29 August 2004

Gaston hits South Carolina, Frances weakens slightly, and Hermine forms...

From Jonathan Vigh: 
In an amazing show of fury and action, the tropical atmosphere has certainly
been living up to its full potential over the last 24 hours. Since
yesterday's update, Hurricane Frances strengthened into a rare cat. 4
hurricane and has since weakened slightly, Tropical Storm Gaston
strengthened and wallopped the South Carolina coast just shy of hurricane
strength, and Tropical Storm Hermine formed from that disturbed area of
weather southwest  of Bermuda that was mentioned 2 days ago (but not
yesterday). Just to recap the records being set -- Hermine is now the EIGHTH
tropical storm to form in August, the first time that has ever happened in
the modern era (beating out 1995's 7 tropical storms). In the last 29 days,
there has been nearly as much activity as would be expected in an entire
normal hurricane season. 

First, the low-down on Gaston. This system had a short, but active life,
having been just upgraded to a tropical storm yesterday morning. Very low
vertical shear and sea surface temperatures to 28 deg C provided favorable
conditions for strengthening. Gaston was clearly on the upswing as it
approached South Carolina and made landfall near McClenanville at 14Z (near
where Hurricane Hugo made landfall in 1989). If it had had a few more hours
over water, it most likely would have become a hurricane. Gaston's
circulation (eyewall?) raked far western Charleston County and Berkley
County with high winds and heavy rains. Some beach erosion was reported
along the coast, as well as peak gusts to 82 mph in downtown Charleston and
81 mph at Isle of Palms. Trees were downed and other minor damage was
reported (preliminary), but the main problems, as expected, came from the
extremely heavy rains. Radar indicates that as much as 13-15" may have
fallen in some areas, and considerable flash flooding was occurring this
afternoon. As of 03Z, Gaston has weakened to a tropical depression, but is
continuing to drop very heavy rains as its impressive rain shield moves
northward through North Carolina. Heavy rains will continue as the remnants
of Gaston track up the Eastern Seaboard over the next couple days.

Hermine appeared on the scene today from that area of disturbed weather
south of Bermuda that was mentioned a couple days ago. Deep convection has
persisted with this storm, although the system is under some moderate
vertical wind shear. Hermine is currently at 33.2N 71.3W with an estimated
central pressure of 1005 mb. The system is moving towards the NNW at 13 kt,
and should continue northward over the next couple of days with only slight
strenghthening forecast. The remants of Gaston, a cold front, and Hermine
may mix together to make for some stormy, wet weather for New England by

Finally, onto Frances. Yesterday, Frances presented a spectacular eye which
fulfilled every expectation of a major hurricane. With the impressive
appearance, the storm was upgraded to the somewhat rare status of cat. 4,
with winds of 115 kt. Overnight and today, the storm has looked somewhat
less impressive at times. It seems that Frances has felt some southerly to
southwesterly vertical wind shear. The first aircraft flew into the storm
this afternoon and evening and verified that the storm weakend slightly this
afternoon, to 110 kts (the upper end of cat. 3) and 954 mb (the pressure was
949 mb when the aircraft first arrived in the storm). With sea surface
temperatures as warm as 29.5 deg C and a strong ridge building to the north,
Frances could strengthen again, assuming that the wind shear is low enough
to be a nonfactor. The official forecast is for Frances to remain a strong
cat. 3 hurricane through the next five days. The track forecast is more
problematic than the intensity forecast at the moment. Frances is now moving
slowly westward at 7 kt. The storm size has increased over the last couple
days, and with a track just north of due west, the storm is forecast to pass
close enough to the northernmost Leeward Islands to bring some ill effects
including tropical storm conditions. Thus, Tropical Storm Watches have been
issued for Antigua, Barbuda, St. Maartan, Anguilla, Nevis, St. Kitts, St.
Eustatius, and Saba. These are generally the northern islands of the U.S.
and British Virgin Islands and Anguilla. With the inherent uncertainty of
forecasting, a more threatening southward track cannot be ruled out at this
time. Therefore, Hurricane Watches have also been issued for the British and
Northern U.S. Virgin Islands including St. Thomas, St. John and environs,
and the islands of Culebra and Vieques.        

Some of these islands took a beating several times during the past decade.
To briefly summarize this area's recent hurricane history: in 1995, Luis hit
Barbuda and St. Martin as a cat. 4 hurricane and later in the season,
Marilyn caused tremendous damage to the St. Thomas (U.S. Virgin Islands).
Hurricane Georges raked pretty much the entire area, including Puerto Rico,
and caused large loss of life in the Dominican Republic in 1998. And in
1999, 'wrong-way Lenny' caused more trouble late in the season. 

In the longer time frame, the storm is expected to move west northwest and
menace the Bahamas. A general threat now exists to the U.S. East Coast, but
it is still too far in advance to even say which state(s) may be threatened.
If the WNW track develops, the threat to South Florida may be reduced (and
increased for areas further north), but it is too early to say. All
residents of Florida, Georgia, South Carolins, and North Carolina should
monitor and track the progress of Frances through the week.

At 03Z, Frances was near 18.9N 56.2W, moving west at 7 kt, with maximum
sustained winds to 110 kt and aircraft-measured central pressure of 954 mb.

Please visit my tropical Atlantic headquarters.

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