John Bacon, Rick Jervis and Michelle Homer

USA TODAY NETWORK

HOUSTON — Helicopters plucked desperate flood victims from rooftops Sunday while boats and trucks swept hundreds more residents to safety as Tropical Storm Harvey fueled historic rains and devastating flooding across a wide swath of East Texas.

Late Sunday night,Texas Gov. Greg Abbott said another 1,000 National Guard members will be sent to Houston on Monday, joining 3,000 already activated.

Flooded highways and streets left cars abandoned as weather alerts went off continuously, warning of possible tornadoes — all under a steady, pelting rain.

The unrelenting rain was forecast well into the week, and the Texas Gulf Coast braced for days of catastrophic flooding. The National Weather Service said some areas could be slammed with an "unprecedented" 50 inches of rain by week's end as the storm lingers in the region.

"This event is unprecedented & all impacts are unknown & beyond anything experienced," NWS tweeted. "Follow orders from officials to ensure safety."

Abbott activated the National Guard troops in addition to hundreds of other state emergency personnel aiding local first responders. He said 600 boats were aiding rescue efforts, and the Coast Guard said at least 16 helicopters were tapped for air rescues, with more coming into the area by Monday.

Tropical Storm Harvey: What we know

Convoys of buses and a mobile hospital unit were on the way to Houston and the Gulf Coast, as were truckloads of food and volunteers, Abbott said.

“They now know the cavalry is coming,” the governor said, adding: "Our top priority is to protect human life."

From Louisiana, the so-called "Cajun Navy," a group of volunteers with boats, were mobilizing to help with rescues. And from New Jersey, Gov. Chris Christie announced he was sending NJ Task Force 1 — a team comprised mainly of police, fire and emergency personnel — as members of the National Urban Search & Rescue Response System to arrived in Texas on Monday

More:Harvey threatens to make natural disaster history

More:Desperate for help, flood victims in Houston turn to Twitter for rescue

Flooding was overwhelming the Houston metropolitan area. Scenes of families being shuttled to safety played out in scores of neighborhoods. The Coast Guard said it rescued more than 100 people from rooftops and conducted more than 2,000 multi-person rescues, its three-boat teams searching block-by-block for stranded residents.

"If you are in a flooding situation, stay calm, do not panic," the Coast Guard said in a statement. "Do not go into the attic, rescuers from the air cannot see you."

The storm had claimed at least two lives, but it was to soon to know the full extent of the death and destruction as power and cellphone outages made communications difficult.

"The flooding in and around America's 4th most-populous city is going to write world headlines and set records for generations," tweeted meteorologist Roger Edwards of the National Weather Service Storm Prediction Center.

More:'Catastrophic' Houston flooding leads to at least 500 overnight rescues

More:Federal government plans years-long recovery effort in states hit by Harvey

President Trump tweeted support for the agencies battling the disaster and planned to visit the state Tuesday.

“We are coordinating (visit) logistics with state and local officials," the White House said in a statement. "We continue to keep all of those affected in our thoughts and prayers.”

Under persistent, pounding rains, some residents in Richmond, 20 miles south of Houston, took refuge in a Red Cross shelter inside a Catholic Church recreation center.

Austin Herrera, 18, said the water on his family's 10-acre property in nearby Guy, Texas, jumped 1½ feet overnight, as water moccasins slithered under the house. He ushered the horses and other animals to higher ground, then drove the six members of his family to a nearby church who then bused them to the shelter.

"We've seen flooding before," Herrera said. "But never like this."

As of Sunday, the Red Cross shelter in Richmond, Texas, had 49 displaced residents. More could soon be on the way, since Richmond and neighboring Rosenberg sit on the banks of the Brazos River, which experienced catastrophic flooding just last year.

This event could match or surpass that, meteorologists said.

Some residents here were being bused in from another shelter at the Chinese Community Center in Houston, which had already filled to capacity, said Christine Bradley, the shelter manager.

"This is a whole new thing," she said. "It's huge."

More:Exclusive: Get an aerial look at Aransas County after Harvey

Over the next few days, Harvey also is expected to produce total rain accumulations of 5 to 15 inches farther south toward the lower Texas coast, farther west toward the Texas Hill Country, and farther east through southwest and central Louisiana, the weather service said.

But Houston was the focal point of the disaster. The National Weather Service said parts of Harris County had been hit with more than 20 inches of rain in 24 hours. Jeff Lindner, a meteorologist with the county Flood Control District, called the rainfall totals "staggering."

He tweeted late Sunday night that Buffalo Bayou was more than 7 feet above flood stage: "Catastrophic flooding is in progress."

This is “worse than the worst-case scenario for Houston,” tweeted WeatherBell meteorologist Ryan Maue.

Mayor Sylvester Turner said emergency officials had been overwhelmed with about 6,000 rescue calls and more than 56,000 calls to 911; he urged residents not to call unless their situation was life-threatening. He ordered the city's George R. Brown convention center opened as a shelter.

Turner confirmed one death in Houston, saying a woman drowned trying to flee her car in high water. Another death was reported in a house fire in coastal Aransas County.

Turner defended his administration's decision not to call for evacuations ahead of the storm, saying it was too difficult to determine which areas of the sprawling city of 2.3 million people were likely to take the worst hit. The entire city has seen at least some flooding, he added.

“You give an order to evacuate, you are creating a nightmare," he said.

Officials were urging people to stay off of the roads.

"It's so dangerous that people would give themselves the death penalty," said Houston Police Chief Art Acevedo, who was out with his officers making water rescues in his saturated city Sunday.

"Sad, breaks your heart for our city and our state," Acevedo said. "But it's Texas. We'll get through it."

The floodwaters themselves are a hideous, toxic brew, with reports of alligators and swarming fire ants in the water.

The storm made landfall Friday night in Aransas County, southwest of Houston, as a furious, Category 4 hurricane with winds exceeding 130 mph. Sheriff Bill Mills said about 30 people were treated for injuries in his county alone.

Two Aransas County municipalities, Rockport, with a population of 10,000, and Port Aransas, with about 4,000 people, took the brunt of the storm. At least 10 injuries were reported from collapsed roofs in Rockport, which is 25 miles northeast of Corpus Christi and 220 miles southwest of Houston.

More:Corpus Christi breathes sigh of relief, prepares for post-Harvey cleanup

More:Texas homeowner shoots intruder as Harvey makes landfall

Nearby Port Aransas was particularly vulnerable perched on a narrow strip of Mustang Island, which sits at the entrance to Corpus Christi Bay. It registered the strongest wind gust of 132 mph from Harvey, according to the National Weather Service.

The two towns, like dozens over others in the area, reported widespread damage as emergency teams searched for any survivors trapped in low-lying areas or collapsed buildings.

Meteorologists were awed by the scope of the disaster.

“This could easily be one of the worst flooding disasters in U.S. history,” tweeted Weather Channel meteorologist Greg Postel, who said he cannot think of an analogous flood event.

Homer reports for KHOU-TV in Texas; Bacon for USA TODAY in McLean, Va. Contributing: Doyle Rice, Rick Jervis, USA TODAY; Julie Garcia, John C. Moritz, Corpus Christi Caller Times