Update on 93L

UPDATE 2: 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.


Once 93L emerges into the Gulf of Mexico Thursday, all of the models indicate the storm will intensify. This is a very complex forecast situation, since the storm is starting out with the cold core of an ordinary non-tropical low pressure system, and will transition to a warm-cored tropical storm. The transition to a warm core system will probably take at least a day. A storm undergoing such a process cannot intensify rapidly while this is occurring. This means that if 93L hits New Orleans Friday night/Saturday morning as the GFS and GFDL models are predicting, the storm will likely still be below hurricane strength--as predicted by the 8am EDT run of the SHIPS intensity model--or a minimal Category 1 hurricane--as predicted by this morning's 06Z run of the GFDL model. I think a tropical storm is more likely. Such a track would take it just north of the high heat content waters of the Loop Current in the central Gulf of Mexico (Figure 2). If 93L takes a more southerly track as the ECMWF and NOGAPS models predict, it will have an extra day over water, and more time to firmly establish a warm core. A warm core, fully tropical system is capable of must faster intensification rates. A more southerly track would also take the storm over the high heat content waters of the Loop Current, further aiding the transition to a warm core system. Texas could see a Category 1 or 2 hurricane on Sunday in this scenario. Slowing down the intensification will be the presence of plenty of dry air to the northwest, however, and a tropical storm may be all that Texas would see.

The NOAA jet is scheduled to fly this evening and collect data to help with the Wednesday evening (00Z Thursday) model runs. The first Air Force Hurricane Hunter mission is scheduled for Thursday afternoon.

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