HOUSTON, United States (AFP) — Tropical Storm Harvey unleashed catastrophic flooding in Houston Sunday, turning streets in Texas’ largest city into raging rivers as trapped residents climbed to higher floors and the death toll rose.

Overwhelmed emergency services warned residents to head for high ground or climb onto rooftops — not into attics — so they could be seen by rescue helicopters.

“It is bad and growing worse,” said Texas Governor Greg Abbott, who said the storm had inflicted billions of dollars in damage in the Lone Star state.

At least two people have died since Harvey crashed ashore late Friday as a Category 4 hurricane, spawning tornadoes and lashing east and central Texas with torrential rains.

In Houston, a woman drowned when she left a car which had stalled in high water, local media reported, citing police.

Local officials said one person was killed when a house caught fire in the Rockport area, where Harvey made landfall with sustained winds of 130 miles (215 kilometers) per hour.

The National Weather Service said more than two feet of rain fell in Houston in a 24-hour period, and flooding was expected to worsen as the most powerful storm to hit the United States since 2005 lingers over the area.

Harvey slowly weakened as it advanced, but it had the power to rip off roofs, flip mobile homes and leave hundreds of thousands of people in the dark on the Gulf Coast, home to some of the country’s most important oil refineries.

More than a dozen tornado warnings were issued overnight for southeast Texas, including several in the Houston area.

In Houston, a city of 2.3 million, streets turned into fast-moving rivers with officials warning residents to stay home.

“If the highest floor of your home becomes dangerous … get on the roof!” the city’s emergency management agency warned in a bulletin.

Abbott said National Guard troops were deployed overnight in the city, using high-clearance vehicles to help with rescues in inundated areas.

Boats and helicopters also were being deployed throughout the area for swift-water rescues, he said.

Hobby International, one of Houston’s two airports, announced that all flights were canceled “due to standing water on runways,” while George Bush International was operating at limited capacity.

Houston officials said their 911 phone system was overwhelmed with emergency calls, mostly from stranded motorists.

“Cannot emphasize enough how much flooding there is on roadways you are endangering yourself and our first responders by being out,” Houston Police Chief Art Acevedo wrote on Twitter.

The governor visited a shelter for coastal evacuees in the state capital Austin and handed out food, describing the damage to homes and property as “sheer tragedy.”

“Some of them had their homes mowed down. Some of them will not have a place to return to… It is our job to make sure they will be taken care of,” he said.

The Federal Emergency Management Agency (FEMA) said there should be no illusions about the long-term impact.

“This is going to be an unprecedented long and frustrating event for the state of Texas,” FEMA director Brock Long told MSNBC.

“The recovery from this disaster is going to be years.”

Emergency services were struggling to make headway as the rains continued to pour down, although the Coast Guard managed to airlift at least 20 people and a dog to safety.

President Donald Trump, aware of the damage to George W. Bush’s presidency from his tardy response to Hurricane Katrina in 2005, said he was closely monitoring relief efforts from Camp David in Maryland.

“We are leaving nothing to chance. City, State and Federal Govs. working great together!” Trump tweeted after a teleconference meeting with his cabinet to discuss emergency operations.

He will visit Texas next week.

In Rockport, a local school and airport suffered major damage, while some homes burned to the ground as power cables caught fire and the storm prevented fire departments from responding. There were similar scenes in Houston and Corpus Christi.

“I’ve never seen anything like this. We do have strong winds — we’re right next to the bay — but nothing like last night,” Corpus Christi store owner Brandon Gonzalez told AFP.

Brian McNoldy, a hurricane expert at the University of Miami, said a strong ridge of high pressure was preventing the storm from dispersing.

“There’s no sign of it really moving in a foreseeable future,” he told AFP.

Coastal Texas is a fast-growing area, with some 1.5 million people moving into the region since 1999. It is also home to a large number of oil refineries and a number of major ports.

US authorities said about 22 percent of crude production in the Gulf of Mexico, accounting for more than 375,000 barrels a day, was shut down as of Friday.

But Abbott said the oil industry was well prepared.

“They hunkered down and were able to contain the facilities, and they have the ability to ratchet up back up there quickly,” he said on Fox News Sunday, predicting a “one- or two-week downturn.”

In Victoria, a town just north of Rockport, residents were shocked by the storm’s intensity.

“We didn’t know it was going to be a Category Four storm so we thought we’ll just ride it out,” local resident Robby Villa told AFP.

“If I knew it was going to be what it came to be, I might have left sooner,” he said.

Several locals, including Leslie Warner, worried about flooding.

“You know after the storm is done, from Austin and San Antonio, all that water is gonna come down here” on the San Antonio and Colorado rivers, she told AFP.