Saturday, August 18, 2007

[Jyo_hurricane] Hurricane Dean: August 18, Update A

I *knew* Hurricane Dean would be a nuisance! but he is a good-looking
storm at the moment with a nice eye and circulation. He is a very strong
Cat 4, with winds of 150 mph (130 kt) (cat 4: 131-155 mph; 114-135 kt)
and a central pressure of 929mb. An earlier plane reconnaissance
detected a minimum pressure of 924 mb earlier this morning but the NHC
didn't upgrade H. Dean to a cat 5 because the other data they received
from instruments dropped into the storm did not corroborate what the
flight level winds were showing. Hurricane force winds extend out 60
miles from the center, and tropical storm winds are out to 205 miles.

I was wrong yesterday when I said that the storm is no longer being
impacted by the low that was over the Bahamas. It is. The low has slowly
been moving westward and southward, and is currently sitting over
Florida. The low will continue to move westward and southward over the
Gulf of Mexico, and H. Dean should continue to follow it. The current
forecast track has the center of the cone going over Jamaica (as a cat
4) and then the Cayman Islands (as a cat 5). Assuming the storm
maintains it's current speed of 17 mph (which is slower than it was
before, but still quite fast), I agree with the track portion that
includes a direct hit or very close call for Jamaica and the Cayman
islands. By Monday evening though, I think it will be a little more
south (by at least a degree) than the center line is indicating - but
still within the cone. If that's the case, then it will go quite solidly
over the Yucatan Peninsula, and if it survives that, then it will
continue westward to Mexico. Having said this, I'm not a professional
forecaster with computer models or the latest/bestest plots - I'm just
working out the trajectory using trigonometry and a pencil and paper
(and you thought trig wouldn't be useful in 'real' life!) - so for those
in southern Texas, I reckon you should keep to your plans on getting
ready and boarding up your place in the next few days (landfall on that
coast, if it survives the Yucatan, should be on Weds/Thurs). I'll do
another track calculation later in the day or tomorrow.

So, if that is the path this storm will take, what about the intensity?
This is where things get really interesting (depending on your point of
view of course, and as a geeky scientist-type, I think it is fascinating
:) ).
The atmosphere: the wind shear over the central Caribbean (around
Jamaica) is back and is pretty good, but the storm itself will modify
the wind field (because it is powerful), so although the shear might
have some impact, it won't be enough to knock it down by much.
The land: the storm will interact with Haiti, but I don't think the eye
will pass over Haiti at this point, so it won't be enough to knock it
down by a lot either. However, the shear and the limited land
interaction should both help a bit.
The ocean: this is the really naughty player in this game. As I said
yesterday, after the storm crosses about 70W, the water is not only warm
at the surface (29-30 deg C), but it is warm with depth in the western
Caribbean (much more so than the eastern). As the storm churns up water
from below the surface, instead of churning up colder water (and
therefore reducing it's own fuel supply), it will be churning up warm
water - it's like an all-you-can-eat buffet for the storm!
Unfortunately, the warm water is deepest right around Jamaica.

It will be a battle between the forces for good and evil. As it
approaches Jamaica, Hurricane Dean might experience a reduction in
intensity from it's current very strong cat 4 to a very strong cat 3 or
weaker cat 4, but in either case it is going to do some damage. After
Jamaica, at the moment it looks like the wind shear decreases, the ocean
is still v. bad, and there is less interaction with land as it
approaches the Caymans. So we can expect intensification again.

All of the above is based on a forward speed of 17 mph and a consistent
speed/direction movement in that low that it is following. If either of
these change speed (or direction in the case of the low), then the track
and intensity will probably change.

I heard from St. Thomas - they had some wind, no rain. All well in that
part of the world. I still don't see anything in the Atlantic behind H.
Dean - there are some areas with circulation but no convection, and
there are some areas of convection with no circulation. This is what
we've been seeing in the Atlantic so far this season before H. Dean came

More later.

These remarks are just what I think/see regarding tropical storms - not
the opinion of any organization I represent. If you are making an
evacuation decision, please heed your local emergency management and the
National Hurricane Center's official forecast and the National Weather
Service announcements. This is not an official forecast. If I "run away,
run away" (Monty Python), I'll let you know.

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