Saturday, June 23, 2012

Tropical Storm Debby: June 23, Update B

As I thought, the NHC upgraded the Gulf blobette to Tropical Storm Debby at 5pm after getting data back from the plane that went to investigate. I’m not sure I would have waited for plane data (although it is very useful to have) to upgrade her. Also, my earlier estimate of her center (when they still had her at 90% chance of forming) was pretty much on target and she is currently at 26.3N, 87.5W. Officially the wind speed is 50mph, which makes her a mid-level Tropical Storm (TS range: 39-70mph). Her central pressure is 998mb.

At 5pm (EST) the NHC said she was moving N at 6mph, but given that my estimate of her center was made this morning and that’s where she still was this evening, I wasn’t convinced. However, since the 8pm (EST) advisory they changed that and she now continues to officially be ‘stationary’, which gets a gold star and two thumbs up from me. She’s just relaxing there in the middle of the Gulf on a Saturday evening, enjoying the view and having a martini I expect. ;-)

The reason she is stationary is because she is surrounded by higher pressure. You can imagine her as a ball. If you put the ball on a hill, it will move and roll downhill. But if you put the ball in a dip, then it gets stuck and can’t move in any direction. The ‘hills’ in the atmosphere are areas of high pressure, and the ‘dips’ and ‘valleys’ are low pressure. Unlike on solid earth, these pressure areas are continuously changing which is why a storm is almost always moving. I’ll get into overall storm movement some other time, but that’s how to envision what is going on around a stationary storm. Forecasting the track of a stationary storm is trickier than usual because while it is stuck in the dip, the pressure fields and general atmospheric conditions around it carry on changing. So although currently the forecast track has TS Debby moving northwards tomorrow, and then turning towards the west and making landfall in Texas sometime on Friday, I am not certain of this track at all. The current center of cone would take her to the Corpus Christi area of Texas (o.k., I have to say it… at least Debby’s not heading towards Dallas!! Ahem. ;-)). However, I think there is still a chance she could head somewhere along the northern Gulf or even the Florida Big Bend area – i.e. anywhere from Florida to Texas. Tomorrow I’ll be looking for this northward movement that is forecast as opposed to a northeast movement. I would still tell everyone from Florida (central-western side) to Texas to keep an eye on this one. Regardless, the severe weather is currently on the east side of the storm, which means Florida folks should have their raincoats handy when they go for their Sunday sunbathing at the beach (and don’t bother taking a sunshade) ;-). (And if you are in Georgia, Alabama, or Mississippi, you can expect a few clouds too).

Her circulation in the lower half of the troposphere is very strong now, and there is a very small signal in the upper troposphere. If it wasn’t for the wind shear (which has kept all her convection to the east of the center), I think she’d be stronger than 50mph, and would even have been closer to the 65-70mph range. I expect her to intensify tomorrow to a strong TS or weak cat 1 because the wind shear looks like it is decreasing. The convection is also improving with bigger thunderstorms forming off the coast of Florida - you can see this as the red areas in this satellite image:

If you are concerned about storm surge, you can have a look at Tides Online (see the directions on what to do in this post: At the moment most places along west Florida to Texas are showing about 1-1.5 ft above normal tides.

I think that’s it for today. More discussions about the delightful Debby tomorrow!

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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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