Historic Houston flood: 16 inches of rain in under 12 hours — and it's not over
UPDATE: April 19, 2016 at 10:00 a.m. ET:
April 18 was the second wettest day on record in Houston, with 9.92 inches of rain. This beat the 8.13 inches of rain recorded during Tropical Storm Allison in 2001, but didn't quite match the 10.34 inches of rain that fell in the city on June 26, 1989.
A deluge of colossal proportions struck the Houston and Galveston, Texas, areas on Sunday night and Monday, with nearly 2 feet of rain falling in some spots.
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The rain led to widespread severe flooding, prompting the rare issuance of a "flash flood emergency" warning from the National Weather Service.
Houston Mayor Sylvester Turner called the rainfall and associated flooding “unprecedented” for the city. City offices and schools are closed on Monday due to flooded roadways.
The flooding forced Ed Emmett, the county judge for Harris County, which includes Houston, to declare a disaster on Monday morning.
As of 3 p.m. local time, Houston Intercontinental Airport had received 11.75 inches of rain since midnight. If confirmed, this would make today the wettest day on record there, which was 10.34 inches on June 26, 1989.
According to Jeff Lindner, a meteorologist with the Harris County Flood District, rainfall across Harris county on April 18 averaged 7.75 inches, or 240 billion gallons. He tweeted that this "Could run Niagara Falls for over 88 hrs."
The heaviest rain fell along and to the north of the Interstate-10 corridor between San Antonio and Houston overnight Sunday and into Monday morning, with more rain on the way as the weather system slowly moves east. By midday Monday, the heaviest rain pushed toward the coast, increasing the flood threat there.
Rain continued to fall in the hardest-hit areas, with more showers and thunderstorms forecast for Tuesday. According to the Weather Prediction Center in College Park, Maryland, another 3 to 6 inches of rain may fall across southeast Texas before the storm system moves far enough to the east to allow the precipitation to taper off.
As of 1 p.m. local time, 17.6 inches of rain had fallen at Little Mound Creek at Mathis, the National Weather Service reported. Numerous reports of more than a foot of rain were received.
The rain brought the metro area to a standstill as water submerged highway guardrails, stranding drivers who defied the Weather Service's lifesaving advice of "turn around don't drown."
Although not an official measurement, the Weather Service reported that a rain gauge near the intersection of Huffmeister and Highway 6 north had "surpassed more than 20 inches." At least 60 water rescues were underway at the same time across the Houston metro area on Monday morning, according to weather.com. A total of at least 1,200 rescues occurred through Monday afternoon, the Weather Service reported.
Rainfall rates in some areas reached 4 inches per hour, according to the National Weather Service.
The scope of the flash flood warning the Weather Service issued was unusually expansive, covering more than 21,000 square miles, and meteorologists at the agency's Houston office called the event a "worst-case scenario" in a discussion posted online.
The rain is the result of a storm system spinning over Colorado, where it dumped up to 50.6 inches of snow in the mountains northwest of Denver. The circulation around the storm has sucked a pipeline of moisture-rich air northward from the Gulf of Mexico.
Inside this storm system
A cold front that is barely moving east is helping the warm, moist air to rise, cool and condense into clouds and precipitation. Near the heaviest rain bands, onshore winds from the southeast were converging with winds from the north, accentuating the rainfall rates.
A key factor contributing to the flooding in the Houston area is that the storm system is barely moving. Instead, the heavy thunderstorms are sitting atop the city, continuously dumping extreme amounts of rain.
As a study released on April 15 found, when weather systems get stuck in traffic jams, extreme events can often result.
This storm was well forecast, with the heavy rainfall threat discussed at least three days in advance. However, pinpointing the specific areas that will receive the heaviest rains is a difficult task, even with all the tools of modern meteorology.
The heavy rains have caused rivers and creeks to rise, and prevented the low-lying cities drainage system from clearing water fast enough to spare homes from flooding. Reports indicate that the flooding in some spots is comparable, if not worse than, flooding that occurred during Tropical Storm Allison in 2001.
That storm caused 22 deaths and more than $5 billion in property damage in Harris County alone, according to the Harris County Flood Control District. Southeast Texas also saw extreme flooding in May and June of 2015.
It’s likely that this event will run a tab in the hundreds of millions, if not eclipsing the $1 billion mark given the amount of property involved.
Across much of the U.S. and around the world, extreme rainfall events are becoming more common as the world warms and the atmosphere holds more moisture.
Andrew Freedman is Mashable's Senior Editor for Science and Special Projects. Prior to working at Mashable, Freedman was a Senior Science writer for Climate Central. He has also worked as a reporter for Congressional Quarterly and Greenwire/E&E Daily. His writing has also appeared in the Washington Post, online at The Weather Channel, and washingtonpost.com, where he wrote a weekly climate science column for the "Capital Weather Gang" blog. He has provided commentary on climate science and policy for Sky News, CBC Radio, NPR, Al Jazeera, Sirius XM Radio, PBS NewsHour, and other national and international outlets. He holds a Masters in Climate and Society from Columbia University, and a Masters in Law and Diplomacy from The Fletcher School at Tufts University.
