28 August 2004

Frances strengthening slowly, TD 7 becomes Tropical Storm Gaston...

From Jonathan Vigh: 
The depression that formed yesterday off the South Carolina coast has become
much better organized in the past day and is upgraded to Tropical Storm
Gaston, the 7th named storm of August (a tie with 1995 for the most storms
to ever form in August). The storm is drifing slowly westward and forecast
to make landfall in South Caroline in 24-36 hours. Tropical Storm Warnings
are now in effect from the Savannah Rive to Little River Inlet, and watches
are in effect for areas on either side of the warning area. A reconnaissance
aircraft is currently flying the storm and has found that Gaston is a
healthy tropical system with a central pressure of 996 mb. The storm is in
an environment favorable for strengthening, so it will likely continue to
intensify until landfall. A ridge to its northeast should prevent it from
making a quick escape out into the Atlantic. Although the storm probably has
at least a one in four chance of making it to hurricane strenth, the primary
threat from this system is still the heavy rainfall that it will be
producing over the next several days. 

At 18Z, Gaston was at 31.3N 78.7W with winds estimated at 40 mph, but
possibly climbing. 

Meanwhile, Hurricane Frances has been keeping things interesting over the
mid-Atlantic. Frances has strengthened only slightly since yesterday, to 105
kts. Overnight, the storm completed an eyewall cycle, which often happens in
intense hurricanes. Although eyewall replacement cycles are not completely
understood, basically, outer rainbands form a second outer eyewall which
then contracts and chokes off the inner eyewall. This process temporarily
halts intensification and can cause a brief weakening. An added side effect
is that the storms size can be increased to some extent. Once the inner eye
has died, the storm continues strengthening and the new eyewall can shrink
in size -- sometimes this cycle occurs multiple times over a storms
lifetime. Some more internal fluctuations are expected as Frances continues
moving towards the northwest. The vertical shear is still low and the sea
surface temperatures are still getting warmer, so Frances is forecast to
strengthen into a cat. 4 hurricane. The area of vertical wind shear
mentioned yesterday is still present, but seems to be decreasing with time.
By the time Frances reaches it tomorrow and Monday, it could be nothing more
than a speed bump to the storm. 

Maybe even more interesting than the intensity is the storm's forecast
track. A strong ridge should force the storm to make a turn to the left
starting tomorrow. The storm is already slowing down, so this may be an
indication that it is getting ready to make the turn. The official forecast
still calls for Frances to pass well to the north of Puerto Rico, the Virgin
Islands, and the Dominican Republic, but if the ridge is stronger, the turn
could occur sooner and the storm could come uncomfortably close to those
locations. This is still a distinct possibility (about a 1 in 10 chance), so
people in those locations should monitor Frances closely over the next
couple days. After that, Frances is forecast to continue west northwest
towards the Turks and Caicos islands, which may eventually feel the wrath of
Frances starting next Wednesday and Thursday. If the strong ridge continues
to hold, the storm could eventually threaten the United States. A potential
landfall is nearly a week away, and there are still many factors that could
prevent Frances from ever threatening the U.S. Nevertheless, if you live in
those locations, it doesn't hurt to review your hurricane plan and resupply
your disaster kit.      

At 15Z, Frances was located at 17.4N  51.9W and moving NW at 8 kts. Maximum
sustatained winds were estimated at 105 kts, with a  central pressure of 958

Please visit my tropical Atlantic headquarters.

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