Swollen rivers in east Texas aren't expected to crest until later this week, but federal officials are already predicting Tropical Storm Harvey will drive 30,000 people into shelters and spur 450,000 victims to seek some sort of disaster assistance.
And yet, forecasters say, more rain is coming. Lots more.
Gov. Greg Abbott activated the entire Texas National Guard to respond to the storm. The total number of deployed is about 12,000, he said.
PHOTOS: Harvey strikes Texas
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. By then, rain totals could reach another 2 feet -- with isolated instances of 40 to 50 more inches -- along the upper Texas coast.
"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, he said. Around 13 million people from Corpus Christi to New Orleans are under flood watches and warnings as of Monday morning as Harvey's storm bands repeatedly pummel the same areas.
Early Monday, Harvey was just barely clinging to tropical storm status, but the danger is far from over. The storm is forecast to head back into the Gulf of Mexico and pick up additional moisture before sliding back over Galveston and Houston, cities it's already hammered.
-- Early tallies indicate at least 5,500 people have arrived at shelters in Houston and another 1,000 in Friendswood.
-- The average annual rainfall in Houston is 50 inches. The city has seen 25 inches of rain in two days. Another 25 could fall by Saturday.
-- Several cities, including Alvin, Friendswood, League City, Pasadena, Pearland, Seabrook and Webster, have issued 11 p.m. curfews.
-- A mandatory evacuation order was issued for areas along the Brazos River in Fort Bend County.
-- 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 for the district's 215,000 kids.
-- President Donald Trump will travel to Texas on Tuesday, press secretary Sarah Huckabee Sanders said. Gov. Greg Abbott, Sen. John Cornyn, R-Texas, and possibly Sen. Ted Cruz, R-Texas, are scheduled to tour the Coastal Bend region Monday.
-- Energy provider CenterPoint says 96% of its Houston customers have power, but more than 93,000 are without electricity as the company's crews experience difficulty reaching the affected areas.
-- The storm killed two people in Texas -- a woman in Houston and a man
Flooding continued in and around Houston on Sunday 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.
"It was shocking," daughter Pam Jones said of the floodwaters. "It just creeped up."
At Monday's press conference, Long encouraged more citizens to come forward, saying the rescue and recovery efforts would require community involvement. He said the National Voluntary Organizations Active in Disaster website would direct residents to religious and nongovernmental agencies through which residents can help out.
"It's going to require the citizens getting involved," Long said. "Donate your money. Figure out how you can get involved as we help Texas find a new normal."
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 US Army Corps of Engineers began the controlled release of water from the Addicks and Barker dams in west Houston early Monday, 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.
Harrowing tales from Houston
"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.
Stuck in the Texas floods? Here's what to do
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.
How social media is helping
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.
Keep track of Harvey
"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."