Above is the latest water vapor satellite image, which shows the center of a middle and upper-level weather disturbance over northern Mexico, a couple of hundred miles West of the Texas border (red circled area on the image).
The red arrows on the same image show where stronger middle and upper-level jet stream winds are flowing out ahead of the system, which are ejecting weaker pieces of "energy" toward the Texas border at this time. This is confirmed by the latest radar mosaic image below, which shows shower and thunderstorm activity breaking out along the Big Bend in southwest Texas:
The system is moving East/Northeastward a bit ahead of schedule (as far as the midday computer models are concerned). Below are images showing where the GFS and NAM computer models were forecasting the center of the main upper disturbance to be located at 6pm CST this evening:
The black "X" over northwest Mexico depicts where each of the respective models were forecasting the center of the middle and upper-level circulation to be located at 6pm CST. I've placed a red dot on each image to approximate where the center will actually be located at that time, based on its progression on the water vapor imagery at the top of the post. The GFS model has done a slightly better job estimating the 6pm position, but it appears that both models will be about 150-200 miles too far to the West at 6pm CST.
Model variance aside, showers and thunderstorms will continue to increase along the Big Bend and eventually Southeastward along the Rio Grande of the southwest Texas border during the mid through late evening hours, as energy increases into the region from the West. Ample low level moisture and instability are in place for the disturbance to work with, which will result in widespread rain for many areas later tonight.
Widespread 1 inch rainfall amounts are forecast (as shown by the lighter blue shaded areas on the image below), with localized amounts of near 2 inches possible in heavier showers and thunderstorms tonight and early Tuesday:
Severe weather will also be a threat tonight, mainly in the form of hail and wind gusts to severe limits. This threat will initially develop toward or just after Midnight along the Rio Grande/Big Bend region, and progress Eastward through the Hill Country and toward the Austin/San Antonio corridor by dawn or shortly after.
I'll make another updated post later this evening as it may become more clear exactly when certain areas are likely to be impacted. At this time, I would caution everyone along I-35 from Waco on Southward through Austin and San Antonio to prepare for a possible rough commute tomorrow morning due to locally heavy rain and strong to severe storms. With that said, I know that no one will be complaining about the rain, no matter what an inconvenience it may cause in morning traffic...
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