The unofficial last summer weekend of the year is shaping up to be a wet one across Central Texas as a slow-moving cold front surges into the state before stalling out across the Austin metro area.
The past several days have been hot and humid, priming the atmosphere with a lot of moisture for the cold front to work with, which means we could see some locally heavy rainfall.
“Moisture convergence along the frontal zone will increase PWs (precipitable water amounts) to 150% of late-August averages,” the National Weather Service said in a forecast posted online Friday. “Locally heavy rains (are) possible through Monday with the main threat being urban and small stream flooding with a marginal risk of flash flooding.”
First, you need a stalled frontal boundary, or convergence zone, where air masses collide and then warmer air rises. This lifts heat and moisture into the atmosphere, making the air unstable. That’s when you start seeing those towering cumulus clouds that eventually turn into rain-making thunderstorms.
Second, available moisture in the atmosphere — what the forecasters had called “PWs” or precipitable water — will reach a peak of 2 to 2.25 inches from Saturday afternoon through Monday afternoon, according to the weather srevice.
Third, the winds that usually steer the storms are weak, so the storms will be slow-moving. Also, the outflow of air and gust fronts of individual storms will collide, push warm air upward and spark new storms along those boundaries, or cause storms to merge.
All these ingredients will be common each afternoon through the holiday weekend, producing rounds of showers and thunderstorms with the greatest threat of heavy rainfall Saturday night through Labor Day.
Rainfall totals will vary but with slow-moving or training thunderstorms (a continuous series of storms over the same spot), widespread rainfall totals of an inch to 3 inches are likely with isolated amounts up to 5 inches. The weather service flood forecasters at the Weather Prediction Center have placed Central Texas, including the Hill Country, under a level 1 of 4 risk, or at least a 5% chance, of flooding from excessive rainfall on Saturday. But they have increased the threat to a level 2 of 4 risk, or at least a 15% chance, of flooding on Sunday.
Outside of the flash flood threat, widespread severe weather is not expected, although the strongest storms could produce isolated wind gusts of up to 50 mph.
On Sunday, a stronger northerly flow will push the cold front south of Austin, spreading the rain chances to South Texas and ushering in cooler air behind it. Saturday will still be warm, with highs in the low to mid-90s, depending on where and when storms develop. By Sunday, with the front through and north winds in place, highs will cool into the 80s.
If you have plans to travel this weekend, be sure to keep an eye on any flooded roads through ATXFloods. Also, be sure to sign up for weather alerts through Warn Central Texas, a city of Austin program preparedness program.