Day 12 of San Antonio, TX (KSAT)

Despite southeasterly winds, it cooled down to 67F before low clouds came in last night, going below some model guidance unlike most recent nights. The low clouds quickly broke up, with a rapid warmup to a high of 91F and a maximum wind of at least 22 kt. However, mid to upper-level clouds could have tempered the warmup slightly. Day 12 will overall be substantially cloudier than the past 2 weeks. A surface low-pressure approaching from the west will lead to stronger and deeper moisture advection from the Gulf of Mexico tonight with southeasterly winds, leading to a warmer low than we’ve seen in the past 2 weeks, likely near or above most model guidance. However, I am hesitant to go too warm after last night’s surprise low. Also, there’s still some uncertainty of timing and thickness of the low clouds, with HRRR not becoming overcast until 12z, and not very thick, while NAM 3-km shows thickening low clouds by 03z. Reality will probably be in between. There will also be some mid to high-level clouds that could hamper cooling regardless of the low-level clouds. 

 

How much the clouds break up tomorrow is also highly uncertain, with HRRR quickly clearing out the low-level clouds, leading to a hotter high, while the NAM 3-km has the clouds persisting for most of the day, leading to a cooler high. There’s even a positive feedback making it especially tricky, since any sunshine will help heat up the ground, helping to burn off the low clouds faster, etc. Even if the low clouds clear out, mid to upper-level clouds will remain to temper the warmup, and HRRR tends to warm it up too much with these clouds. Still, there may be enough heat and low-level moisture for scattered showers to occur along a subtle pre-frontal trough/low-level convergence zone in the vicinity, though models indicate very few and weak storms if any at all in the vicinity, with the strong forcing with the real cold front still staying to the northwest. 

 

The greater cloud cover could also inhibit winds slightly, making it more difficult to mix stronger winds aloft down to the surface. Any showers that do form tomorrow afternoon could briefly increase winds somewhat but they do not look nearly deep enough to produce strong cold pools with strong gust fronts like along the real cold front to the northwest. All models agree that the main squall line associated with the cold front will still be at least 100 miles (160 km) to then northwest of San Antonio even at the end of the period (Sunday 06z), which means that even if the cold front and squall line advance more quickly to the southeast than modeled, which sometimes happens, it is still unlikely to reach San Antonio in time to affect day 12.

 

Source: PivotalWeather

 

Source: PivotalWeather