The latest radar composite image above shows thunderstorm activity forming along the Rio Grande river along the Texas/Mexico border. The pink outlined area shows where the Storm Prediction Center (SPC) is monitoring for a possible severe weather watch this evening.
This activity will increase as it moves Northeast, spreading toward the I-35 corridor from Austin to San Antonio by late this evening. Scattered storms may form ahead of this activity and produce a brief downpour across the region this evening, but the main "show" for the Austin-San Antonio corridor will come after 10pm and into the overnight hours, as several rounds of showers and heavy thunderstorms move across the region.
Very heavy rainfall, frequent lightning and strong (possibly damaging in some cases) wind gusts will be the primary threats with the activity late this evening and tonight. A few tornadoes are also possible, but they should be generally brief in duration. While the brevity is nice from a hazardous weather standpoint, it certainly makes identifying them on radar very difficult as the circulations are usually very short lived. It's also not much consolation if the "brief" tornado happens to hit your house, or the home of someone that you care about, so please don't let the word "brief" catch you off guard.
If you're staying up late tonight (or are awakened in the night), keep an eye out for "dog leg" or "comma head" configurations on radar. They look something like this (within the white circled area):
Those signatures are often associated with a broad circulation within the lower levels of the thunderstorm, which often produces damaging winds and/or a brief tornado at the surface. (That image is from the event that took place almost one year ago, 1-25-12, and produced an EF-1 tornado in northeast Austin just before 3 a.m. CST).
In addition to their relative brevity, these types of circulations tend to be hidden within large areas of heavy rain, which make them difficult (if not impossible) to view, not to mention the fact that it will be dark outside. So, these types of situations should not be taken lightly. If threatening weather approaches your area, take shelter as if a tornado had been sighted, that way you know you are protecting yourself and your family as best you can until the storm passes.
If you live along I-35 in southcentral Texas, including the Austin/San Antonio areas, please make sure that you have a way to receive weather warnings overnight tonight. It's still not too late to do so even at this hour, as you can make a quick trip to the store or even download an app or plug-in on your computer and/or phone!
Rainfall will be torrential at times, with widespread amounts of 2-4 inches likely, and localized amounts of 4-6 inches possible by the time the precipitation ends, which will be between 10am and 12 Noon tomorrow:
Based on the forecast, this will be the most substantial rainfall event for the I-35 corridor from Austin-San Antonio in almost 1 year (possibly more if greater than 5.66 inches of is received). Looking at the records from the Austin Bergstrom Airport, the last time that a significant rainfall has taken place is as follows:
0.50" or more: 10/11/12 (0.50")
1" or more: 9/29/12 (1.55")
2" or more: 8/18/12 (2.85")
3" or more: 1/25/12 (5.66")
Note the last date being the 1-25-12 event once again (as noted with the "brief" tornado comparison above).
The threat of severe weather and widespread heavy rainfall will shift East into east-central and southeast Texas, including the Houston area, tomorrow, so folks down that way should continue to keep an eye to the sky!
Stay tuned for more information on this situation as it continues to unfold. I will most likely "pseudo-live" blog for the Austin-San Antonio areas later this evening and tonight as time allows between client updates. If you don't already follow me on facebook, Google+ or twitter (see links below), please do so as those are the most likely places that I'll post short, more frequent updates (which is typical during an ongoing severe weather situation).
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