HOUSTON (AP) — Power was expected to be restored by late Wednesday to nearly all Houston area homes and businesses left in the dark by last week’s deadly storm, according to officials.

But possibly up to 20,000 customers might have to wait until the weekend before their lights come back on.

Around 98% of residents and businesses were set to have their power fully restored by the end of Wednesday, Brad Tutunjian, vice president of regulatory policy with CenterPoint Energy, told members of the Houston City Council during their weekly meeting.

At the height of the power outage, around 922,000 customers were without service. By Wednesday evening, that was down to about 42,000.

“We are working around the clock to try to get them on as quickly and safely as possible. I would also like to recognize the patience of our customers,” Tutunjian said.

The May 16 storm left at least eight dead, caused widespread destruction and brought much of Houston to a standstill. Thunderstorms and hurricane-force winds tore through the city, reducing businesses and other structures to piles of debris, uprooting trees and shattering glass from downtown skyscrapers. The storm spawned two tornadoes — one near the northwest Houston suburb of Cypress and the other in Waller County, located northwest of Houston.

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The National Weather Service has determined that the storm was a weather event known as a derecho, said meteorologist Jeff Evans. A derecho is a widespread, long-lived windstorm that’s associated with a band of rapidly moving showers or thunderstorms, according to the weather service.

The derecho had hurricane force winds across a 5-mile (8-kilometer) to 8-mile (13-kilometer) swath through the Houston area, with some pockets of winds reaching up to 100 mph (160 kph), Evans said. The straight-line winds from a derecho can be more damaging than a tornado because they can last for several minutes compared to the instant hit of a tornado, Evans said.

Tutunjian said the 2% of customers — about 20,000 — likely to still be without power after Wednesday live in areas with “significant damage” to equipment.

Others need transformers or fuses in their neighborhoods repaired and that can take time, Tutunjian said. And other customers will have to repair damaged electrical equipment they are responsible for in their homes before they can get power back.

“The 98 (percent) is impressive, unless you’re one of the 20,000,” said Mayor John Whitmire, adding CenterPoint had done a good job overall in restoring power.

Tutunjian said he expects crews will continue working through the weekend.

The Houston area was expected to have potentially dangerous heat this weekend, with temperatures and humidity levels higher than normal for May, according to the National Weather Service.

The deadly storm took place as the Houston area and several Texas counties to the north were still recovering from flooding caused by heavy rainfall in late April and early May.

Houston City Council Member Abbie Kamin said last week’s storm should prompt discussion about fortifying power lines and other infrastructure in Houston.

The city of more than 2 million people is one of the most flood-prone metro areas in the country and has long experience dealing with devastating weather, including hurricanes.

Tutunjian said CenterPoint has worked to make its system more resilient, but more study is needed.

“We have some tough questions that we have to talk through. And again, we’re not even into hurricane season yet and we still have thousands of customers without power right now,” Kamin said.

Hurricane season starts June 1.

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