Heavy rain inundated the Houston area with as much as 10 inches of rainfall in just six hours and triggered flash flooding early Monday.
Jeff Lindner, a meteorologist with the Harris County Flood Control District, said that "significant widespread flooding" was reported northwest of Houston. He added that a total of between 12 and 16 inches of rainfall was expected in the area by daybreak.
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In a tweet, he warned of a "life-threatening situation" due to flash flooding developing in northwest Harris County. "Stay where you are," Lindner added.
Authorities confirmed early Monday that 5-10 inches of rainfall dropped in six hours in Harris County and neighboring Waller County.
Rosie Torres, a spokeswoman for the Harris County Office of Homeland Security and Emergency Management, said high water rescues had occurred but the number was not immediately clear.
"The situation is pretty serious," Torres added.
Torres' agency advised residents to "stay home and off the roads."
In a statement, it added: "If your home or apartment takes in water, DO NOT LEAVE. You are safer inside your home. Straying into deep or fast moving flood waters means certain death."
Michael Walter, a spokesman for the Houston Office of Emergency Management, said early Monday that "tremendous rainfall" had been reported over western parts of the city.
Videos and images posted online showed cars stranded in flooding and hail in Houston.
The Weather Channel warned that "heavy rain and thunderstorms remain a threat from eastern Texas northward to the Dakotas," adding that "scattered severe thunderstorms may develop in southern Texas through Monday night" with "damaging wind gusts, hail and isolated tornadoes" all being possible.