Gulf of Mexico disturbance 96L poorly organized, but may develop

Posted by: Dr. Jeff Masters, on June 22, 2012

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An area of low pressure and heavy thunderstorms in the Southern Gulf of Mexico (designated 96L by NHC Thursday afternoon) is a threat to become a tropical depression this weekend, and all interests along the Gulf of Mexico coast should pay attention to the progress of this disturbance. The disturbance is bringing occasional heavy rains to Western Cuba, South Florida, and Mexico’s Yucatan Peninsula. Top winds measured in the surrounding ocean areas this morning were 27 mph, gusting to 34 mph, at the Yucatan Basin buoy between Mexico’s Yucatan Peninsula and the Cayman Islands. Our wundermap for the surrounding ocean areas show a ship that measured sustained winds of 30 mph near the western tip of Cuba this morning. Satellite-based surface wind measurements from the newly-available Oceansat-2 scatterometer, courtesy of India, show no signs of a surface circulation. Visible satellite loops show that 96L is less organized than it was Thursday evening, with only a little low-level spin apparent, and a modest area of disorganized thunderstorms. The decrease in organization is probably due to the moderate to high levels of wind shear of 15 – 25 knots over the region. Water vapor satellite loops show a modest region of dry air over the Central Gulf of Mexico, which is interfering with development. Ocean temperatures are 81 – 83°F in the Western Caribbean and Southern Gulf of Mexico, which is about 1°F above average, and warm enough to support formation of a tropical storm. A hurricane hunter mission is scheduled to investigate 96L this afternoon, but this mission will probably be cancelled due to the disturbance’s lack of organization.

Figure 1. Morning satellite image of the tropical disturbance 96L in the Southern Gulf of Mexico.

Forecast for 96L
Wind shear is predicted to remain in the moderate range through Saturday night, which is likely low enough to allow 96L to develop into a tropical depression; NHC gave 96L a 70% chance of developing into a tropical depression by Sunday morning, in their 8am EDT Tropical Weather Outlook. By Sunday, wind shear is predicted to increase, limiting 96L’s potential for intensification. Where the storm might go is anybody’s guess. The GFS model has consistently been predicting that a trough of low pressure pushing off of the U.S. East Coast will be capable of grabbing the disturbance and accelerating it to the northeast across Florida north of Tampa Bay on Sunday or Monday. However, an ensemble of forecasts from the model created by running the model with slight perturbations to the initial conditions shows a wide range of possible tracks, both to the east over Florida, and to the west towards Texas and Louisiana (Figure 2.) The latest ECMWF model run (00 UTC) predicts that the trough will not be strong enough to pull 96L northeastwards across Florida. The ECMWF predicts that a ridge of high pressure will build in over the Southern U.S., forcing the disturbance westwards across the Gulf of Mexico and into South Texas by Thursday. The UKMET model also favors a track west towards Texas. The NOGAPS model takes 96L to the northwest into Louisiana/Texas by Monday.

Figure 2. Which way will 96L go? The GFS model, when run at low resolution with 20 slightly different perturbations to the initial conditions in order to generate an ensemble of different forecasts, shows two distinct possibilities: a sharp east turn to move over Florida, or a west or northwest motion towards Louisiana or Texas. The high-resolution official GFS forecast is shown in white.

Jeff Masters

from:    http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2128