The thunderstorm that ripped through San Antonio on Friday night brought winds approaching 80 miles per hour, deadly flash flooding, and knocked out power for over 100,000 residents — all with relatively little warning.
Fewer than 2,000 San Antonio residents remained without power Monday in sweltering heat. The city said it has repaired more than 1,500 traffic signals across the community. And San Antonio Police are investigating the deaths of two people that may have been related to the flash flooding produced by the storm.
The first alert from the National Weather Service came around 9 p.m. Friday as the storm moved into the area.
According to Harrison Tran, a meteorologist with the NWS Austin-San Antonio office, that's largely because scattered summer thunderstorms fueled by heat and instability in the atmosphere are hard to warn people about ahead of time.
"Oftentimes, when you get a lot of these storms clustering together on a day like that, it increases the potential for stronger winds, especially with the hot weather we've been having throughout the summer," Tran said. "That usually creates an environment that's more favorable for these storms to produce pretty strong gusts."
San Antonio was under a marginal risk for severe weather on Thursday, when isolated showers moved through the area but left most of the city dry. The National Weather Service issued no such advisory for Friday, however.
NWS and local forecasters did note a chance for isolated thunderstorms Friday afternoon and evening. But no one was forecasting anything as strong as the storm that moved through the city Friday night.
That evening, scattered storms supplied by August heat — "popcorn showers," as Tran described them — congealed together just northeast of San Antonio. Compared to spring storms, these are much harder to predict days ahead of time since they aren't the result of a larger weather pattern like a cold front, Tran explained.
"They're a lot more fickle, oftentimes a lot more isolated and scattered in nature, rather than a more widespread event," he added. "It can be tricky to issue a watch or messaging well in advance."
The San Antonio International Airport reported a wind gust of 77 mph, and damage surveys were consistent with such speeds. These were straight-line winds, Tran said, and there were no confirmed tornadoes produced by the storm.
The wind and rain may have been enhanced by a microburst — a strong downward column of wind and rain that can occur during strong thunderstorms — though confirming that will require more investigation, according to Tran.
These kinds of summertime storms are often short-lived, producing lots of rain, wind and lightning over a shorter period. They're not unusual for the area this time of year, Tran said, they just don't often strike populated urban areas.
"It's just rare in this circumstance that this went directly over the San Antonio metro area," he said. "But we do get plenty of storms that show similar signatures on radar that appear similar in more rural areas."
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