Monday, January 7, 2013

Update on Texas Heavy Rainfall, Severe Weather Event for Tuesday and Wednesday...

All of the ingredients necessary for a widespread heavy rain event are coming together in central and eastern Texas late this afternoon, including the Austin-San Antonio corridor region along I-35.

Above is the latest surface weather map, with the green shaded areas showing rich moisture building up in the Gulf of Mexico.  Surface winds (as noted by the purple lines) are shifting to the East/Southeast, which will pull the moisture Westward into the region overnight tonight and into Tuesday.

Later tonight, a "low level jet stream" will form just above the surface across southcentral and southeast Texas, pulling moisture into the region at speeds of around 50 mph, which will further make the atmosphere moist and unstable for the upcoming event.

Meanwhile, in the upper levels of the atmosphere, a strong area of low pressure is currently located just South of Yuma, AZ, over extreme northwestern Mexico:

Strong jet stream winds are already spreading East ahead of the system across the Lone star state, as indicated by the yellow arrows on the water vapor satellite image above.

The system is forecast to swing toward the Southeast tonight and move across Texas Tuesday and Wednesday, resulting in widespread shower and thunderstorm development.

Both locally heavy rainfall and severe thunderstorms are likely in association with this system.  Computer forecast model guidance is suggesting a total of 2-4 inches of rain will be common along the Austin to San Antonio corridor, with localized amounts up to 6 inches possible, and I agree with that assessment.

Here is how the latest high resolution NAM computer forecast model is forecasting the precipitation situation to shape up over the next 24-48 hours, and I agree with the overall trends that are depicted by this model.  First, accumulated rainfall from 6am to 6pm CST on Tuesday:

...through 6am CST Wednesday:

...and through 6pm CST Wednesday:

As you can see, the model is depicting a maximum rainfall swath of 5-6 inches along and East of I-35 between Austin and Dallas.  Widespread 3-5 inch rainfall amounts are indicated surrounding that bullseye.

The heavy rain may cause flash flooding in the urban areas of Austin and San Antonio, particularly tomorrow night and Wednesday morning.

In addition to the threat of heavy rainfall and flash flooding, severe thunderstorms will be a distinct possibility across mainly southcentral, southeastern and east-central Texas, including the Austin/San Antonio corridor, the Corpus Christi area and the Houston Metro area.

Below are the latest severe weather outlooks for Tuesday into Tuesday night (first) and Wednesday (second), from the Storm Prediction Center (SPC) in Norman, OK:

Severe weather will primarily take place along and to the South of a surface warm front across Deep South and southcentral Texas on Tuesday afternoon.  This threat will spread Northward into the Austin/San Antonio region on Tuesday night, and then shift East into southeastern Texas (including the Houston area) on Wednesday.

Damaging wind gusts and hail up to 1 inch in diameter will be the primary severe weather threats during both periods of time.  Isolated tornadoes are also possible, including during the overnight hours on Tuesday night in the Austin / San Antonio area.

If you live across the severe weather threat areas for Tuesday and Wednesday, please be alert.  Listen to NOAA Weather Radio, local media or other trusted sources for the latest information, watches and warnings.  If you live in the Austin/San Antonio and adjacent areas, please make sure that you have a way to receive severe weather warnings at night.

Stay tuned for more on this situation tomorrow and Wednesday.  If you don't already follow me on facebook, Google+ or twitter, please do so as I am likely to make more frequent, short posts on those outlets during an ongoing severe weather situation.

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