A severe thunderstorm watch is expected to be issued for the Edwards Plateau, South-Central Texas, South Texas, and the Coastal Bend in the next hour or two. A line of severe thunderstorms is organizing in northern Mexico. That squall line will move across the Rio Grande and into the United States by 3 AM. The strongest storms may produce large hail up to the size of golfballs, although larger hail cannot be ruled out. Damaging straight-line winds of 60 to 70 MPH are also expected in the stronger storms. The threat for damaging winds may increase later this morning as the squall line makes its way closer to the coast and into the Coastal Bend.
Mesoscale Discussion 0272 NWS Storm Prediction Center Norman OK 0100 AM CDT Sun Apr 07 2019 Areas affected...portions of southern TX Concerning...Severe potential...Watch likely Valid 070600Z - 070730Z Probability of Watch Issuance...80 percent SUMMARY...Large hail and damaging wind gusts will be possible overnight into the early morning hours across much of southern Texas as a cluster of storms in Mexico cross the Rio Grande and shift eastward. DISCUSSION...The severe threat will continue to increase overnight as a cluster of storms over the higher terrain of Mexico shifts east across parts of the Hill Country and points south. This convection is being forced by increasing ascent as the southern stream shortwave trough ejects over northern Mexico. The downstream environment is characterized by a very moist boundary layer with upper 60s to low 70s dewpoints beneath midlevel lapse rates around 8-8.5 C/km. This is resulting in MUCAPE values around 2000-3000 J/kg. Low level easterly flow through the lowest 1km or so is relatively weak, ranging from about 10-15 kt, but should increase slightly with time. A quasi-stationary front is draped across the region from northern Maverick County east/northeast into east-central TX. Hi-res guidance suggests convection will organize and travel roughly along and south of this boundary in the more pristine warm sector airmass. Given very steep midlevel lapse rates, hail will be likely in embedded stronger updrafts/supercells. Storm mode could temper a more significant hail threat, though some very large hail cannot be ruled out. While boundary layer inhibition is quite strong at this point, strong forcing should be sufficient to weaken low level inhibition sufficiently for some damaging wind threat as well. Convection will track eastward across much of southern TX through the overnight hours and a watch will be within the next hour or so.