How extraordinary rainfall caught Texas by surprise
Meteorologists had cautioned there was potential for flooding across Central Texas in the overnight hours late Thursday and early Friday morning. But there was little indication of just how torrential and unrelenting the downpours would become in the predawn hours, killing at least 27 people, many of them children at camp.
Radar and precipitation data and National Weather Service warnings show the floods were the result of extraordinary atmospheric conditions that sent intense plumes of Gulf of Mexico moisture into parts of Texas long known to be vulnerable to flash flooding, when bursts of heavy rain cause water to rise rapidly.
And unlike a typical summer thunderstorm that can cause quick flooding, this system formed in a way that allowed it to stall, creating deluges that repeatedly poured several inches of rain on the same areas within a matter of hours.
“The flooding damage is catastrophic,” Kerrville Police Officer Jonathan Lamb told The Washington Post. “It’s the worst flood that we’ve ever seen.”
It occurred against the backdrop of rising global temperatures, as surging fossil fuel emissions trigger the greenhouse effect,
causing the most intense rainfall to become even more extreme, scientists say. Warmer air is capable of holding greater amounts of moisture, and with warming bodies of water evaporating more vapor into the air, it is raising the risks of heavy downpours like the ones hitting Texas.
Rain was still falling Saturday, with some storms pouring on the Austin and San Antonio regions at rates exceeding 5 inches per hour, the Weather Service said. North of Austin, authorities were evacuating apartment buildings along the fast-rising San Gabriel River.
Around Kerr County, where many of the deaths are believed to have occurred, the downpours dumped a widespread 10 to 15 total inches late Thursday into Friday morning, in an area west of Austin and northwest of San Antonio, a region that typically averages 28 to 32 inches of rain in a year. Instead, four months of rainfall came down in four hours.
The rain had fallen in droves in the surrounding area: In Hext, 3.25 inches of rain were reported in the hour between 6 and 7 a.m.; Mason, got 2.63 inches of rain in the hour between 5:10 and 6:10 a.m., including 0.82 inches in 15 minutes; Brady Creek got 5.35 inches between 12:30 and 2:30 a.m.; Ranch got 2.35 inches in an hour.
All that water flowed into only a few rivers, causing extreme rises. The Guadalupe River
surged from seven feet to 29 feet in only a few hours in Hunt — its second-greatest height on record,
according to the National Weather Service, and higher than levels reached when floodwaters rose in 1987.
At least 1.8 trillion gallons of rain fell over Texas Hill Country and the Edwards Plateau on Friday morning.
On Thursday, there was some indication of a flooding risk across this region of Central Texas through the overnight hours. But the severity of the rainfall was far from certain, and warnings did not suggest extraordinary rainfall was ahead, according to National Weather Service forecasts.
Meteorologists were watching a drifting low-pressure system for the potential for what are known as “training” storms, when downpours repeatedly regenerate over the same area. Known as a “mesoscale convective vortex,” or a weak swirl in the atmosphere about 20 or 30 miles across, the system was circulating intense Gulf moisture northward like a pinwheel, feeding the storms with a whirlpool of robust tropical moisture.
Meanwhile, Tropical Storm Barry — which dissipated over northeastern Mexico about five days earlier — left behind an intensely moist air mass. National Oceanic and Atmospheric Administration models suggest that water vapor wafted into Texas.
Thursday afternoon, forecasters at the Weather Service office in San Angelo noted that there was potential for the storm system and all of that moisture to converge, but it was far from clear that could produce catastrophic floods.
“If this happens in this air mass [with precipitable water values soaring well above normal values] rainfall could be torrential and flash flooding would develop very quickly,” forecasters at the Weather Service office in San Angelo wrote Thursday afternoon. “Still, these features are so weak and the interaction so complicated, if and where this band develops remains uncertain.”
They issued the flood watch through early Friday acknowledging “the potential for a lower probability but much higher impact flood event overnight.”
A flood watch issued across the region through 7 a.m. Saturday cautioned 1 to 2 inches of widespread rainfall was likely, and that a narrow band of rainfall totaling 3 to 5 inches could develop, likely causing flooding.
But by 4 a.m., it became clear the rainfall was far more intense than that.
In a 4:26 a.m. Facebook post, the San Angelo meteorologists wrote: “**This is a life-threatening situation** 6 to 10 inches of rain has fallen and an additional 3 to 4 inches is expected through daybreak. Expect rapid rises on creeks, rivers and arroyos.”
One rain gauge in Mason County reported more than 18 inches of rain within 24 hours, the Weather Service confirmed.
Ocean temperatures in the Gulf of Mexico have
remained above average for months, likely adding extra fuel — in the form of water vapor — to intensify the flooding disaster.
Total atmospheric moisture was in the top 0.5 percent of historical observations for this region of Texas. Every column of atmosphere was holding 2.25 inches of moisture that could be squeezed out by downpours — but that air mass was continually fed into storms. It’s as if all the water from a sponge had been squeezed out, before the sponge was dipped back into the bucket of water again.
Every squeeze brought more rain.
Storms continued to linger over Central Texas on Saturday because there was a dearth of winds strong enough to push it away entirely.
A moderate (Level 3 out of 4) risk for excessive rainfall continues in the region through early Saturday morning, with the potential for rain and thunderstorms to reintensify, according to the Weather Service.
The devastation beared similarities to another flood along the Guadalupe River almost 40 years ago.
Ten teenagers were killed and 33 others were injured on July 16, 1987, when a bus and van leaving a church camp encountered floodwaters caused by 5 to 10 inches of rainfall in the upper headwaters of the Guadalupe River basin, according to the National Weather Service. A massive flood wave traveled through Ingram, Kerrville, and Comfort, driving the evacuation of hundreds of people along the river and its tributaries.
