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Monday’s record-setting rainfall a ‘one-in-a-thousand-years event’

The previous one-day record for rainfall in Dallas County was set in July 2004.

Many parts of North Texas saw historic rainfall on Monday, leading multiple counties, including Dallas and Tarrant, to issue flash flood warnings.

Meteorologists described the storm as a one-in-a-thousand-years event, with some parts of Dallas-Fort Worth receiving more than 10 inches of rain in less than a 24-hour period. Some parts of eastern portions of Dallas got more than 13 inches of rain, according to Dallas Water Utilities.

Those figures would break a one-day rainfall record from July 2004, when a flood station at Joe Pool Lake reported 12.05 inches of rain, said John Nielson-Gammon, climatology professor at Texas A&M.

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“It seems like 10-inch rainfall totals over a day are more common right now in the area, and those would correspond to about a 100-year rainfall event,” said Nielson-Gammon, who is also the Texas state climatologist, a governor-appointed position.

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Areas of Dallas-Fort Worth that got more than 10 inches of rainfall experienced a 1,000-year event, an estimate based on historical weather data, Nielson-Gammon said.

And some of the data used to calculate 100-year and 1,000-year rainfall estimates could be outdated, he added.

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“You have a roulette wheel with a thousand slots, and it just happened to land on the money this time, although with climate change, you would probably need two or three slots to properly estimate the risk of rainfall coming up like this,” Nielson-Gammon said.

North Texas was experiencing record levels of drought and heat in the weeks leading up to Sunday’s storms.

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Higher temperatures increase the atmosphere’s capacity to hold water vapor, leading to more intense rainfall, Nielsen-Gammon said.

Matthew Cappucci, a meteorologist for The Washington Post, said the storm that moved through North Texas were among multiple extreme rainfall events in the country in the past month.

“It reminded me so much of what happened in St. Louis this month, and what happened in eastern Kentucky and in Death Valley in California, and in Effingham, Illinois,” he said. “To have five 500- or 1,000-year rain events within a month’s time is virtually unheard of.”

Human-induced climate change contributes significantly to a growing tendency for more and more frequent heavy rainfall events, Cappucci said.

“In the past, this might have been a 1,000-year event, maybe now it’s a 200-year or 500-year rain event, or even a 50-year rain event,” he said. “We just don’t know how much more frequent it is becoming.”

In May 2015, flooding caused by multiple days of unprecedented rainfall in Dallas led to multiple deaths and millions of dollars in damage.

Earlier this year, the U.S. Corp of Engineers and the city of Dallas announced work on a $223 million project to improve the city’s floodway along the Trinity River.

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In a written statement Monday morning, Terry Lowery, director of Dallas Water Utilities, said the city did not have any concerns with levee segments, and that crews are working to address drainage issues related to the downpour.

“With these conditions, many neighborhoods are experiencing street flooding conditions and staff has been working around the clock to address blockages from debris, grass clippings and other impediments,” Lowery wrote.