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Fox News Weather Center

Downpours to aggravate Texas flooding as storm crawls across US

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A storm that unleashed deadly flooding across central and eastern Texas on Monday continues to crawl across the central United States, threatening additional rounds of rain for the flood-weary region into midweek.

It will take until Friday before enough dry air punches in from the northwest to end the rounds of downpours. By Friday, the large storm will have drifted into the eastern part of the nation, in a weakened state.

"The worst of the rain is already over," according to AccuWeather Senior Meteorologist Henry Margusity. "However, there will still be occasional downpours that can aggravate existing flooding at the local level into Thursday night."

Motorists and residents will need to remain vigilant over the next few days as less-intense storms could cause new rapid flooding incidents along streets, highways and other low-lying areas.

Additional rounds of showers and thunderstorms will form nearby or travel across portions of Texas hit hard by torrential rainfall to start the week.

Due to saturated ground and streams, rivers and bayous filled with water, it will take a smaller amount of rain to cause new flooding.

The period from Friday into Sunday will be mainly rain free in hard-hit flooding areas of central and eastern Texas.

Showers and thunderstorms will return to much of the Plains Sunday night into the middle of next week. Along with the return of the downpours will come the chance of localized flooding and severe weather.

"Overall the pattern looks to remain very active in terms of downpours and severe weather for the Central states and other areas in the coming weeks," Margusity said.

The heavy rainfall from Sunday into Monday almost immediately resulted in flooding of small streams and bayous. During the next few days, water levels will rise to or remain at moderate to major flood levels along some of the large rivers in the region. These rivers include the San Jacinto, San Bernard, Trinity and Colorado.

A large complex of thunderstorms with intense rainfall formed over north-central Texas Sunday night and drifted slowly eastward into Monday. The thunderstorm complex was a spinoff of the massive storm system drifting east of the Rockies.

"Had that huge complex not formed or been less intense, we would have not had the widespread flooding disaster," Margusity stated.

The complex produced two to four times the average rainfall for April in less than 24 hours in some locations.

During Monday, at Houston Intercontinental Airport, Texas, slightly more than 10 inches of rain fell in about 12 hours. This amount occurs, on average, about once every 100 years, according to the Rainfall Frequency Atlas of the United States.

Rainfall rates of 2-4 inches per hour occurred in some communities, which was greater than the drainage system could handle.

In Harris County, Texas, alone, more than 1,200 high-water rescues were performed during the event.

It was during last May when portions of northern Texas, northern Louisiana, Oklahoma and Arkansas received one to five times their monthly rainfall and experienced major flooding. That rainfall occurred in several rounds which spanned one to three weeks.

In the spring and into the summer there is always the risk of a large complex of storms forming and producing training effect rainfall. The majority of these complexes are neither as large, nor as intense as the one that hit parts of Texas to start the week.

Motorists are advised to never attempt to drive through flooded roadways. The water is likely to be much deeper than it appears and the road may have been compromised beneath the water. A foot or two of water is enough to cause the vehicle to temporarily float, drift into deeper water and sink.

Attempting to drive through flooded areas not only puts the driver and occupants of the vehicle at risk, but also the would-be rescuers.