Rain was still falling yesterday, with some storms pouring on the Austin and San Antonio regions at rates exceeding 127mm/h, 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 255mm to 380mm early on Friday local time, in an area west of Austin and northwest of San Antonio, a region that typically averages 710 to 810mm 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.
All that water flowed into only a few rivers, causing extreme rises.
The Guadalupe River surged from 2.1m to 8.8m in only 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 6.8 trillion litres of rain fell over Texas Hill Country and the Edwards Plateau on Friday.
There was some indication of a flooding risk across this region of Central Texas through the overnight hours local time.
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 30 or 50km 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 vapour wafted into Texas.
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 on Thursday afternoon local time.
“Still, these features are so weak and the interaction so complicated, if and where this band develops remains uncertain.”
They issued the flood watch on Friday acknowledging “the potential for a lower probability but much higher impact flood event overnight”.
A flood watch issued across the region cautioned 25 to 50mm of widespread rainfall was likely, and that a narrow band of rainfall totalling 75 to 125mm could develop, likely causing flooding.
But it became clear the rainfall was far more intense than that.
In a 4.26am local time Facebook post, the San Angelo meteorologists wrote: “**This is a life-threatening situation** 6 to 10 inches [152-255mm] of rain has fallen and an additional 3 to 4 inches [75-100mm] is expected through daybreak. Expect rapid rises on creeks, rivers and arroyos.”
One rain gauge in Mason County reported more than 457mm 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 vapour - to intensify the flooding disaster.
Total atmospheric moisture was in the top 0.5% of historical observations for this region of Texas.
Every column of atmosphere was holding 57mm 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 yesterday 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, with the potential for rain and thunderstorms to re-intensify, according to the Weather Service.
The devastation bore 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 125 to 255mm of rainfall in the upper headwaters of the Guadalupe River basin, according to the National Weather Service.
A massive flood wave travelled through Ingram, Kerrville, and Comfort, driving the evacuation of hundreds of people along the river and its tributaries.
- Janice Kai Chen, Arelis R. Hernández, Kyle Rempfer and Daniel Wu contributed to this report.
