Tropical weather analysis - August 10, 2010

Tropical Depression Five

Invest 94L developed into Tropical Depression Five this afternoon in the eastern Gulf of Mexico. As of the 7:30 PM EDT advisory from the National Hurricane Center, the center was estimated to be near 26.0°N 84.1°W, moving NW at 6 mph. The minimum central pressure is estimated at 1007 mb. Visible satellite animations suggest that though the overall circulation is quite vigorous, large, and well-defined, the cyclone lacks deep convection near the assumed center.

The environment across the Gulf of Mexico is characterized by light easterly shear and some minor subsidence (dry air), as noted on water vapor imagery. Water vapor imagery also depicts a weak upper low centered about 100 miles to the north of the depression. This low appears to be moving rapidly toward the west, however, and this feature is forecast by the 18z GFS to quickly lift out and deamplify while moving westward toward the upper Texas coast. This upper low is currently having a slight negative effect on TD5, with a small dry slot seen to the northwest of the center on water vapor imagery. As the low lifts out, however, the environment will become more favorable, characterized by less subsidence and more uplift. This should allow the cyclone to begin intensifying at a more steady rate in around 12 hours.

Further supporting the GFS' forecast of a rapidly weakening and deamplifying upper low is 200 mb vorticity data from CIMSS, which strongly implies that said upper low has significantly weakened over the last six hours, while having moved westward. There are also indications within the mid- upper level steering that the upper low will move away from the depression at a very rapid rate. I am expecting a general W motion of the upper low throughout the next two days until it reaches the upper Texas coast and dissipates, although a slightly S of due W component of motion cannot be ruled out in the near-term.

Regardless, it appears that the upper low will have a negative influence of TD5 for about the next 12 hours. My own forecast track is in best agreement with the 12z UKMET, which moves the system WNW within the next 6-12 hours, followed by a steady WNW motion until landfall. My track takes the system inland near Cocodrie in two days. I feel the UKMET is a little slow in bringing this system inland, however (60 hours). Based on the current and forecast steering, I would expect to see a bit of a westward shift of the NHC's track over the next day or so, along with the models. At this time, a landfall between Intracoastal City and Buras cannot be ruled out, but the greatest threat appears to be central Louisiana at this time.

Nevertheless, given the uncertainty, residents from Intracoastal City, Louisiana eastward to Pensacola, Florida should closely monitor the progress of the depression over the next couple of days. Should the depression take a day or so longer to move ashore, as predicted by some models, it would likely become stronger than is currently being forecast by the NHC or myself.

All in all, the NHC's current track looks good, but I'm just a bit to the west of that. Some moderate easterly should could develop over the system in about 24-30 as hours as the high pressure ridge to the north strengthens. However, given that the cyclone will be moving roughly in tandem with the upper flow, I am not expecting this to slow intensification significantly. In fact, I think the current NHC forecast of only 45 mph is much too low. I'm going with 60 mph at the moment.

Watches and warnings

A tropical storm warning has been issued from:

Destin, Florida to Intracoastal City, Louisiana. This includes the city of New Orleans and Lake Pontchartrain.