Through early this evening, 9.91 inches of rain had fallen at Houston Intercontinental Airport*, breaking the record for the wettest April day set in 1976. As it stands, it is the second wettest day on record in Houston for any month, just shy of the record of 10.34 inches from June 26, 1989, although additional rain is possible this evening.

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The torrents of rain, which were heaviest early Monday, inundated homes in at least 100 neighborhoods, with the west side of Houston hardest hit. Schools were shut, and tens of thousands of people were without power.

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The National Weather Service serving Houston called the event “historic” and “severe” and described it as a “worst case scenario.”

The City of Houston advised its residents to stay home while the National Weather Service warned of a drowning risk and exposure to chemicals and snakes.

“It is not an overstatement to say this was the Houston region’s worst flooding event in nearly 15 years, since Tropical Storm Allison deluged the upper Texas coast and dumped in excess of 30 inches of rain over parts of the city,” wrote Eric Berger, author of the website SpaceCityWeather.com.

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The event has proven remarkable for both its size and intensity. The flash flood warning declared by the National Weather Service Monday morning covered a region encompassing more than 21,000 square miles, the largest in the last decade, at least.

A huge area west of Houston was impacted by rainfall amounts that occur only once every 100 to 200 years, on average.

Rainfall rates in some areas reached three to four inches per hour and a foot in less than 12 hours. Storm totals exceeded 17 inches just west of the city.

The downpours caused dramatic rises in water, with some creeks rising 20 feet in a flash.

In addition to rescuing stranded cars and people, emergency responders were seen frantically trying to rescue horses in Cypress Creek, where the water shot up 20 feet in just five hours:

Before and after photos show the landscape in and around Houston utterly transformed by the swelling floodwaters.

Entire retaining walls collapsed.

And, following the worst of the rains, sinkholes formed.

This flood was set up along a front sprawled across south Texas where extremely warm and moist air from Gulf of Mexico interacted with cooler air flowing in from the north.

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Intense thunderstorms repeatedly formed in this convergence zone, which moved little over a course of six to 12 hours.

The astonishing radar loop of the event shows intense thunderstorms blossoming west and north of Houston before sinking over the metropolis, stalling and unloading copious rainfall:

The quantity of moisture drawn into the front was made possible by a giant area of high pressure over the eastern United States and the clockwise flow of air around it. This huge high pressure center, part of a continental-scale blocking pattern, directed air from the Gulf of Mexico smack into south Texas.

A second radar view, which shows the accumulation of rainfall over time, illustrates the incredible influx of moisture from the south:

The threat of severe flooding diminished some in the Houston area late Monday afternoon as the heaviest rain sagged southeastward.

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Still, additional rounds of showers and storms are possible over the next several days which could cause flooding to resume.

“Forecaster confidence is moderate to high on the re-occurrence of periods of high rainfall exacerbating flooding concerns,” the National Weather Service wrote it in its Monday evening discussion. “It will not take much more than 1 to 2 inch per hour rates to again create urban sheet (street) flooding and lift local area rivers, creeks, and bayous up above low/moderate flood levels.”

A flash flood watch is in effect for the region through Tuesday morning and may require extension.