03 September 2004

Weaker Frances strolls towards Florida, Ivan named, gaining strength...

From Jonathan Vigh: 
Tropical Storm Ivan became the 9th named tropical storm of the season today.
With a beautiful curved band wrapping into the storm and excellent outflow,
it is easy to see that Ivan is a tropical cyclone with ambition. This
afternoon's visible satellite imagery showed a fascinating thin cloud line
stretching back to Africa -- this curved line, sometimes called the
'umbililcal cord', is due to the convergence of the circulation of the storm
with a surge of low-level easterlies out of Africa's Sahel region. These
Sahel surges, which often contain Saharan dust, are characterized by warm,
stable, dry air which can be detrimental to developing systems. Ivan doesn't
seem to be minding so far, and the storm is forecast to cross the central
Windward Islands in less than 5 days as a major hurricane. 

At 21Z, Ivan was at 8.9N  34.6W, moving west-southwest at 19 kts. Maximum
sustained winds were estimated at 45 kt and the lowest central pressure was
estimated at 1000 mb.

It appears that there were some chinks in Frances' armor. The storm came
under some moderate westerly vertical wind shear starting yesterday. The
cause of this shear is not obvious, but rather due to subtle changes in the
environment. First of all, the upper level winds changed from light
southeasterly (moving with the storm) to light southwesterly (moving against
the storm). This wouldn't be enough to cause the disruption that has
resulted, however, so there's more to the story. At the same time, the ridge
to the north of the storm built around to the northwest of the storm as
well, slowing it down to a crawl. The low level steering flow behind Frances
didn't know about this, and continued trying to push the storm
northwestward. With the combination of the upper level winds and the pushing
from below, the shear has become considerable, stressing the storm. Along
with the unfavorable winds, Fracnes ingested a large amount of dry air from
the west. In just a few hours' time, the convection died out on the
southwest side of the storm and the circulation started slowing down. It
takes a long while to slow down a cat. 4 hurricane, and it wasn't until this
evening that the winds dipped down to cat. 2 strength. Unfortunately, the
hurricane is sitting directly over the northwest Bahamas, continuing to lash
these islands with sustained winds as high as 100 mph at times. Yesterday,
as she passed over San Salvador Island, Frances caused wind gusts up to 152
mph! Today, there have been several reports of sustained winds to 100 mph on
Eleuthera and Great Abaco Island (which also reported gusts to 115 mph).
Nassau, the capital of the Bahams, recently reported a gust to 110 mph, and
this is on the weak side of the storm, so Frances still packs a punch.   

Frances is forecast to move very slowly towards the west or northwest over
the next day, finally making landfall in Florida sometime tomorrow afternoon
or evening. Although the core of the hurricane is weaker, the outer
circulation is still very broad and capable of spreading hurricane force
winds over a large area. Because of this, winds to at least tropical storm
force could still be felt from south of Miami up to the Georgia border. The
outer rain bands have started to reach eastern Florida this evening, and
have been strong enough to knock down trees and cause isolated power
outages. People in the hurricane warning area should not let their guards
down, as a cat. 2 hurricane can still cause death and destruction. If the
shear lets up a bit, it is quite possible that Frances could gain strength
again. The massive bursts of convection blowing up right over the center of
the storm are a sign that the Frances still has spunk. If Frances crosses
Florida the short way, she could reemerge into the Gulf to cause more
trouble there early next week.    

At 00Z, Frances was at 25.9N 77.5W, moving very slowly towards the
west-northwest with maximum sustained winds of 90 kt and a minimum central
pressure of 960 mb.

For more Frances links and information including storm surge maps, local
weather statements, and technical information on Frances, please see

Please visit my tropical Atlantic headquarters.

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