One of the nation’s most flood-prone cities is in the bull’s eye for potential flooding again Friday into the weekend, with heavy tropical downpours and storms set to move over the Houston metro area for days. Up to half a foot of rain is possible in northwestern parts of the metro area by Sunday morning, the product of deep tropical moisture pooling along a stalled cold front.
Some places farther west could approach double-digit totals.
Rainfall rates of 2 to 3 inches per hour are possible in the heaviest downpours, which will train, or repeat over the same areas, and bolster the risk of rapid onset flooding. A flash flood watch is in effect for the greater Houston-Galveston metropolitan area and much of south coastal Texas through Sunday.
As of midmorning Friday, a number of areas north and west of Houston had measured 2 inches or greater thus far as the axis of heaviest downpours shifted closer to the city. Closer to 4 inches had come down in Walker County.
A flash flood warning had been issued for areas midway between Houston and College Station on Friday morning, where up to 5 inches of rain had already fallen.
“The storms are expected to stall and heavy rainfall is likely to continue,” warned the local National Weather Service office. “Roads may flood very quickly with this intense rainfall.”
The National Weather Service Weather Prediction Center had hoisted a level 3 out of 4 moderate risk for excessive rainfall for much of the midcoast of Texas, the greatest risk centered southwest of Houston and including cities like Victoria, Rockport and Port Lavaca.
“All of the ingredients one would look for in regards to flash flooding (outside the presence of a tropical cyclone) exist Friday into Saturday morning in South and Southeast [Texas],” forecasters wrote.
The agency noted model simulations of up to 15 inches falling in Southeast Texas, with shorter-term models highlighting a roughly fifty-fifty shot of Houston exceeding 5 inches in 24 hours.
The flooding stems from a low pressure system centered over the mountains of Coahuila, Mexico. Counterclockwise flow around the low has induced southerly winds, the wind fetch acting like a conveyor belt to truck Gulf moisture ashore. Meanwhile, a stalled cold front draped parallel to the Texas coastline has been acting to focus downpours, cold air near the ground forcing the mild air mass upward and generating bands of heavy rain.
There’s little indication the band will move any time soon, continuously regenerating and back-building as southerly winds replenish its moisture source.
“Much of the guidance indicates storms getting focused over the southwestern areas extending up into the Houston Metro and along the I-10 corridor late morning through early afternoon,” wrote the Weather Service in Houston.
Rainfall looks to taper in intensity during the late afternoon and evening hours Friday before filling back in Saturday morning. By Saturday afternoon, more scattered thunderstorms will be the story.
All told, the metro is looking at a broad 3 to 6 inches of rainfall, while areas farther southwest, like Victoria, could see totals as high as 6 to 10 inches. While rivers in the greater Houston area have plenty of capacity for additional rainfall, the concern primarily centers on urban flash flooding. That comes despite dry soils.
The city is packed with concrete, streets and highways, the surfaces impermeable basins that prevent water from draining.
“The more urbanized corridors of south-central to Southeast Texas will most be susceptible to enhanced runoff concerns,” wrote the Weather Prediction Center.
Complicating matters further is the risk for severe weather – including isolated hail, microbursts and tornadoes – primarily southwest of Houston on Friday, but expanding across the metro by Saturday. Pockets of spin along the front will overlap with a weakly to moderately unstable airmass to brew a couple embedded rotating thunderstorms. The Storm Prediction Center identified a level 1 out of 5 “marginal risk” for severe weather.
Houston is one of the most vulnerable places in the nation for high-end flooding. That stretch of Texas routinely receives excessive rainfall of ten inches or more at a time, and the city’s infrastructure simply can’t handle it. And human-induced climate change is making it worse.
As air temperatures warm, the atmosphere can hold more water. Since a warmer world is a wetter world, that means more flooding.
High-end rainfall events have doubled in frequency in just the past 50 years in Houston, while annual rainfall has increased 4 to 8 inches. That’s like getting an extra month or more of rainfall every year.
What once was a rarity for Houston is now routine as the “new normal” shifts to include more flooding. And as the city continues to boom and populations expand, more high-end flooding and flood disasters are increasingly inevitable.