TAOI 07-22-2010

Overview:
Two tropical waves in the Atlantic are continuing to be monitored, PGI-16L/AL98, PGI-17L/AL97/TD3.  AL98 is located in the SW Gulf of Mexico near 20N, 96W.  TD3 is located at 22N, 75W.  PGI-18L is no longer being tracked on the Montgomery website.  Operations have focused on TD3 for the past several days, and it was upgraded by the National Hurricane Center (NHC) from an invest to a tropical depression at 15 UTC today after satellite imagery began to indicate a closed circulation had developed.  Tropical Storm warnings have been issued for the Keys and southern Florida coasts.  Ft Lauderdale is currently under a tropical storm watch.  The system continues to be under the influence of a strong upper tropospheric cold low which is helping to maintain a strong shear gradient near the center of TD3.  1500 UTC CIMSS analysis shows shear greater than 20 kts within a few degrees NW of the storm center.  However, to the east of the storm and over the center, shear is only 5-10kts. Due to the shear influence, convection is limited to the eastern half of the storm.  Upper level divergence is also only marginally favorable, however some good outflow is present in the eastern half of the storm.  SSTs and atmospheric moisture content over the storm center remain conducive for development, however Ocean Heat Content is not particularly high, and dry air is present to the west of the center of circulation.  Compared to yesterday, the low level vorticity is far more concentrated, and well stacked vertically.  As long as the low level center does not move ahead of the convection, it appears that there is a good chance for development into a tropical storm in the short term.  Flight operations including all 3 NASA aircraft as well as NOAA aircraft are planned over the next 3 days for this system.

The NHC has also issued an invest on PGI-16L, now AL98, and there is a 50% probability of development within the next 48 hours.   The 06UTC Ocean Prediction Center places a 1010hPa low in AL98’s location, and widespread convection is occurring around the low.  Windshear is low in this region and SST’s are sufficiently high to support genesis.  However, the low is very near land, and should reach the coast of Mexico within the next 24 hours, limiting its chances to develop.  Elsewhere in the Atlantic, a pair of 1027 hPa subtropical highs dominate the flow resulting in largely easterly surface winds from Africa through the Gulf of Mexico.  A tropical wave which separated from TD3 is being analyzed in the Caribbean at 15N, 80W, however no convection is occurring with this system. 



Forecast:



The primary forecast concern is TD3 which is centered at 22N, 75W, moving WNW at 13 kts.  Overnight it began to show signs of development and a closed circulation formed.  However, several factors are still hindering the development of TD3.  This includes dry air to the west of the storm, land influences from Cuba, and the persistent shear to the W.  The cold low is moving faster than the depression, and has moved south and west away from the storm, effectively decreasing the shear immediately over the center of the depression, and changing the direction of the shear from SW to S.  As the cold low continues to move SW, the shear is expected to eventually turn to the SE, as indicated by the SHIPS text forecast.  While the shear is not expected to be particularly favorable for development, it also is not expected to be extremely unfavorable either.  Another consideration of the cold low is that as it moves farther ahead of the depression, the downstream ridging supporting convection near Hispaniola may set up over TD3, creating a more favorable upper level environment.

The model track forecasts for TD3 have a fairly good consensus.  Almost all of the 12Z models bring TD3 between Cuba and the Keys, with the northern most tracks bringing the center over the Keys.  Within 36 hours, the system will have emerged into the Gulf, continuing a steady NW track.  A gradual northward turn is then predicted, resulting in an eventual landfall along the Louisiana or Northern Texas coasts within 72-96 hours.  The National Hurricane Center is predicting a track near the center of the model spread with a landfall in about 84 hours at the Louisiana-Texas border.   This track seems reasonable, and there is no reason to expect any substantial deviation.

There continues to be a split in the model forecasts of intensity.  The statistical-dynamical models continue to strengthen the system into a moderate tropical storm before it makes landfall, while the dynamical models show little to no strengthening, and in fact suggest the system will gradually weaken.  Because the system is now a depression, the DSHIPS/LGEM developmental data is now applicable and more faith can be placed in those forecasts.  The NHC is taking the middle road with very slight strengthening over the next several days before landfall as a 45kt tropical storm, while SHIPS and LGEM suggest a landfall intensity closer to 55kts.  00Z Pouch tracking in the GFS and NOGAPS models show high initial OW and Zeta values, however these values quickly decay, however both level off at levels around the nominal threshold for development, suggesting that development is a possibility.  While substantial development is not anticipated, it seems likely that the system will intensify into a weak tropical storm within the next 24-48 hours.