20 September 2002

Isidore now a CAT2 hurricane.

At the time of yesterday's update, Isidore was a strong TS with a
strengthening trend.  At 21Z yesterday, he reached CAT1 hurricane
status, then at 09Z this morning, reached CAT2 status and low-lying
western Cuba (and perhaps the occassional eyewall cycle as the storm
re-organizes itself) is the only obstacle to further intensification. 
The central pressure fell 24mb in the past 24 hours, and 13mb in the
past 12 hours, so the storm is not yet qualified as a rapid intensifier
(some classic definitions can be found at
http://www.nrlmry.navy.mil/~chu/chap6img/tab605.jpg and a recent
definition is +31kts/day).  It will be a very well-observed storm, in
terms of aircraft flights into it, around it, and sensors dropped into
the ocean ahead of and under it to record the depth of the warm water
(latest data I know about: 29.5-30.0C running 50-70m deep, which is an
amazing amount of energy available to this storm... it's a 230'-deep
bathtub of 86F water!).

At 15Z, Hurricane Isidore was located at 21.7N 83.9W (just a few miles
south of the western Cuban coast) and tracking WNW at 7kts.  Maximum
sustained winds have reached 90kts and the MSLP has plummeted to 966mb. 
Despite this low pressure, an eye has still not formed, which from my
experience is very unusual.  However, the inner precipitation structure
-as revealed by microwave imagery- does contain the mature features one
would expect from a 966mb hurricane.  A Hurricane Watch and Tropical
Storm Warning are in effect for most of the northern Yucatan Peninsula,
and a Hurricane Warning is still in effect for all of western Cuba and
the Isle of Youth.  You can download and print a high-quality map from
http://www.mcwar.org/gallery/tropics/MESO_atl_blank.ps if you wish to
keep up with the track of this storm.

The intensity forecast is relatively simple for a change... it's in a
very favorable environment; both the ocean and the atmosphere will allow
it to continue to strengthen, seemingly well into the major hurricane
domain (CAT3, 4, 5).  However, just to keep the operational tropical
meteorologists on their toes, the track is far from trivial to
forecast.  It's hard to find even two models that agree on a scenario,
let alone a concensus.  The official NHC forecast takes Isidore past
western Cuba and into the southeastern Gulf, slows him down and turns
him slightly westward.  This seems quite reasonable, and after that, I
suspect some sort of a more northward turn, in advance of the next
trough.  At any rate, the US is not threatened by landfall through the
end of the weekend, but should be taking that time to prepare for one.

Elsewhere, there's a broad circulation with a 1010mb Low and scattered
deep convection at about 29N 52W (roughly 14 degrees ESE of Bermuda). 
Vertical shear is fairly low, the SST there is about 27.5C, but the
vorticity center is still somewhat elongated and disorganized.  Should
this develop, the next number/name on deck is 12/Kyle.

Please visit my tropical Atlantic headquarters.

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