(Bloomberg) -- The worst windstorm in Houston in 40 years killed at least four people, left more than 800,000 customers without power and grounded hundreds of flights across Texas. 

Wind gusts reached at least 78 miles (125 kilometers) per hour through the Houston metro area Thursday. The fourth-most populous US city hasn’t seen damage like this since Hurricane Alicia in 1983, said Hayley Adams, a National Weather Service meteorologist. It was the worst straight-line wind event to hit Houston since May 1983 as well, she said. 

Utility CenterPoint Energy said restoration efforts in hardest-hit areas are expected to take several days or longer. Damage from the severe weather that resulted in electric service interruptions peaked at nearly 922,000 customers, the company said. 

Houston Mayor John Whitmire called on businesses to tell non-essential workers to stay home as recovery work continues. Schools were closed. San Jacinto College shuttered for the day.  More than a third of Harris County — the worst-hit area — was still without power early Friday, according to utility data. 

Getting power back on may take days and possibly weeks some places because transmission lines are down, the mayor said.

Houston has been hit by a series of devastating spring storms in recent years, with most of the damage coming from flooding. The city’s vast areas of pavement, combined with air that’s warmer because of climate change, have contributed to many of these events that have often clustered in the month of May. 

Along the Gulf Coast, from Texas to as far east as Mississippi, more than 904,075 customers were without power, according to PowerOutage.us. 

Texas was the hardest hit, with close to 800,000. 

In addition to the high winds across Houston, New Orleans reported a gust of 84 mph, said Peter Mullinax, a forecaster with the US Weather Prediction Center.

Flooding rains fell north of Houston, with a wide area getting between 3 to 4 inches (7.6 to 10 centimeters) — some isolated regions got much more.

“The Houston story was wind — the really heavy flash flooding was focused north of Houston,” Mullinax said.

Flights Canceled 

Across the US Thursday, 708 flights were canceled, with more than 530 of them in Texas, according to FlightAware, an airline tracking company. Flight disruptions eased on Friday. 

There were reports of ships being buffeted in the Houston Ship Channel by the winds, Mullinax said. 

Social media posts showed violent squalls and blown-out windows on skyscrapers in downtown Houston. The storm hit just as the grid again warned of the risk of electricity shortfalls as seasonal repair works cut the availability of power plants and hot weather lifts demand. 

Read More: Texas Power Grid Sees Several Days of Supply Strain Ahead

A line of storms developed in west Texas Thursday and a clash of dry air with heavy, moist air from the Gulf gave the system a boost, Mullinax said. This combined with an active sub-tropical jet stream to provide the mix to unleash the terrible windstorms. 

Harris County Judge Lina Hidalgo said 16 libraries are open and can be used for cooling centers through the weekend. The true extent of the damage, the fatalities and the power restoration timeline are still being assessed.

Flood watches and advisories were in place from eastern Texas to western Florida and Alabama early Friday, according to the National Weather Service. Waters near Houston, which is prone to surges, rose earlier this month, closing roads, affecting energy supplies and sending residents fleeing.

Houston is under a flood warning through Tuesday. Only three tornado reports came into officials, but that number may rise as the damage is assessed. Rain will continue around Houston and across the Gulf on Friday, as temperatures soar leaving those without power struggling to keep themselves cool. 

“It’s another busy day on the Gulf Coast,” Mullinax said. 

--With assistance from Brian Eckhouse, Naureen S. Malik and Rob Verdonck.

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