Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

HOUSTON — Heavy rain returned with a fury Monday as up to 20 more inches was predicted to deluge the region following a weekend of widespread flooding and more than 2,000 rescues in the wake of Hurricane Harvey.

A 60-year-old woman in east Montgomery County, north of Houston, died when a tree fell on her while she slept, the sheriff's office said Monday, raising the confirmed number of deaths to three as torrents of water continued to cripple the region.

The catastrophic storm is expected to send more than 30,000 people into temporary shelters and prompt about 450,000 people to file for disaster aid, the Federal Emergency Management Agency said Monday.

"This is a landmark event for Texas," Brock Long, FEMA's administrator, told reporters. "Texas has never seen an event like this."

The Latest on the Storm

Harvey, now a tropical storm with winds of about 30 to 40 mph, is about 25 miles off the middle of the Texas coast and is slowly moving back toward coastal waters. It is expected to remain offshore through Tuesday before resurging in the Gulf of Mexico and turning back north toward southeast Texas on Wednesday.

Bands of heavy rain are expected to persist over the next several days, with parts of Texas and Louisiana facing record rainfall through at least Labor Day weekend, forecasters say.

Flash flood warnings remain in effect for parts of the Houston region through Tuesday morning, and more than 290,000 customers are without power.

The National Weather Service warns conditions are "somewhat favorable" for tornadoes.

Long added that the situation remains a rescue operation, with 30 to 50 counties in Texas potentially affected. The Department of Homeland Security is sending personnel to the state to help bolster security forces, while Gov. Greg Abbott announced Monday that the entire Texas National Guard — about 12,000 members — would be deployed. Already, 3,000 of them had been mobilized to help with rescue and recovery.

"Hurricane force winds have diminished, but we are still not out of the woods," said Elaine Duke, the acting homeland security secretary.

After daybreak, abandoned cars could be seen littering roadways. Drivers tried to leave Houston on Interstate 45, which cuts through the city and stretches south toward the Gulf of Mexico. The highway was still passable, although some roads were completely swamped by flooding and the tops of traffic lights peeked out from above the water.

The Army Corps of Engineers announced early Monday that it had begun releasing water from both major Houston-area dams — something that has never been done before. That decision came after levels in the Barker and Addicks reservoirs "increased dramatically," by more than 6 inches per hour. Earlier Sunday, officials had said they didn't plan to open the Barker dam until 11 a.m. Wednesday.

Related: Hurricane Harvey: Life-Threatening Rain Hammers Houston

While opening the two reservoirs will increase water levels and enhance flooding for those along Houston's Buffalo Bayou watershed, it's a better alternative than allowing the water to spill over and around the reservoirs into additional neighborhoods, Mayor Sylvester Turner told reporters Monday.

He added that the water would continue to be released gradually through noon — the equivalent of 8,000 cubic feet per second — and that the Army Corps is monitoring in real time how much water levels are rising.

As much as 40 inches of rain had fallen on areas near Houston, the country's fourth-largest city, in 24 hours as of Sunday night, the NWS said. And 15 to 25 more inches of rain could fall on a swath of the upper Texas coast to Lafayette, Louisiana, by Friday.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

The National Oceanic and Atmospheric Administration said the wettest tropical storm in U.S. history, Hurricane Hiki, yielded more than 52 inches of rain on Hawaii in 1950.

Rivers, meanwhile, are expected to crest later this week, exacerbating the dire situation.

"Results could be devastating if any of these rains fall where catastrophic flooding has already occurred," the National Weather Service warned.

National Weather Service

More than 290,000 customers in the region were without power at 5:30 a.m. ET Monday, utility companies said.

Dallas was preparing to open its "Mega Shelter" in the Kay Bailey Hutchison Convention Center to handle a stream of people displaced from Houston and other coastal areas. People were already huddled outside on Monday wearing American Red Cross blankets. The city plans to have the convention center ready to accommodate 5,000 people by Tuesday morning, NBC Dallas-Fort Worth reported.

Harvey weakened to a tropical storm since making landfall in Rockport, Texas, on Friday, but it still posed a danger of devastating floods. Around 250 roads and highways in the state have been closed by floodwaters, Abbott said.

"People need to know that there are large and growing rescue teams that will be working around the clock to evacuate people in need," Abbott said on MSNBC.

Forecasters warned that the flooding in Texas could be historic.

The National Weather Service said average rainfall amounts in Harris County had already eclipsed the amounts seen in the devastating Tropical Storm Allison in 2001 — in about half the time.

A Houston woman whose home flooded early Sunday said the water began to build at 3 a.m. and didn't stop. "I was thinking of writing my Social Security number on my arm," she told MSNBC.

Thousands of calls for help came in to first responders, and Turner, the mayor, pleaded with people to call 911 only in life-threatening emergencies.

Houston police issued a call for boat owners to volunteer Sunday night.

Private boat owners coming from all over to help evacuate people from homes around Houston. This man has been going all day. #HoustonFlood pic.twitter.com/mofHwfdvaH — Aaron Franco (@AFrancoTX) August 27, 2017

The U.S. Coast Guard sent assets from as far away as Maine and California to help, Vice Adm. Karl Schultz said. Helicopters and numerous boat crews were being used to assist in rescues, he said.

People were rescued from "pretty much everywhere," said Cmdr. Jim Spitler, commanding officer of Air Station Houston. "Most of them are rooftops, but they've been on top of cars, they’ve been on bridges, they’ve been in their attics."

He repeated warnings from authorities not to retreat to attics amid rising water, because people can be trapped and it makes rescue difficult, and he encouraged people who need help to mark their roofs.

Houston Police Chief Art Acevedo said the department has conducted about 2,000 rescue missions and has about 185 critical requests left. With a lull in the rain earlier Monday, he told MSNBC that crews will also focus on homes that are flooded to the rooftops.

The National Hurricane Center said the Houston area could see a total of 50 inches of rain. The storm was causing "unprecedented flooding" in southeastern Texas, it added.

"This is a life-threatening situation," said Michael Palmer, lead meteorologist for The Weather Channel.

Related: Hurricane Harvey: How to Help Storm Victims

As of early Monday, Harvey was about 25 miles northeast of Port O'Connor, located on the middle Texas coast, with maximum sustained winds of 40 mph, the hurricane center said. The center of the storm was moving at about 3 mph off the middle Texas coast, and was set to meander just offshore through Tuesday before heading back north toward land.

President Donald Trump will travel to Texas on Tuesday. He signed a disaster declaration to help get federal funds to stricken areas. Abbott said Sunday that he expanded an initial request to 18 counties, including Harris County, where Houston is located, and that it had been approved.

HISTORIC rainfall in Houston, and all over Texas. Floods are unprecedented, and more rain coming. Spirit of the people is incredible.Thanks! — Donald J. Trump (@realDonaldTrump) August 27, 2017

Abbott praised the White House response and said he had spoken with Trump personally.

Long said Sunday that the recovery effort would take years.

Lisa Rehr holds her son Maximus as they awaited evacuation from Rockport, Texas, on Saturday. Hurricane Harvey destroyed their home. Adrees Latif / Reuters

"This is a long game. It's going to take a long time to heal," Abbott added. He called Harvey "a horrible tragedy."

Harvey caused damage along the Texas Gulf coast and inland, and the city of Rockport north of Corpus Christi was hit hard, officials said. Port Aransas was reportedly without power, water and telephone service, and every business in town was damaged.

Flood watches from San Antonio to Lafayette, Louisiana, covered 13 million people and were expected to be in place through midweek.

Federal officials said they would be planning for recovery efforts later in the week.

When Hurricane Katrina hit in 2005, about 30,000 people unable to flee New Orleans were sheltered in the Louisiana Superdome. Nearly 74,000 families that were displaced were provided temporary housing through FEMA.

Katrina caused an estimated $81 billion in property damages and required FEMA to help more than 738,000 individuals and households.

Phil McCausland, Stephanie Gosk and Aaron Franco reported from Houston. Phil Helsel reported from Los Angeles. Erin Dean and Erik Ortiz reported from New York. Jason Cumming reported from London.