Swollen rivers in east Texas aren't expected to crest until later this week, but federal officials are already predicting Harvey will drive 30,000 people into shelters and spur 450,000 to seek some sort of disaster assistance.
Several locales have already received 2 feet or more of rain, and forecasters say a reprieve won't arrive till week's end at the earliest.
"This is a landmark event for Texas," said FEMA Administrator Brock Long. "Texas has never seen an event like this."
But, Long warned, Harvey presents a dynamic situation, and "every number we put out right now is going to change in 30 minutes."
Harvey will likely surpass 2008's Hurricane Ike and 2001's Tropical Storm Allison, two of the most destructive storms to hit the Gulf coast in recent memory.
Around 13 million people are under flood watches and warnings stretching from Corpus Christi to New Orleans as the remnants of Hurricane Harvey menace the already drenched Texas and Louisiana.
"Catastrophic and life-threatening flooding" continues in southeastern Texas, where bands of storm have been repeatedly pummeling the same areas.
Over the next few days, Tropical Storm Harvey is forecast to head back into the Gulf of Mexico, where it will pick up moisture before moving back over Galveston and into Houston again, CNN meteorologist Karen Maginnis says, meaning at least four more days of rain.
The National Weather Service (NWS) warns that flash flood emergencies are in effect for some areas and the rain -- which can be measured by feet rather than inches -- is not letting up.
Up to 25 inches of rain could fall through Friday over the upper Texas coast, while "isolated storm totals may reach 50 inches over the upper Texas coast, including the Houston Galveston metropolitan area," according to the weather service.
-- Over two days, Houston got 25 inches of rain -- more than half of its annual rainfall. That number could double this week.
-- 11 p.m. curfews were introduced Sunday in the City of Alvin, City of Friendswood, League City, City of Pasadena, City of Pearland, City of Seabrook and City of Webster.
-- A mandatory evacuation order was issued for areas along the Brazos River in Fort Bend County.
-- Several states and the US military are sending emergency workers and equipment to Texas.
-- Dallas is opening a "mega-shelter" capable of accommodating 5,000 evacuees at the Kay Bailey Hutchison Convention Center by Tuesday morning.
-- The Houston Independent School District has canceled school for the week.
-- President Donald Trump will travel to Texas on Tuesday, press secretary Sarah Huckabee Sanders said.
-- More than 300,000 customers have lost electricity across the state.
Flooding continued in and around Houston on Sunday night as citizens with boats assisted authorities in search and rescue efforts on roads that have turned into rivers.
A CNN crew was with one such volunteer when he used his vessel to rescue an elderly couple, their daughter and two dogs in Dickinson, Texas.
The family became trapped upstairs when floodwaters came through the garage, filling the house with about three feet of water.
"It was shocking," Pam Jones said. "It just creeped up."
Houston resident Louise Walker told CNN she was trapped in her neighbor's apartment.
"Our bottom level is waist-deep in water. We have helicopters that are flying over us rescuing people, we have people who are living in these first-floor apartments like I have. They have been breaking into empty second level apartments just to have somewhere to go because we can't get out. We're simply completely surrounded with water," she said, adding that the helicopters were only rescuing people who were in immediate danger.
"My only plan at this point is to stay out of the water. I've been keeping in contact with family and friends, but other than that we can't do anything. We are literally stuck here," Walker said.
Officials say they have conducted more than 250 water rescues and rescued more than 1,000 people. The US Coast Guard's Captain Kevin Odditt says 19 Coast Guard helicopters are also involved in relief efforts.
As of Sunday evening, between 800 and 1,200 people had been rescued from their homes in Galveston County, County Judge Mark Henry said.
An additional 1,000 National Guard members are being called in to help flood victims in Houston, Texas Gov. Greg Abbott announced via Twitter Sunday evening.
The governor is expected to tour the Texas Coastal Bend region Monday with Senator John Cornyn.
The US Army Corps of Engineers began the controlled release of water from the Addicks and Barker Dams in Galveston early Monday, after water levels began rising quickly, according to Harris County Flood Control District Meteorologist Jeff Linder.
This is ahead of schedule because the water levels at the dam began to rise quickly, Linder says. The engineers are expecting spillways and roads in the area to be impacted, according to a release from the agency.
"Residents adjacent to the reservoirs need to be vigilant because the water in the reservoirs is rising rapidly," said Col. Lars Zetterstrom, Galveston District commander. "Both reservoirs are rising more than half a foot per hour."
The City of Conroe says record levels of water are also being released from Lake Conroe Dam and flooding is imminent in some areas.
"The City of Conroe will be evacuating McDade Estates. Other neighborhoods will be evacuated by the County," it said." Public Safety Officials have been overwhelmed by the number of calls and are currently prioritizing calls as they come in."
The rainfall threatens to exacerbate an already dangerous situation, as Harvey's rains have left many east Texas rivers and bayous swollen to their banks or beyond.
"The breadth and intensity of this rainfall are beyond anything experienced before," the NWS said. "Catastrophic flooding is now underway and expected to continue for days."
In Fort Bend County, a voluntary evacuation order was made mandatory for areas along the Brazos River, with the NWS predicting river levels of 56.1 feet -- nearly two feet above the record during flooding last year.
"Harvey continues to batter Fort Bend County," said County Judge Robert Hebert. "Residents who flooded last year know how serious this situation is."
Fort Bend had worked with the Red Cross to establish shelters for residents, Herbert said.
The storm killed two people in Texas, authorities said, and the death toll will likely rise. A woman who drove her vehicle into high water in Houston was killed, and fire killed a man in Rockport.
Houston Mayor Sylvester Turner warned that some 911 calls are going unanswered as operators "give preference to life-threatening calls."
The 911 dispatchers in the nation's fourth-most populous city have received 2,000 requests for rescue, Turner said. Houston Fire Chief Samuel Peña said his department had responded to 2,500 calls and have about 1,000 more waiting to be serviced.
People are also taking to social media to announce their locations and ask for help.
Many roads impassable
In a Sunday news conference, Gov. Abbott said, "We want to stress when there is heavy rainfall and flooding, the importance of staying off the road. If you drive into water, you're taking your life into your own hands."
Portions of major highways, including Interstates 10, 45 and 610, were submerged and unnavigable. Houston resident Dion Laurent said the White Oak Bayou flooded I-10 and I-45.
Harris County Sheriff Ed Gonzalez said he understands the compulsion to find safer ground, but urged people to think twice before venturing out into high water and to consider unforeseen dangers, such as manhole covers being lifted from their holes.
In Brazoria County, south of Houston, officials set up an evacuation route for all residents living west of State Highway 288 and south of State Highway 6, ordering them to "LEAVE NOW!" under a mandatory evacuation order. Those in need of shelter can take refuge in the Bell County Expo Center in Belton, officials said.
Louisiana in Harvey's sights
Meantime, Louisiana Gov John Bel Edwards wrote to President Trump requesting that he declare an Emergency Disaster for the state.
"The National Weather Service forecasts that remnants of Hurricane Harvey will bring heavy rainfall to Louisiana posing serious danger to life and property of the citizens of our state. Significant lifesaving efforts such as search and rescue, transportation to shelters, logistical support, and shelter operations are particularly needed in Beauregard, Calcasieu, Cameron, Jefferson Davis, and Vermillion parishes," Edwards wrote. The NWS predicted 10 to 20 inches of rain between Sunday night and Monday, he said, with "major river flooding" anticipated.
Harvey blasted ashore as a Category 4 hurricane on Friday night, making landfall just north of Corpus Christi before quickly being downgraded to a tropical storm.
"What is unique in Harvey is that as the storm moved inland, a large high pressure built in to the north and, basically, the steering currents, which guide Harvey, collapsed," said CNN senior meterologist Dave Hennen. "This has caused the extremely slow movement of the storm, moving only around 60 miles, less than 2 miles per hour. This has allowed the bands of storms to move over the same areas over and over."