Monday, September 03, 2012

Tropical Storm Leslie and Tropical Depression 13: September 3, Update A

Dear Bermudians, think I’d forgotten about you? Never! You may be a dot in the Atlantic, but you drink proper tea and probably have yummy Jammie Dodgers and Jaffa Cakes. J (hmm… it must be time for dessert…) I didn’t really get a chance to look at a computer for long yesterday, but I see that Leslie is considering a round of golf on your fair island.

Tropical Storm Leslie
She has definitely got the biggest area of very strong convection I’ve seen in a long time! And it’s persistent… it has been that impressively large since yesterday. Here is the latest infra-red satellite image for the Atlantic so you can see how she could easily blanket the entire state of Florida with strong thunderstorms (red parts of the image)!

But her bark is, at least officially, bigger than her bite... her winds are only 60mph (TS: 39-73mph), so she’s officially a mid-strength Tropical Storm (central pressure 998mb). Leslie has not yet developed into the hurricane that was initially forecast by now by the NHC because of wind shear and a bit of dry air to the west. She’s quite interesting because she’s had a large area of really strong convection for over 24 hours, but her wind speed has not really improved. Although sea surface temperatures are around 29 deg C, which is certainly warm enough for convection and gentle intensification, water warmer than 26 deg C is only in the upper 50m or so of the water column so one would expect that this strong convection wouldn’t persist in the same area for so long. The atmosphere is key in influencing her intensity and once that wind shear decreases she will have room to develop into a hurricane. The only things I can see that would counter this are the dry air to her west and cooler water… between her current location and Bermuda the water warmer than 26 deg C is only in the upper 25-40m and if she doesn’t put her walking boots on she may actually cool the surface water underneath quite substantially (enough to inhibit strong convection anyway). The NHC forecast a hurricane on Thursday. It looks like wind shear will be decreasing within the next day. She might be a hurricane before Thursday. Her circulation is really strong in the lower half of the troposphere, with only a very small signal in the upper troposphere. I think 60-65mph is reasonable.

At the moment Leslie is lolling around in the Atlantic, heading NNW at 3mph, currently officially at 24N, 63.1W. Because of that wind shear and strong convection I find it difficult to see her center. I think it could even be southeast of the official location, closer to 23N, 62W.The forecast track has her slowly inching towards Bermuda, getting close around Saturday/Sunday. The pressure maps I look at don’t really have any information other than she’ll be slow. Not a whopping insight really, is it? It looks like her girth is large enough for Bermuda to be feeling a little breeze already. I’m sure that people are preparing on the island with a pint or two.

Tropical Depression 13
The circulation is not very strong, but there is a bit in the lower half of the troposphere. The main problem this blob is facing is dry air, which is inhibiting it’s convection. It is officially at 25.6N, 42.2W, moving NW at 3mph. The winds are officially estimated to be 35mph, central pressure 1012mb. They think this will be a named storm by tomorrow, but as you can see in this IR satellite image, it would have to really get its act together to pull that one off!

That’s it for now. It’s half-past wine-and-cheese o’clock!

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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.

1 comment:

diverdutch said...

You forgot to mention that Bermuda has the best rum in the world as well!