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Gulf of Mexico disturbance 93L less of a threat

By: Dr. Jeff Masters, 7:41 PM GMT on September 19, 2007

Recent Satellite loops and the Tampa Bay long range radar show that a non-tropical low pressure system (93L) appears to be reforming off the southwest coast of Florida. This is an important shift, since it brings the surface low underneath the upper level low pressure system aloft--the same kind of situation one finds in ordinary non-tropical "cutoff lows". This is an unusual event in September over the Gulf of Mexico, and is good news for those potentially living in the path of 93L. A surface low pressure system vertically aligned with a cold-cored upper level low will usually take a day or two to make the transition to a warm-cored tropical storm. During this kind of transition, rapid strengthening is rare, and the chances of 93L ever reaching hurricane strength now appear dim. The latest (12Z, 8am EDT) intensity forecasts from the GFDL and HWRF computer models keep keep 93L below hurricane strength, as does the 18Z (2pm EDT) SHIPS intensity model. The HWRF model indicates that 93L will come ashore at tropical depression strength, and this is entirely possible.

The forecast tracks from the latest cycle of model runs all show a landfall between central Louisiana and Pensacola, Florida on Saturday morning. The exceptions are the NOGAPS and ECMWF models, which show a Sunday morning landfall near the Texas/Louisiana border.

Figure 1. Current long range radar out of Tampa Bay, Florida.

Secondary low develops off Cape Canaveral
A secondary low pressure system developed late this afternoon just north of Cape Canaveral, Florida, and has moved inland. This second low is now being referred to as "93L" by NHC, not the low developing off the Southwest Florida coast. Both of these lows need to be tracked, however. The new low north of Cape Canaveral has generated some impressive pressure falls of up to 9 mb in the past 24 hours, and has brought rainfall amounts of five inches along the coast from Daytona to Jacksonville. Heavy rains will spread across northern Florida Thursday as this low tracks westward. If the low pops out over the Gulf of Mexico south of the Florida Panhandle, it could intensify into a tropical depression and bring very heavy rain to the Florida Panhandle.

Figure 2. Latest radar estimated rainfall from the Jacksonville, FL radar.

The NOAA jet is scheduled to fly this evening and collect data to help with the Wednesday evening (00Z Thursday) model runs. The models should have a much better handle on both of these lows early Thursday morning. It will be interesting to see which low ends up being dominant.

Remains of Ingrid
The remains of Tropical Storm Ingrid are still kicking up some heavy thunderstorm activity a few hundred miles north of the northernmost Lesser Antilles Islands. There is not much spin evident on satellite loops, but wind shear has dropped from 30 knots yesterday to 10 knots today, and we will need to watch this area for development. Steering currents are weak in this region, and the remains of Ingrid will move little over the next 3-5 days.

I'll have an update Thursday morning. I haven't quite finished my wind shear tutorial yet, but I will post that as soon as I finish it.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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