Houston storm: At least 5 killed, more than 600,000 without power

At least five people were killed when a powerful wind storm tore through Houston on Thursday night, officials said.

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

The National Weather Service (NWS) in Houston said it determined this was a derecho event, based on the intensity and path length of the storm. A derecho is a powerful wind storm that produces straight-line winds that can cause major damage, and often accompanies rapidly moving showers or thunderstorms, according to the NWS.

Straight-line winds peaked at around 100 mph in downtown Houston, and residents told ABC News the winds sounded like a freight train.

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

One tornado -- rated EF-1, with peaks winds of 110 mph -- was confirmed near Cypress, the National Weather Service said.

Houston Mayor John Whitmire urged residents to stay home Friday following the "exceptionally" strong storm, noting 2,500 traffic lights are not functioning.

Houston schools closed on Friday and more than 600,000 customers remain without power.

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

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 White House on Friday declared that "a major disaster exists in the State of Texas" due to the storms and their aftermath, which makes federal aid available for recovery efforts. The statement also said President Biden had directed the Federal Emergency Management Agency (FEMA) "to coordinate Federal recovery operations in the affected areas."

White House press secretary Karine Jean-Pierre said earlier that FEMA is working with state and local counterparts and is "ready to provide federal assistance as needed," and that the White House is "praying for four people who tragically lost their lives in Houston" and "also thinking of those who were injured and the communities that were affected by this extreme weather."

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 flood watches in effect from Louisiana to the Florida Panhandle. The heaviest rain is expected Friday and this weekend in parts of southern Mississippi, Alabama and the western Florida Panhandle.

Jean-Pierre said Friday, "We continue to monitor the storm's path as it moves east, and more severe weather is likely across the Gulf Coast today. Residents in the affected area as well as those in the path of the storm should heed warnings from state and local officials."

ABC News' Mireya Villarreal, Justin Ryan Gomez and Daniel Amarante contributed to this report.