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Posted at 11:15 AM ET, 07/28/2011

Tropical storm Don takes aim at thirsty Texas; intensification iffy

Tropical storm Don continues to take aim at the drought-plagued Texas Gulf Coast. It is moving west-northwestward at 14 mph across the Gulf of Mexico and at the current pace will make landfall somewhere between Brownsville and Galveston late Friday. As a result, a tropical storm warning is in effect for Port Mansfield to San Luis Pass and a tropical storm watch south of Port Mansfield to the mouth of the Rio Grande.

Track forecast for Don. Tropical storm warnings in blue along Texas coast, watches in yellow.
Maximum sustained winds are estimated to be near 45 mph. But with tropical storm force winds confined to a small area within 60 miles of the center, and a rather ragged satellite presentation, Don is not a healthy tropical cyclone.

Don’s prospects

Though some of the large-scale factors involved in determining its fate could try to encourage further strengthening until landfall (e.g., relatively weak wind shear across some altitudes, and high ocean heat content), there are others that will fight this tendency.

Strong north to northeast winds around the hot and dry upper-level high over the Southeast U.S. – reaching speeds as high as 50 mph near 40000 feet-- are being drawn into Don’s circulation.

Wind shear affecting Don (yellows and reds indicate high values) (Florida State University)

These winds are not only providing potentially destructive wind shear at levels not typically sampled for shear estimates (areas shaded in red in image to the right over the Gulf of Mexico indicate precisely that), but they are also beginning to dehumidify the recently moist atmosphere high above the southern Gulf of Mexico.

Evidence of their impact was already noticeable late Wednesday, as Don’s once durable thunderstorm clusters began to collapse and create the kind of outward-flowing cool pools of air commonly seen in midlatitude storms. These “gust fronts” tend to raise the surface pressure and create a low-level spin in opposite sense.

But Don’s troubles have been, and continue to be, well-predicted by the global weather models. Perhaps because they’re properly simulating the injection of the windy and dry continental airmass from southern U.S. into the tropical system, none of them ever really make much of it. Given that they’ve consistently given little hope for intensification, these developments … or lack thereof … shouldn’t be surprising.

Intensity forecast

With these pieces of evidence, it’s hard to imagine Don ever becoming a hurricane. Chances of that happening are about 20%. It will most likely remain at minimal tropical storm strength, if not devolve completely into a tropical low before making landfall Friday.

Storm impacts

The National Hurricane Center projects 3 to 5 inches of rainfall from the central Texas coast westward to south central Texas with isolated amounts to 7 inches. As the latest drought monitor shows 75 percent of Texas in exceptional drought, this is welcome news. However, NOAA says Texas needs 15 inches of rain to erase drought conditions.

Given the exceedingly dry soils in place, and the relatively fast storm motion, the risk for widespread flooding appears to be low but isolated flooding could occur.

CWG’s Jason Samenow contributed to this post

By  |  11:15 AM ET, 07/28/2011

Categories:  Latest, Tropical Weather

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