HOUSTON — More than 30,000 people will need shelters as a result of the unrelenting rains and flooding of Tropical Storm Harvey overwhelming Texas, FEMA Administrator Brock Long said Monday.

"The sheltering mission is going to be a very heavy lift," Long said in a news conference in Washington, adding that up to 50 Texas counties are dealing with the impact of the storm.

Louis Uccellini, director of the National Weather Service, said some areas of southeast Texas around Houston have already seen more than 30 inches of rain. A wide swath of the region has been hit with 15-20 inches of rain, he said.

"We are seeing catastrophic flooding, and this area is likely to expand," Uccellini said.

Harvey, now spinning near Port O'Connor, Texas, is forecast to move back into the Gulf of Mexico today, the National Hurricane Center said. It will meander over the Gulf for a couple of days before making a second landfall somewhere near the Texas/Louisiana border, likely on Wednesday.

Harvey is then expected to slowly move northeast across Louisiana and Arkansas as a tropical depression from Thursday into Saturday.

As it spins offshore, the storm is expected to dump an additional 15 to 25 inches of rain through Friday over the upper Texas coast and into southwestern Louisiana, exacerbating the life-threatening, catastrophic flooding in the Houston area, the hurricane center said. Isolated storm totals may reach 50 inches over the upper Texas coast, including the Houston/Galveston metropolitan area.

"Reliable weather forecasts still show taking whatever rain has already fallen around Houston and doubling it over the next 4-5 days," said WeatherBell meteorologist Ryan Maue.

Residents of this embattled city spent the night in dark, flooded homes, continued rescuing neighbors throughout the night and braced for days of heavy rains.

From Katy to Dickinson to downtown Houston, the waters continued rising to new heights and plunged the greater Houston area and its nearly 6 million residents into uncharted chaos. The pounding rains were remnants of Harvey, which roared ashore last week as a Category 4 hurricane and has lingered as a tropical storm. They were forecast to continue through the next few days.

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In an unprecedented move, the U.S. Army Corp of Engineers early Monday began releasing water from the overfilled Addicks and Barker reservoirs in west Houston. The release was necessary, officials said, to avoid a collapse of the reservoirs’ dam and inundate downtown Houston but puts several thousand homes in the area at further flood risk.

“The idea is to prepare ... pack up what you need and put it in your vehicle and when the sun comes up, get out,” Jeff Lindner, of the Harris County Flood Control District, said Sunday. “And you don’t have to go far, you just need to get out of this area.”

More than 1,000 rescues had been conducted since the floods picked up early Sunday. Throughout Houston, drivers abandoned cars overtaken by flood-swollen streets and emergency alerts on radios and cell phones continuously warned of possible tornado activity – all as rain continuing pelting the city in steady sheets.

Besides steadily rising water, residents in the greater Houston area on Sunday contended with the potential of tornados ripping homes apart.

Early Monday, Tammy Elizondo was in her house with her family in Richmond, 30 miles southwest of Houston, when the wind suddenly revved up and the ceiling began to buckle.

“It was crazy, very loud,” she said in an interview with the Canadian Broadcasting Corporation. “Lots of cracking and popping and the roof was trying to suck up into it … As soon as I heard the roof starting to go, I started yelling, ‘Go to the bathroom! Go to the bathroom!’”

Minutes later, when it died down, the family looked outside to see six rows of fences destroyed and a large chunk of their neighbor’s roof gone, Elizondo said.

They are deciding whether or not to evacuate but that might also be challenging.

“The street looks like a river,” she said.

The deluge from Harvey was so intense that authorities were urging residents to seek refuge on roofs as emergency crews struggled to make their way through the city by land, water and air amid desperate pleas for help.

Interstate 610, a freeway forming a 38-mile long loop around downtown Houston, was engulfed in floodwaters that were creeping closer to overhead highway signs — another sign of how dire the situation was.

Houston Mayor Sylvester Turner said authorities had received more than 2,000 calls for help and would be opening the city’s main convention center as a shelter.

"I don’t need to tell anyone this is a very, very serious and unprecedented storm,” Turner said at a news conference. “We have several hundred structural flooding reports. We expect that number to rise pretty dramatically.”

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Residents were being told to stay on roofs instead of climbing into attics — and to wave towels or sheets to flag down rescuers.

Harris County sheriff’s spokesman Jason Spencer said flooding throughout the county that includes Houston was so widespread that it’s “difficult to pinpoint the worst area.”

Spencer said the department has high-water vehicles and airboats but “certainly not enough.” He says authorities are prioritizing hundreds of phones calls for help to ensure life-and-death situations were at the top of the list.

The situation was “heartbreaking,” he said.

Contributing: Susan Miller