Thunderstorms will pose a significant threat for flash flooding across parts of Texas and Louisiana over the next couple of days, with the threat for more than half a foot of rain possible in some communities this evening through Thursday. Stubborn thunderstorms already produced destructive flooding across Louisiana on Monday evening, and some of those same areas could see even more heavy rain to come.
Flash flood watches cover a significant portion of the southern United States this evening, including a huge chunk of Texas, most of Louisiana, and parts of Oklahoma, Arkansas, and Mississippi.
The greatest threat for widespread flash flooding appears to exist in Texas and Louisiana, where the Weather Prediction Center issued a moderate risk for excessive rainfall for Tuesday, Wednesday, and Thursday. The WPC mentioned on Tuesday that they might upgrade parts of Texas to a high risk for flash flooding on Wednesday—these high risks are reserved for days when forecasters are confident that excessive rainfall could lead to significant flash flooding (and it usually does).
The agency's latest rainfall forecast calls for five or more inches of rain across a huge portion of eastern Texas, with isolated pockets of extreme rainfall totals that could lead to major flooding problems in vulnerable areas. This includes the cities (and suburbs) of Houston, San Antonio, Austin, Dallas, and Corpus Christi.
We've already seen significant flash flooding over the past 24 hours in Louisiana, where persistent thunderstorms led to flash flood emergencies around Lake Charles and Baton Rouge. Local officials had to conduct water rescues to save residents trapped by waters that rose too fast to escape. The flooding in Lake Charles is especially gut-wrenching given that the community was hit so hard by category four Hurricane Laura in August 2020 and again by Hurricane Delta two months later.
The region is at risk of excessive rainfall because of a slow-moving upper-level trough that's meandering over the southern Rockies. This is the same trough that brought a tease of rain to California over the weekend and led to so many photogenic supercells on the Plains.
Southerly winds flowing between the trough to the west and a building ridge of high pressure to the east will pump humid air northward from the Gulf of Mexico. Any thunderstorms that form in this soupy airmass will have a deep reserve of moisture to tap into, and these heavy storms can linger over the same areas for hours without much in the way of prevailing winds to steer them along.
The northern and western Gulf Coast is exceptionally vulnerable to flash flooding during high-intensity rainfall events like this. Low, flat, and soggy terrain makes drainage a challenge even during a routine summertime thunderstorm. The combination of persistent high rainfall rates, poor drainage, and communities spreading ever farther into flood-prone areas all leads to fairly routine flash flood emergencies these days. The increasing frequency of heavy rain events (likely influenced by a changing climate) makes matters worse.
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