HOUSTON – Frank and Anthony have been keeping you posted this past week on the upcoming rain for this weekend. While the storm system that is bringing us this washout is not a tropical storm, the rain we get will seem like one.
Tropical storms form over the ocean, not over land. Our weather-maker this weekend is taking shape today over West Texas and will really get its act together over the Hill Country and South Texas tonight and early Saturday. A surface low pressure will develop overnight tonight and track eastward more or less along Interstate 10 passing through our area sometime on Sunday. The focusing mechanism for this storm is what we weather people call an upper level short wave, or trough of low pressure which is moving out of the southern Rockies today and across southeast Texas into Louisiana over the weekend. This type of storm is our primary rainmaker during the winter and we see these most often between November and March. The strong wind shear, colder temperatures and presence of drier air are just the opposite of what one associates with a tropical storm.
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So, what is the tropical connection? As the surface low develops, a counterclockwise circulation will increase. On the east side of the low winds from the south will bring very moist air into Texas from the Gulf. Being that is still September and the Gulf water is still hot, the moisture content of the air will be much higher than in the typical winter season storm. As the low pressure moves towards southeast Texas, it will trigger widespread thunderstorms, and given the tropical like moisture flowing in from the gulf, these storms will easily produce very heavy rainfall rates. The graphic shown here is the current NWS forecast for average rainfall totals, showing widespread 2-4 inches. It would not surprise me to see a few folks get in excess of 10 inches as several rounds of thunderstorms seem likely before the storm moves east of our area on Sunday. That kind of rain is tropical like and is what can give us serious flooding.
Stay tuned to KPRC for updates over the weekend!
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