It has been a loud last couple of hours around the D/FW Metroplex as strong storms erupted shortly after 2 PM. The strongest storm over the ninety minutes was in Cleburne where hail larger than the size of golfballs developed. Here’s a photo from TSC’s Jason Cooley of a severe storm a bit earlier near Waxahachie.
— Jason Cooley (@operation_chase) April 10, 2017
Over the last thirty minutes we’ve seen that discrete supercell evolve into a line segment from Waxahachie to just north of Hillsboro to Cranfills Gap. Sub-severe storms continue from West Tawakoni to Clarksville. Isolated storms are firing up in Central Texas. The atmosphere south of a cold front (extending from south of San Angelo to southeast of Brownwood to D/FW) remains quite unstable and contains plenty of moisture. Both of those ingredients are seen in this graphic displaying surface fronts, radar, surface instability values, and dewpoint temperatures. Dewpoints south of that cold front are in the upper 60s and 70s – summertime type of humidity.
The line of storms over Hill and Ellis counties are likely the start of what will become a larger squall line in the coming hours. By the early evening this squall line will likely be progressing south/southeast into Central Texas and East Texas. Some of the storms within the line will probably contain hail up to the size of golfballs and damaging straight-line winds over 60 MPH. Not all the storms in this squall line will be severe. However, most of the storms will be efficent rainfall producers. The threat of severe weather will start to come down by late evening, but localized strong storms will certainly be possible through the late night.
A dangerous flash flooding event may evolve this evening across South-Central Texas and the Hill Country. The line of storms developing across southern sections of North Texas now will probably be the primary cause of this flooding threat. As the storms move south, away from the stronger upper level forcing, the southward motion of the squall line will slow. Storms will continue to produce very heavy rains, so the longer they sit on one area, the more rain accumulates. As the HRRR simulated radar image shows above, we could see that squall line put its brakes on around the San Antonio metro – but the locations aren’t set in stone. However, when we put the brakes on the forward motion of the storms, those under those storms could get 4-8 inches of rain in a short period of time (2-3 hours). Should that occur, and we think it will, a rapid flash flood event will develop tonight. Flood waters are very difficult to see at night and can rapidly become dangerous. Be ready to act if a flash flood warning is issued for your location tonight. I’ll leave you with a discussion from the National Weather Service in San Antonio.
However, the main concern we are messaging will be the heavy rainfall
potential that could lead to flash flooding. Precipitable water
values of 1.6-1.8 inches are pooling over the region south of the
front. This is nearly 2 standard deviations above average. Initial
slow motions of the storms and complex will yield pockets of intense
rainfall rates this evening through portions of the overnight, as
indicated by runs of the HRRR. Models the past three runs have
trended farther south and with higher QPF (rain totals) amounts into much of the
southern and eastern Hill Country as well as I-35 corridor. GEFS (Global Ensemble Forecast System)
plumes for both Austin and San Antonio have trended up, with the
lowest member at Austin around 2″, highest at 4.3″, and mean 2.87″.
Some of the high resolution guidance suggests the highest amounts
farther south toward the San Antonio metro area. It is difficult at
this time frame to determine where the heavier pockets may occur as
there is uncertainty with the mesoscale evolution of the system.
But generally 2-4″ is expected across the Flash Flood Watch area
with isolated amounts up to 7″.