Sunday, August 26, 2012

Tropical Storm Isaac: August 26, Update A

Up here in Tampa Bay it looks like a perfectly fine British summer day out there at the moment. In the Keys, things are a little damper… from Lisa T. in the Keys around 11.30am: “Just rain in Grassy Key, waiting for squall line”.

Tropical Storm Isaac is officially at 23.9N, 80.8W, heading WNW at 18mph. This is about right by my books. He is well and truly in the Straits of Florida, just north of Cuba and south of the Keys. If you want to look at a radar loop and watch his eye cross the Straits, go here:, and then click on the blue circle that is located closest to the area you want to see. In the case of Isaac, click on the radar data from Key West (the blue circle just southwest of mainland Florida). To get the movie loop, on the left side click on the word ‘Loop’ next to ‘Composite’.

As expected, Isaac is moving a little west of yesterday evenings official center of cone track. The official track has now been shifted so he will pass close to or just west of Key West later today. However the rainiest weather is mostly on the north and east side of the storm, so the Keys and south Florida are getting a few bouts of heavy rain and thundery weather. Luckily, although he is crossing the warm waters of the Straits, there is still some wind shear and dry air that is inhibiting his development so I agree with his current designation as a strong Tropical Storm (range: 39-73mph) with winds of 65mph, central pressure 995mb. The circulation has not changed at any level of the troposphere since yesterday, and remains strong in the lower half, so until that convection improves in all quadrants, I really don’t think they should upgrade him to a hurricane. Here is the latest water vapour satellite image of him so you can see the wind shear with the clouds still streaming off to the north and east and the dry air (shown in brown) to the south.

What factors will influence his intensity in the Gulf?

1. Wind shear: at the moment this looks like it will decrease within the next day or so. This will help him to get organized.
2. Interacting with Cuba and Florida: Cuba is having a larger effect than Florida, because that is what his circulation is interacting with and that inhibits his structure from developing. Because of wind shear and dry air, Florida is only really impacting his convection (which helps). So as he moves away from Cuba in the next day, he will have an opportunity to improve.
3. Dry air to his south: As he moves a little farther north, this will have less of an impact. It will also be less important once he gets better organized.
4. Sea surface temperature: The water is around 28-29 deg C, which is definitely warm enough to allow slow intensification.

5. The Loop Current: He is essentially over this already because he is in the Straits of Florida, where this current is called the Florida Current. (If you need a refresher on this and why it is important, check out the science alert here: The upper 75-100m of the water is warmer than 26 deg C…. but the good news here is that as he gets closer to the Keys, this gets shallower, and we are lucky because at the moment it looks like the Loop Current is not very far north in the Gulf at all! That’s the biggie in the Gulf as far as intensification goes. There are a couple of warm eddies out there in the northern/western Gulf, but overall I don’t think he’ll be more than a cat 1, if that (depending on the path he takes as he crosses the Gulf).
6. Upper troposphere structure: It did look like there was a chance that this would be a factor in helping him get stronger, but it looks like the conditions for him to improve his upper tropospheric structure is decreasing. (If you need a refresher on what this is, check out the science alert here:

At the moment it looks like that  high is continuing to build in the northern half of the Gulf. There is room for him to continue to move WNW/NW for the rest of today, but he will either slow down tomorrow from his brisk 18mph forward speed – if he does, that’s a sure sign of high pressure to the north and west of him, or he may take a more westward track (so really, everyone in the Gulf needs to continue to keep an eye on him!). I’ll have to assess his track options again once he has crossed the Keys region.

If you have to evacuate, there are signs that mark out the evacuation route, complete with a plan attached. Here is an example of one of these so you know what to look out for (I got this via Fred G. in Florida):

I’ll be back later! Time for a cup of tea.

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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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