Wednesday, August 22, 2012

Tropical Storm Isaac and TD 10: August 22, Update A

Thank you to John L. for sending me a check for $1M. Unfortunately it is dated April 2002, so I’m not sure if I can still cash it. Bother. ;-)

Tropical Storm Isaac
His intensity has decreased a bit since last night. Convection is still good as you can see in this satellite image of convection with reds and greys meaning lots of strong thunderstormy weather:

Water temperatures (27-28 deg C) are still warm enough to sustain and slowly increase the convection and no change in the depth of the 26 deg C isotherm (still in the upper 75-100m). The Lesser Antilles are getting a dollop and a half of rain and thunder. However his circulation has actually decreased a bit in the lower half of the troposphere since yesterday - indeed a sign that the islands and some dry air is having an impact. Currently his winds are 45mph, central pressure 1006mb.

They moved his center slightly southward overnight to 15.5N but this morning he is officially at 15.9N, 59.3W, moving W at 21mph. I still think he is slightly south of this, at around 15.4N, 59.3W. We might both be correct! The NHC also don’t have a good handle on his center because they think it has become elongated and not very well formed and they can’t really see it from the local radar either. Here is another satellite image (black/white) – can you see his center?

Having a good center is important in figuring out the track and intensity and I think we will see some track adjustments.

<Forecasting alert> Why are the center coordinates important? The way it works is that the center coordinates are entered into the models as one of the initial pieces of information it needs (called <jargon alert> initial conditions). Then the models are let loose (run) and take a few hours to produce the new track (or intensity, depending on the model). And then the NHC takes the latest set of tracks and produces it’s center of cone track, which is why it is shifted a little every few hours. So if the center is slightly off or the plane returns some new data, these get fed into the model and it takes a few hours (sometimes as much as 8 hours) for changes in the track to materialize. Sometimes there is not much shift in the track because the surrounding pressure fields are so well developed that a little change in the initial conditions doesn’t make much difference. However, in this case the pressure fields are evolving, so things will change. <end Forecasting alert>  

The official track has shifted slightly northward as it approaches Hispaniola, with the storm now passing over Hispaniola and eastern Cuba on Sat. and heading to south Florida on Monday, and then up towards the Tampa Bay area on Monday evening. This is the current OFFICIAL track. However, I think this will change. It may even change by quite a lot so I am not worried about this storm hitting Tampa Bay at the moment, but of course it doesn’t hurt to be prepared!  

Track from now until the Puerto Rico region (around Friday): When they figure out his center, if it is south of their estimate then I think he will initially track south of the current forecast track. If it is actually at 15.9N, then the northern track they have is possible.

Track after PR: There is actually a chance that he will remain on a more westward track. I won’t know this for sure for another day (Thursday) because I need to see where and what he does once he crosses the Lesser Antilles (which he is doing now) but I say this because he is following a high pressure and yesterday it did show a break in the Gulf region, but today it looks like the break has gone. If that break re-appears, it gives him a chance to curve to the north so the other possibility is that he will be on the eastern side of Florida, crossing the Bahamas and into the Atlantic. The uncertainty is if and where this break will occur – will it be over Florida?

All these possibilities in track will change his intensity forecast of course. If he is a hurricane over Hispaniola (if), there is a good possibility he will emerge in the Bahamas as a Tropical Storm. If he crosses Cuba and into Florida or the Gulf, there is a good possibility that he will be a Tropical Storm by the time he gets there. If he remains on a more westward track, then he will be a hurricane and may track into the Yucatan and the Gulf as one. The NHC also recognize that the intensity forecast is very uncertain after Friday/Sat. The only reason I can see to keep him as a hurricane as he crosses these islands is that the upper troposphere is currently looking good to allow him to grow … remember the divergence/convergence/ hurricane structure science alert? If not, I cut and paste here (because I’m nice like that)…

<science alert> A storm is a low pressure system at the bottom of the atmosphere (close to the ground). Air flows from high to low pressure, so it flows into the center. We call this low level air flow inflow or convergence (converging to the middle). But if it flows in from all sides, then where can it go? It can only go one way, and that is up. At the top of a tropical storm it has room to flow outwards… so, interestingly, a tropical storm with a good structure actually has high pressure at the top because the air is flowing out horizontally from that point. We call this outflow or divergence. A good storm has low pressure at the bottom and high pressure at the top. But to set this up there has to be ‘room’ around the top of a storm for the air to flow out, which depends on the conditions of the surrounding atmosphere at the top. With Ernesto, it looks like the conditions in the upper troposphere were not good enough to continue setting up the outflow circulation and that suppressed the entire storm from intensifying. You can look at the lower level convergence here: and the upper level divergence here: For a good storm structure, both would show concentric circles. The more (closer and circular) the circles, the better the structure. <end science alert>

So to sum up, pretty much everyone from the northern Gulf to the eastern Seaboard need to keep an eye on this one. The future track is one of the most uncertain I’ve seen in a long time (even Debby to me was an easier call!). Along with the track, the intensity is uncertain.

Tropical Depression 10
This morning they had this with 100% chance of becoming a Tropical Storm in the next 48 hours, so we are definitely looking at Tropical Storm Joyce two or three days behind Isaac! Since then they have upgraded this to Tropical Depression 10. Joyce will depend on what Isaac does so I’m not going to spend too much time on this for now. Except to say that she doesn’t have much convection at all because of dry air, but her circulation is not bad in the lower half of the troposphere. As I said, she will be named once she gets past this SAL region. She is currently at around 12.4N, 36.3W, moving WNW at 16mph. Winds are 35mph, central pressure 1007mb.   

More later!

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DISCLAIMER: 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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