Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 11:42 PM GMT on July 29, 2011
Tropical Storm Don continues to make its way west-northwest toward the Texas coast this evening. Don has increased steadily in pressure over the past 18 to 24 hours. Wind speed remained 45 knots in the 5pm EDT advisory. The strongest thunderstorm activity continues to be south of the center, southeast of Brownsville, Texas, but the strongest winds are to the north according to Hurricane Hunter data. A mesoscale vortex developed on the southern end this afternoon with high radar reflectivity. Radar estimated rainfall rates are as much as 5 inches per hour to the south, and around 2 inches on the north side of the storm, but these are likely an overestimate, although rainfall rate is definitely higher on the southern end of this storm. It began raining in Brownsville, Texas in the 2pm CDT hour, and so far the airport has received 0.3 inches of rain. The heaviest thunderstorms still look to be to Brownsville's southeast.
Figure 1. Radar reflectivity from Brownsville, Texas at 5pm CDT.
Forecast for Tropical Storm Don
A Hurricane Hunter is currently in the storm, and center fixes suggest Don will make landfall south of Baffin Bay, Texas, in the next couple of hours. Tropical storm-force winds, should there be any, will most likely be seen in a swath from Baffin Bay to Corpus Christi. Once Don moves inland it is expected to dissipate quickly, and without having produced enough meaningful rainfall to impact any drought conditions. The Hydrometeorological Prediction Center isn't predicting any rainfall that would exceed flash flood guidance, and no flood watches or warnings are in effect.
NHC Invest 91L
Moderate thunderstorm activity continues in Invest 91L this afternoon, and the Hurricane Center estimates that the axis of the wave is located near 42W. Estimates from satellites show that 91L has a moderate circulation at all levels, and the Advanced Scatterometer resolved a surface circulation early this morning.
Figure 2. Advanced Scatterometer pass over 91L at 7am EDT this morning. The wind barbs show a low-level circulation had formed near the most intense thunderstorms—a good sign of potential development.
Some of the reliable models—GFS, UKMET, and CMC—are developing 91L into at least a weak tropical cyclone. The ECMWF continues to show no development, but a more interesting solution, one that takes the wave on a southerly track. The GFDL does not develop Invest 91L, but the HWRF forecasts category 1 hurricane strength by August 1st. The track for 91L is likely a turn to the northwest when it reaches the Caribbean and a curve to the northeast around the Bahamas. The models that are developing the system do not suggest this is a U.S. landfall threat, although it's pretty early in the game to predict that, and I imagine the track will fluctuate over the next few days. Today the Hurricane Center gives the wave a 30% chance of development (now 50% in the 8pm EDT outlook) over the next 48 hours, and I'm around 40-50% over its lifetime.
I'll have a post tomorrow afternoon on the aftermath of Don as well as NHC Invest 91L.
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