Tuesday, June 26, 2012

Tropical Storm Debby: June 26, Update A

Ok, ok, I know. *I* might be enjoying this fine British weather (although it could be a little cooler), but you guys are fed-up with it and you want your sunshine and calm air back. So, as a favour to you all, I have spent the last few hours on the seawall here with all the fans going and I think we’ve managed to bring landfall forward to tomorrow morning (Weds) at the latest! ;-)

At the moment Dawdling Debby looks like she’s moving to the Northeast at a faster clip. The NHC in their last advisory (1.5 hours ago) had her moving eastward at 3mph, but I think that’s a little slow and with the wrong heading. Officially she’s at 28.9N, 84.2W, but from the satellite images I’m looking at, she’s at 29.01N, 83.99W. It may not seem like a huge difference, but it is when you are this close to shore.  She’s heading to the Big Bend area of Florida, near/just north of Cedar Key. The current official center of cone track has her making landfall on Cedar Key as a tropical storm at 8am Wednesday.

As she gets closer to land, her winds are going to be a little stronger of course. But she is also weakening. The circulation in the lower half of the troposphere is transitioning from being tropical to more of a low pressure front, so I expect she’ll be a Tropical Depression by the end of today.  The NHC currently have her winds at 40mph and a central pressure of 993mb, based on aircraft and other observations.

Here is the latest IR image of Debby:

The convection is way off to the northeast and you can clearly see the remains of the eye, just offshore.

<Science Alert!> On Sunday, when the forecast track was still heading west to north Texas, Karen M. from St. Petersburg sent me a question: “with so many models pointing toward the big bend region, and what seems to be fewer models pointing toward TX (it's a little hard to tell on the scale of the model map I'm looking at), why isn't the predicted track further north, or even eastward.  Do they think the high pressure will have that great an effect?  Do they trust one model more than others?”.

This is a really interesting question. They do use many different models, and some they think are better than others. But some models are also run many times, each time with a slightly different set of atmospheric or oceanic conditions (called <jargon alert> initial conditions <end jargon alert>). The output from these variations on a model form part of an ensemble of model runs of that particular model. With this storm some ensemble runs from one of their most trusted models was showing landfall to the west, and some ensemble runs were showing landfall to the east. However, other models (not ensemble) were also showing landfall to the west. The reason for the divergence was because they were not sure how strong the storm would be… so the forecast track in this case depended on their ability to forecast the intensity. And the reason why the intensity was important is because if Debby was stronger, the pressure fields higher up in the atmosphere would have played a bigger role in how she moved (which is the high pressure part of Karen’s question). However, she turned out to be weaker than they were expecting, so the model runs that had a weaker system turned out to be closer to the track. <end Science alert> Gosh, I hope everyone followed that… if not, please let me know and I’ll see what I can do.

<begin soapbox alert> ;-) The underlying tale to take away from this is that in order to improve the track forecast, we NEED to know the intensity! And in order to improve the intensity forecast we need to know the track because (for example) whether she is over land or deep warm water will impact her intensity. The two cannot really be divided when it comes to the research and prediction of storms. <end soapboax alert> J (hmmm… why is it called a soapbox? I can’t imagine I’d be much more imposing and commanding of your attention if I stood on a 1 inch soap box, not to mention the technical difficulties of balancing on something that small without crushing it…).

That’s all for now. 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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