After hours of teetering on the edge, the National Hurricane Center finally took the plunge and declared a swirling mass of thunderstorms in the Gulf of Mexico a tropical storm, the second of the Atlantic hurricane season. The greatest threat with Bill is very heavy rainfall that could lead to flash flooding.

Bill currently has 50 MPH winds as it swirls towards the Texas coast, where a tropical storm warning is in effect for just about everyone from around Corpus Christi up through the Houston/Galveston area. Tropical storm force winds are approaching the coast and will overspread the area through the nighttime hours. The system could strengthen a bit overnight, with the NHC’s latest advisory calling for Bill’s maximum sustained winds to reach 60 MPH before making landfall on Tuesday morning.

While winds are always a hazard—trees and power lines are especially vulnerable in this area due to the saturated soils—the heavy rain will likely be the big story with this storm. The Weather Prediction Center continues to call for a wide swath of five or more inches of rain from Texas up through the Midwest, with much higher totals in areas caught underneath training showers and thunderstorms.

The track of the storm is key, since the heaviest rain will likely fall along and to the east of the storm’s track. If Bill performs as the NHC (and WPC) currently expect, the heaviest rains will likely fall in a corridor between Corpus Christi and Houston north through Austin, Waco, Dallas-Fort Worth, and eventually up through Oklahoma and into the Midwest.

Just about everyone under the threat for extreme rainfall over the next couple of days is under a flash flood watch by now, and watches and warnings will continue to fill in as forecasters get a better idea of what will happen once Bill makes landfall. Even if the main rain shield associated with the storm falls apart, the deep tropical moisture will still remain in the atmosphere and loop towards the Mid-Atlantic around the ridge of high pressure keeping the southeast locked in a heat wave. Any thunderstorms that develop in this pulse of tropical moisture will be prone to dumping a large amount of rain in short order, hence the threat for heavy rains far beyond the Texas coast.

The National Hurricane Center’s next update comes out at 1:00 AM CDT, with a complete forecast at 4:00 AM CDT. The Vane will have updates through the day on Tuesday as Bill and its remnants plague Texas and neighbors with more rain than it can handle.

[Maps: author]

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