(HOUSTON) — At least four people were killed when an “exceptionally” strong storm tore through Houston on Thursday night, according to Mayor John Whitmire.

Fallen trees appeared to cause at least two of the deaths, according to officials.

The winds — which reached 78 mph — sounded like a freight train, Houston residents told ABC News.

The winds were so powerful, Harris County Judge Lina Hidalgo said they were comparable to Hurricane Alicia in 1983.

Houston schools are closed on Friday and more than 770,000 customers lost power across the state.

For some, the power could be out for weeks, Hidalgo said.

Whitmire urged residents to stay home Friday, noting 2,500 traffic lights are not functioning.

The mayor said Houston is in “recovery mode.”

“Please … stay away from downtown — it’s dangerous. There’s broken glass in every direction,” Whitmire said.

The intense winds came after a rare “high risk” warning for flash flooding was issued in Texas and Louisiana, with the states bracing for up to 9 inches of rain in 24 hours.

“The high risk area has seen over 600% of their normal rainfall for the past two weeks alone,” the National Oceanic and Atmospheric Administration warned, and the flash flooding could be life-threatening.

“High risk” days account for just 4% of days, but they are responsible for more than one-third of flooding deaths, according to the Weather Prediction Center.

The storm in Houston is now over, allowing residents to begin to cleaning up on Friday.

The severe weather threat has now moved east, with damaging winds and large hail possible from Louisiana to Georgia.

ABC News’ Mireya Villarreal and Daniel Amarante contributed to this report.

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