What triggered the cataclysmic deluge in Texas? A meteorologist breaks down the weather behind the devastation.

Boston Globe

07-07-2025

Officials arrive to assist with a recovery effort at Camp Mystic along the Guadalupe River after a flash flood swept through the area.
Julio Cortez/Associated Press
As a meteorologist, it is very difficult to look at the science of weather when lives are lost and
communities devastated. But reviewing what unfolded is essential to providing clues that could lead to better warning times and more accurate predictions going forward.
You may have read that questions have been
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Those issues will likely be scrutinized for some time. But for now we know of a few factors that primed the atmosphere for the devastating flooding that continues at least through Monday night.
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A stalled frontal boundary provides subtle 'forcing'
First, let me define a term. A stalled frontal boundary provides steady and persistent lift to produce storms over the same area, rather than a passing front that moves through. It's like a car stuck in neutral with it's engine running. It just stays in one place rather than driving down the road.
In Texas, a stuck frontal boundary slowly deteriorated and lingered directly over the areas that saw the heaviest rainfall and most intense rainfall rates. Some rates reached a jaw-dropping 6 inches per hour, which is like a wall of water falling from the sky. For reference, the term 'torrential rainfall' is typically used when rainfall rates reach about 2 inches per hour.
Cold and warm fronts are
so-called 'forcing mechanisms' that march across the country, lifting air vertically to condense and wring out precipitation. There was
no dominant weather feature present in Texas to prompt the deluge. If there had been a strong system, it would have led to a more progressive storm, one that would have been more widespread but less intense.
But the setup Thursday night
held a weak, rather common forcing mechanism called 'synoptic forcing,' which was all that was needed to trigger an atmosphere that was primed with copious amounts of moisture. Like throwing an ember onto a pool of gasoline, it
didn't take much to erupt into flames.
This weak forcing created an additional complication as well,
as the stuck front allowed for storms to develop over the same spot again and again.
Surface map on July 3 (left) and July 4 (right) shows a crawling stalled front fading over Central Texas.
WPC
Copious moisture converging from the Gulf and remnants of Tropical Storm Barry
Atmospheric moisture was
notably high over Central Texas, and the European forecasting model started picking up on this late Thursday night.
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A strong northerly flow funneled moisture from the Gulf that was already holding remnant moisture from Tropical Storm Barry.
You can see the result below — the deep purple pushing through Texas. You may ask why it didn't rain then in Oklahoma and to the north? There wasn't a lifting mechanism. That moisture stayed suspended high in the atmosphere while Central Texas remained in that unfortunate sweet spot.
Copious moisture pushed through Texas over the holiday weekend.
Pivotal
The stalled frontal boundary was the atmospheric wedge that lifted all of this moisture above the area, resulting in a cluster of storms repeatedly dropping heavy rain. Unlike normal storms passing through the region and fizzling out, these storms regenerated on top of each other because the lifting mechanism and the fuel supply (moisture) remained uninterrupted.
Just take a look at the radar loop below to see how the storms hammered the same area on the Fourth.
Lots to unpack in this loop from yesterday's flood - notice how the Kerr County floods were initiated by stationary thunderstorms with torrential rainfall rates, followed by a second round as a cluster of storms to the north grew into an intense mesoscale convective system (MCS):
— Tomer Burg (@burgwx)
The terrain made matters worse, with rolling hills and shallow rivers and creeks prone to launch walls of water downstream in the event of extreme flooding. That is exactly what happened.
A warming atmosphere over decades
The atmosphere across Texas has evolved over time, growing warmer and holding much more water vapor. Remember, for every degree warmer it is, the atmosphere can hold 4 percent more water vapor.
Texas has seen a dramatic increase in flooding events in recent decades and it's directly linked to a warming atmosphere, especially coming out of winter and spring.
The average temperature across Texas has increased 1.6 degrees in the past 40 years during the five-month period of January to May. That means 6 percent more water vapor heading into much hotter summers and prompting more extreme and severe weather events.
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The average temperature from January to May has increased by over a degree and a half over the last 40 years.
NOAA
Ken Mahan can be reached at