Sunday, August 26, 2012

Tropical Storm Isaac: August 25, Update B

It was a relatively calm in the world of forecasting today as we all watched Isaac continue on that NW path along the northern coast of Cuba. He didn’t quite touch the coast enough and remained off-shore, so there has been a very slow improvement in his circulation throughout the troposphere in the last few hours. Thunderstorm activity has also improved in the last couple of hours, but it is confined to the north and east. He is still being impacted by dry air and wind shear (as well as interactions with land – Cuba) because there is very little convective activity on the west or south side of the center, as you can see in this satellite image:

His center is officially at 22.1N, 77.2W and he is moving NW at 17mph. I agree with this – and this is also exactly on the center of cone forecast track the NHC have had posted in the past couple of advisories. No wonder it feels so calm! His winds are still 60mph, central pressure 997mb. I also agree with maintaining his status as a mid-intensity Tropical Storm.

The high pressure that the models are basing their future westward movement on is, as expected, developing (which means that it is expanding from the Atlantic back over Florida). Isaac is following the edge of this high pressure (if you want a refresher on what steers storms, check out the <science alert> here:, and here: He is also being influenced very slightly by low pressure to his southwest, but this is only temporarily helping him move on his current NW track so we are more concerned with the high pressure for the long term track.

The latest pressure fields show me that he will continue on a NW path tomorrow, remaining along the coast of Cuba, pretty much as forecast, as he goes to visit the Keys (maybe now as west as Key West). The problem is what happens after the Keys. The complication in trying to figure out the landfall in the Gulf is that the high pressure looks like it is building up across the northern Gulf. So either he will be forced more westward than the forecast track as he gets close to/passes the Keys, or he will slow down until there is a path for him to move north in some manner. We will revisit this again tomorrow evening, as he approaches the Keys region, because of the fluctuating high pressure field. Do not place any weight on the center of cone from the Keys northwards… the entire cone is a possibility – and maybe even beyond! And be prepared of course!!

There is still some wind shear to his north and over Florida, so his convection is not very symmetrical around the center (see satellite image). This wind shear could help to slow down his intensification as he crosses the Straits of Florida where the sea surface temperature is 30 deg C and the upper 100m is warmer than 26 deg C but, as his circulation is improving, I think something around a strong Tropical Storm/weak cat 1 in the Keys is possible. I will tell you the factors that play into his intensity in the Gulf tomorrow morning… time for a nap now. J


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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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