lights and signs. One man was injured due to a tree falling
on his car, and many roads were blocked by fallen trees. Another
report indicated that four people were injured in the Galicia region
with 100,000 homes losing power. Portugal also experienced heavy rain
and wind gusts that downed trees and caused roof damage.
In Ireland around 1500 homes lost power due to the stormy conditions
wrought by ex-Gordon, and there was one injury reported. In southwestern
England more than 1000 homes lost power in Truro, Cornwall, and a rail
line between Exeter and Plymouth was damaged by high surf. Up to
100,000 homes in Northern Ireland lost power as tree branches fell on
power lines, and fallen trees blocked many roads with flooding reported
in some areas.
Additional information can be found in the online Wikipedia report
from which some of the above information was obtained. The link is:
http://en.wikipedia.org/wiki/Hurricane_Gordon_%282006%29
(Report written by Gary Padgett)
HURRICANE HELENE
(TC-08)
12 - 26 September
-------------------------------------
A. Introduction and Storm Origins
---------------------------------
Hurricane Helene, the fourth hurricane of the 2006 Atlantic season,
was a classic Cape Verde hurricane in every sense of the word, forming
as a tropical depression south of the islands and becoming a large,
intense hurricane. Fortunately, unlike many Cape Verde hurricanes of
the past, Helene recurved well to the east of Bermuda and did not
threaten any populated areas in the western Atlantic basin. Also, the
storm was the only one of the September hurricanes which did not affect
Newfoundland or the Azores, passing well between those islands. In some
aspects the hurricane was similar to its namesake in 1988. However, the
Hurricane Helene of that year was somewhat more intense, becoming a
Category 4 hurricane as it moved northward through the central Atlantic.
This year's Helene became the strongest hurricane of the season, peaking
at 110 kts on 18 September--a strong Category 3 hurricane on the Saffir/
Simpson scale (although there is some possibility this value was a
little too high--see discussion below).
A strong tropical wave emerged from the western coast of Africa on
11 September and immediately began to show signs of developing as a
tropical cyclone. By the morning of the 12th the system had become
sufficiently organized that advisories were initiated on Tropical
Depression 08, located about 160 nm south-southeast of the southernmost
Cape Verde Islands and moving west at 16 kts. A well-defined curved
convective band had formed over the western semicircle and two ship
reports of northerly and southerly winds confirmed the existence of
a closed surface circulation. Although it was anticipated that TD-08
would become a tropical storm by early on the 13th, intensification
proceeded somewhat more slowly than forecast. The depression was large
and somewhat elongated with multiple low-level swirls within a larger
circulation gyre. By the evening of 13 September the depression's
convective pattern had gradually become better organized and Dvorak
T-numbers had reached 2.5 from all agencies; hence, the system was
upgraded to Tropical Storm Helene at 14/0300 UTC. Helene was located
about 490 nm west of the Cape Verdes and moving quickly to the west-
northwest at 19 kts along the southern periphery of a deep-layer ridge.
B. Synoptic History
-------------------
After reaching tropical storm intensity, Helene was slow to intensify
further. Winds remained at 40 kts through 15/0900 UTC, but were bumped
up to 50 kts at 1500 UTC after satellite imagery revealed that the cloud
pattern had become better organized. With low shear and warm SSTs ahead
along the anticipated track, Helene was forecast to reach hurricane
intensity within 24 hours. The cyclone was upgraded to the season's
fourth hurricane at 16/1500 UTC while centered approximately 1000 nm
east of the northern Leeward Islands. Once having reached that milestone
Helene began to steadily intensify as it continued to churn northwestward
along the southwestern side of the deep-layer ridge toward a weakness in
the ridge. Helene became the season's second major hurricane at 18/0300
UTC when winds were upped to 100 kts, and the storm reached its peak
estimated intensity of 110 kts at 1500 UTC while centered about 950 nm
east-southeast of Bermuda, moving northwestward at 8 kts. Earlier that
morning Helene had reached the longitude of Gordon and the storm had
temporarily moved on a north-northwestward track for several hours, but
by advisory time it appeared that an anticipated westward turn, forecast
by the global models, was about to begin.
A surprise was in store later that afternoon. A NOAA research
aircraft made a pass through the center of Helene and found that the
hurricane was not quite as strong as previously estimated by satellite.
Based on the reconnaissance findings, the MSW was lowered to 100 kts at
2100 UTC. (It seems very possible to the author that in post-storm
analysis Helene's peak MSW may be lowered to 105 or even 100 kts.) On
the 19th Helene's motion became westerly as a mid-tropospheric ridge to
the north built, but by the 20th the storm was moving northwestward once
more. A NOAA aircraft made a pass through Helene early on the afternoon
of the 19th and measured a CP of 956 mb. The peak FLW was 98 kts at
850 mb in the northwest quadrant, and two eyewalls were present at radii
of 40 nm and 120 nm. Based on these findings Helene's MSW was reduced
to 95 kts. This brought the hurricane down to Category 2 status, and
even though brief re-intensification to a Category 3 hurricane was
forecast, this never materialized. Helene at this stage was a fairly
large hurricane with hurricane-force winds extending outward 55 nm to
the northeast of the center and 35 nm to the southwest. Gales covered
a zone over 300 nm in diameter.
Over the next few days Helene continued to weaken very slowly as it
moved over gradually cooling SSTs and encountered some drier air. The
storm's heading gradually turned to the north and then northeastward
ahead of an approaching trough in the westerlies. The MSW was reduced
to 85 kts at 21/0300 UTC based upon data from a NOAA P-3 mission during
the previous afternoon--satellite intensity estimates at the time ranged
from 77 to 90 kts. The storm passed about 485 nm east of Bermuda around
0900 UTC on 22 September, moving northeastward at 13 kts. The intensity
was lowered to 70 kts at 21/1500 UTC but was bumped back to 75 kts twelve
hours later as cold cloud tops had expanded and become more concentric
around the center. However, the satellite presentation soon began to
deteriorate once again and Helene was reduced to minimal hurricane
intensity at 22/2100 UTC and downgraded to a 60-kt tropical storm six
hours later. A QuikScat pass from around 2200 UTC showed 50-60 kt
winds southeast of the center.
In a manner somewhat similar to Hurricane Gordon, Helene pulled a
surprise over the reaches of the North Atlantic. The TPC/NHC discussion
at 23/0900 UTC noted that a small burst of convection with cloud tops
to -60 C had persisted over the LLCC for more than six hours, and given
the forward speed of 22 kts, the intensity could be higher than the
advisory MSW of 60 kts. At 23/1200 UTC a special advisory was issued,
upping the winds to 80 kts. A recent QuikScat pass had indicated that
Helene possessed a large area of hurricane force winds southwest of the
center and the data showed some vectors to 80 kts. So Helene was
re-upgraded to a hurricane, even though the storm appeared to be in
the process of transforming into an extratropical cyclone. The 2100 UTC
discussion noted that the cyclone displayed both tropical and extra-
tropical characteristics. Helene had a frontal-like and asymmetric
appearance in satellite imagery with limited deep convection, yet FSU
cyclone phase space diagrams diagnosed the storm with a deep-layer warm
core.
The MSW remained 80 kts for the 2100 UTC advisory but began to decline
after that. The final TPC/NHC advisory on Helene was issued at 1500 UTC
on 24 September, placing the center about 515 nm west-northwest of the
Azores and moving northeastward at 19 kts. The previous advisory had
treated Helene as a 75-kt hurricane, but the final one reduced the system
to a 60-kt extratropical storm. Helene by this time had acquired
definitive frontal features with extensive cold-air advection over the
southwestern semicircle. Also, a QuikScat pass around 0900 UTC had
suggested that the strongest winds were becoming removed from the center.
Interestingly, microwave data still showed a warm core, but this was
believed to be due to a warm seclusion that is common in strong extra-
tropical cyclones. The post-tropical Helene continued to speed east-
northeastward across the North Atlantic and by 25/0000 UTC was generating
hurricane-force winds of 70 kts around a 964-mb center. Thereafter, the
system began to slowly weaken and the last information available to the
author placed a 980-mb center well north of the Azores at 26/0000 UTC.
C. Damage and Casualties
------------------------
No casualties or damage are known to have resulted from Hurricane
Helene.
(Report written by Gary Padgett)
HURRICANE ISAAC
(TC-09)
27 September - 4 October
--------------------------------------------
A. Introduction and Storm Origins
---------------------------------
Hurricane Isaac, the final hurricane of September and (as of this
writing--9 November) the final tropical cyclone of the 2006 Atlantic
season, followed a similar trajectory to its two predecessors, Gordon
and Helene, recurving well east of Bermuda. Isaac, however, didn't
recurve quickly enough to miss Newfoundland, and the storm brushed the
extreme southeastern tip of the island while still a fairly strong
tropical storm shortly before becoming extratropical. If indeed there
are no more tropical cyclones this season, 2006 will be the first
season since 1963 to produce exactly 9 tropical storms or hurricanes
(although in that season one tropical storm was unnamed). The last
year to see exactly 9 named storms (down through the letter "I") was
1964. However, during post-analysis 3 unnamed storms were added to
the official roster, making a total of 12 tropical storms/hurricanes.
(These statistics are based upon the current Best Track database. When
the ongoing reanalysis is complete these records may no longer hold.)
Isaac's origins can be traced back to a tropical wave which emerged
from the African coast on 18 September. By early morning of the 20th
the wave was accompanied by a broad surface LOW centered about 390 nm
south-southwest of the southernmost Cape Verde Islands. Convective
activity had increased and additional development was anticipated over
the next couple of days as the system moved west-northwestward. As
things turned out, a week elapsed before the system was upgraded to
a tropical depression. On the 23rd convective activity became better
organized, but by the next day upper-level winds had strengthened and
the potential for a tropical depression to form had diminished. During
the evening of 25 September the wave was looking a little healthier once
more and this trend continued through the 26th and into the 27th.
Environmental conditions gradually became more conducive for tropical
cyclogenesis and the first advisory on Tropical Depression 09 was issued
at 2100 UTC on 27 September. Visible satellite pictures indicated
that the LLCC had gradually become better defined and there was
sufficient deep convection for the system to be classified as a 30-kt
tropical depression. The center of TD-07 was located about 705 nm east-
southeast of Bermuda and moving northwestward at 12 kts. The depression
was upgraded to Tropical Storm Isaac at 1500 UTC the next morning (28th),
located about 575 nm east-southeast of Bermuda and moving northwestward
at 7 kts. The upgrade was based primarily on data from a QuikScat pass
at 0845 UTC which had shown evidence of 35-kt winds in the system.
Isaac was exhibiting some characteristics of a subtropical storm with
the radius of maximum winds near 75 nm and with an upper-level LOW almost
on top of the system. Some modest intensification was forecast. Even
though SSTs in Isaac's forecast track were near 26 C, upper-level
temperatures were also colder than normal, which would allow for more
instability and more convection.
B. Synoptic History
-------------------
The subtropical-like features continued on into 29 September.
Convection was becoming more consolidated around the center, but the
convection was rather weak with only a few cloud tops colder than -50 C.
By afternoon of the 29th Isaac was showing definite signs of increasing
strength: convection was increasing and attempting to form a ring
around the center and outflow was expanding and becoming more symmetric.
One factor which had earlier had some inhibiting effect on Isaac's
intensification was that it had been moving over cooler waters upwelled
by Gordon and Helene. The tropical cyclone was now leaving these cooler
waters behind and moving into a region with warmer SSTs and low vertical
shear. The 29/2100 UTC advisory was the first to forecast that Isaac
would reach hurricane intensity.
Isaac was upgraded to a hurricane at 1500 UTC on 30 September while
centered about 325 nm east-southeast of Bermuda. The cyclone was still
moving on a rather slow northwesterly track at 6 kts. Isaac had formed
a ragged eye, and a high-resolution QuikScat pass at 1000 UTC showed one
believable wind vector of 63 kts. Since that time, the storm's satellite
signature had improved and TAFB was reporting a T-number of 4.0. By
0600 UTC on 1 October Dvorak ratings from TAFB and SAB had reached T4.5,
so Isaac's MSW was bumped up to 75 kts--the peak for the storm's history.
Hurricane Isaac was then located about 245 nm east-northeast of Bermuda,
moving north-northwestward at 8 kts. The cyclone was situated between
the southwestern flank of a rather small deep-layered subtropical ridge
and a very large trough along the U. S. Atlantic Seaboard. A northward
turn was forecast, followed by acceleration to the northeast.
Following its peak in intensity Isaac's MSW slowly began to decrease.
Twenty-four hours after peaking in intensity Isaac had weakened into a
minimal 65-kt hurricane. Microwave imagery revealed that the LLCC was
displaced about 30 nm southwest of the mid-level center, and Dvorak
ratings from TAFB and SAB had decreased to T4.0 and T3.5, respectively.
The storm had recurved and was moving north-northeastward at 26 kts.
Isaac was downgraded to a 50-kt tropical storm at 02/1500 UTC, centered
about 165 nm south-southwest of Cape Race, Newfoundland, racing north-
northeastward at 36 kts. The cyclone passed just south of Cape Race
during the afternoon of 2 October, and by 2100 UTC Isaac was beginning
to take on extratropical features to the point that TPC/NHC issued their
final advisory on the system, which was then located about 45 nm east-
northeast of Cape Race. The extratropical remnants of Isaac continued
to speed northeastward across the North Atlantic, gradually turning east-
northeastward, and by 04/1800 UTC had weakened below gale force several
hundred miles west of Ireland.
C. Meteorological Observations
------------------------------
The only land area affected by Isaac was the island of Newfoundland.
Following is a summary of Isaac's impact in the area sent by Chris
Fogarty of the Canadian Hurricane Centre. A special thanks to Chris
for sending this write-up.
"The centre of Tropical Storm Isaac passed approximately 45 km south-
east of Cape Race, Newfoundland, late in the afternoon of October 2nd.
Isaac was moving very quickly to the northeast near 70 km/hr. The storm
circulation was very small and strong winds were confined to only a short
distance to the north of the centre. The storm had maximum sustained
winds of 50 knots (92 km/hr) and a central sea level pressure near 993 mb
at its closest approach to the Avalon Peninsula. Rain from Isaac quickly
spread across the Avalon but did not extend beyond Trinity and Placentia
Bays. Isaac was still in its tropical cyclone phase when it passed by
the Avalon as seen in radar imagery. There was a rain-free area near the
centre, perhaps indicating the remnants of the storm's eye.
"The strongest winds on land were reported at Cape Race where gusts
reached 96 km/hr (52 knots) with the highest sustained winds of 74 km/hr
(40 knots). A peak wind of 76 km/hr (41 knots) was reported at a private
weather station on Cape Pine about 30 km west of Cape Race. Because of
the compact circulation of the storm and its rapid forward motion, winds
were much lighter over most of the Avalon Peninsula. The highest wind
gust at St. John's International Airport was 54 km/hr (29 knots). Peak
winds from an offshore buoy south of the storm reached 56 knots or
104 km/hr.
"Rainfall amounts directly associated with Isaac were generally less
than 25 mm due to its rapid forward motion and diminishing amount of
moisture. The highest measured rainfall was 26 mm at Cape Race. 19 mm
was measured at the Cape Pine location and only 10 mm in the St. John's
area.
"Forecasting the track and speed of the storm was very challenging,
as are most tropical storms and hurricanes moving into our region. Given
that Isaac was still a strong tropical storm very near the Avalon
Peninsula, a tropical storm warning was issued for the potential for high
winds. If the storm had tracked only 50 to 100 km farther north, the
St. John's area would have experienced much stronger winds. Isaac also
moved faster than originally expected, thus rainfall amounts were less
than forecast."
D. Damage and Casualties
------------------------
No casualties or significant damage are known to have resulted from
Hurricane Isaac.
(Report written by Gary Padgett and Chris Fogarty)
*************************************************************************
NORTHEAST PACIFIC (NEP) - North Pacific Ocean East of Longitude 180
Activity for September: 2 tropical depressions **
1 tropical storm
1 major hurricane
** - both the non-developing tropical depressions formed in the Central
North Pacific region
NOTE!!! The Northeast Pacific basin will be covered in the second
installment of the September summary.
*************************************************************************
NORTHWEST PACIFIC (NWP) - North Pacific Ocean West of Longitude 180
Activity for September: 1 possible tropical depression or storm **
3 tropical depressions ++
1 tropical storm ##
2 typhoons
2 super typhoons &&
** - system repeatedly received Dvorak ratings of T2.5 from both JTWC
and AFWA but was never started as a tropical depression, nor so
classified by JMA
++ - one of these was classified as a tropical depression by JMA only;
another formed late in month and became named tropical cyclone in
early October
## - system was classified as a minimal tropical storm by JTWC only
&& - one of these was a visitor from the Central North Pacific
NOTE!!! The Northwest Pacific basin will be covered in the third
installment of the September summary.
*************************************************************************
NORTH INDIAN OCEAN (NIO) - Bay of Bengal and Arabian Sea
Activity for September: 1 depression **
1 tropical cyclone of gale intensity ++
1 severe cyclonic storm
** - no warnings issued on this system by JTWC
++ - system was treated only as a depression by IMD
NOTE!!! The North Indian Ocean basin will be covered in the second
installment of the September summary.
*************************************************************************
SOUTHWEST INDIAN OCEAN (SWI) - South Indian Ocean West of Longitude 90E
Activity for September: No tropical cyclones
*************************************************************************
NORTHWEST AUSTRALIA/SOUTHEAST INDIAN OCEAN (AUW) - From 90E to 135E
Activity for September: No tropical cyclones
*************************************************************************
NORTHEAST AUSTRALIA/CORAL SEA (AUE) - From 135E to 160E
Activity for September: No tropical cyclones
*************************************************************************
SOUTH PACIFIC (SPA) - South Pacific Ocean East of Longitude 160E
Activity for September: No tropical cyclones
*************************************************************************
SPECIAL FEATURE - SOURCES OF TROPICAL CYCLONE INFORMATION
The purpose of this section is to list some websites where many and
varied types of tropical cyclone information are archived. Many readers
will know about these already, but for the benefit of those who don't,
I wanted to include them.
(1) Aircraft Reconnaissance Information
---------------------------------------
Various types of messages from reconnaissance aircraft may be
retrieved from the following FTP site:
ftp://ftp.nhc.noaa.gov/pub/products/nhc/recon/
Information regarding how to interpret the coded reconnaissance
messages may be found at the following URL:
http://www.nhc.noaa.gov/reconlist.shtml
Links are also included to websites with further information about the
U. S. Air Force 53rd Weather Reconnaissance Squadron and the NOAA Air-
craft Operations Center.
(2) Archived Advisories
-----------------------
All the advisory products (public advisories, forecast/advisories,
strike probabilities, discussions, various graphics) issued by TPC/NHC
are archived on TPC's website. For the current year (using 2004 as an
example), the archived products can be found at:
http://www.nhc.noaa.gov/archive/2004/index.shtml
Links to tropical products archives for earlier years are available at
the following URL:
http://www.nhc.noaa.gov/pastall.shtml
JTWC warnings for past storms are archived on the NRL Monterrey website:
http://www.nrlmry.navy.mil/tc_pages/tc_home.html
On the NRL site, the link to past years can be found in the upper left
corner of the screen.
I am not aware at the moment of any other TCWC which archives all
its tropical cyclone warning/advisory products for public access, but
if I learn of any, I will add them to this list.
(3) Satellite Imagery
---------------------
Satellite images of tropical cyclones in various sensor bands are
available on the NRL Monterrey and University of Wisconsin websites,
courtesy of Jeff Hawkins and Chris Velden and their associates. The
links are:
http://www.nrlmry.navy.mil/tc_pages/tc_home.html
http://cimss.ssec.wisc.edu/tropic/tropic.html
On the NRL site, the link to past years can be found in the upper left
corner of the screen. For the CIMSS site, a link to data archives is
located in the lower left portion of the screen.
Additional tropical satellite imagery, along with looping ability for
composite microwave imagery for the Western Hemisphere north of the
equator, can be found at:
(1) For the Eastern North Pacific:
http://www.ssd.noaa.gov/PS/TROP/trop-epac.html
(2) For the Atlantic, Gulf of Mexico and Caribbean Sea:
http://www.ssd.noaa.gov/PS/TROP/trop-atl.html
I'm sure there are other sites with available imagery available, and
as I learn of them, I will add the links to this list.
*************************************************************************
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 first included in the August, 1998
summary. I don't normally include the Glossary in most months in
order to help keep them from being too long. If anyone would like to
receive a copy of the Glossary, please e-mail me and I'll be happy
to send 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
from the archive sites listed below. (Note: I do have a limited e-mail
distribution list for the track files. If anyone wishes to receive
these via e-mail, please send me a message.)
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. Back issues can be obtained from the following websites
(courtesy of Michael Bath, Michael V. Padua, Michael Pitt, Chris
Landsea, and John Diebolt):
http://australiasevereweather.com/cyclones/
http://www.typhoon2000.ph <http://www.typhoon2000.ph/>
http://mpittweather.com <http://mpittweather.com/>
ftp://ftp.aoml.noaa.gov/hrd/pub/landsea/padgett/
http://www.tropicalcyclone2005.com/
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.metoffice.gov.uk/weather/tropicalcyclone
TROPICAL CYCLONE REPORTS AVAILABLE
JTWC now has available on its website the Annual Tropical Cyclone
Report (ATCR) for 2004 (2003-2004 season for the Southern Hemisphere).
ATCRs for earlier years are available also.
The URL is: http://199.10.200.33/jtwc.html
Also, TPC/NHC has available on its webpage nice "technicolor"
tracking charts for the 2005 Atlantic and Eastern North Pacific
tropical cyclones; also, storm reports for all the 2005 Atlantic
and Eastern North Pacific cyclones are now available, as well as
track charts and reports on storms from earlier years.
The URL is: http://www.nhc.noaa.gov <http://www.nhc.noaa.gov/>
A special thanks to Michael Bath of McLeans Ridges, New South Wales,
Australia, for assisting me with proofreading the summaries.
PREPARED BY
Gary Padgett
E-mail: garyp@alaweb.com
Phone: 334-222-5327
Kevin Boyle (Northwest Pacific)
E-mail: newchapelobservatory@btinternet.com
Simon Clarke (Northeast Australia/Coral Sea, South Pacific)
E-mail: scla4255@bigpond.net.au
*************************************************************************
*************************************************************************
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