HOUSTON — More than 5,500 weary refugees of Tropical Storm Harvey's fury sought refuge in the city's cavernous convention center and other shelters Monday as local, state and federal officials warned that the human crisis in southeast Texas was just beginning.

Mayor Sylvester Turner said he expects the number of people seeking shelter to rise sharply by Tuesday. FEMA Administrator Brock Long said more than 30,000 people ultimately could need shelter as the storm continues to pound the region with unrelenting rains and flooding.

"The sheltering mission is going to be a very heavy lift," Long said. About50 Texas counties and parts of Louisiana will face serious repercussions from the "landmark event," he said.

Meantime, officials confirmed at least two deaths while KHOU-TV, citing interviews with relatives, said four children, ages 6 to 16, and their great-grandparents drowned in a white van Sunday afternoon while trying to flee from rising floodwaters. The driver, a relative, survived, the report said.

The bodies have not been recovered, a man who identified himself as the children's great-uncle told The Chronicle.

On Monday afternoon, it was still raining in Texas and expected to do so for days. Local officials put out a call for boats, and Long said people across the nation could help with financial donations or volunteer efforts.

"We need citizens to be involved," Long said. "You could not draw this forecast up, you could not dream this forecast up."

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Across much of the city, Harris county and southeast Texas, the waters rose to new heights Monday, plunging the greater Houston area and more than 6 million residents into uncharted chaos.

Evacuees described turmoil and uncertainty.

Surrounded by people taking a smoking break outside the George R. Brown Convention Center Monday night, Danielle Brown hugged her boyfriend, Lorenzo Harps. They arrived at the shelter Sunday night after being rescued by boat from a ground-level Houston-area apartment.

They awoke about 11 p.m. Saturday amid water a few inches deep.

“Everything was already flooded,” Brown said late Monday, huddled beneath a relatively dry overhang as rain poured down. “It was about to our feet, and it just kept rising. We had to move to the top floor, to a neighbor.”

Before moving up, they tried putting electronics, clothes and keepsakes on counters and shelves. The water climbed to 3 feet within hours, and rescuers evacuated them by boat the next day.

“They were strictly business,” Harps said of the rescuers’ efforts.

Now, the couple is stranded at the shelter. They shared a phone, which fell in the water during the chaos — something they can only laugh about now. Brown’s recently purchased Saturn sedan was destroyed.

The wrath of Harvey is Brown’s first brush with a tropical storm or hurricane. And she had one message to the untold numbers of residents experiencing a similar crisis as rescue efforts intensify.

“Be prepared. Be cautious,” she said. “Grab only what you need. Grab your life.”

City Police Chief Art Acevedo said officers had rescued more than 2,000 people trapped by the overwhelming waters; another 185 rescue requests were still pending. The city has grappled with 75,000 911 calls, and the system has backed up but never went down, authorities said.

The Coast Guard said it had conducted 300 air and 1,200 boat rescues and had 20 helicopters and more than 20 boat teams in the Houston area. Coast Guard Capt. Kevin Oditt called Harvey an "all-hands-on-deck event."

Gov. Greg Abbott activated the state's entire National Guard force, increasing to 12,000 the number of guardsmen deployed to flooded communities.

"Texas (officials) and FEMA will be involved here for a long, long time," Abbott said. "Until we can restore things as back to normal as possible. But we have to realize it will be a new normal for the region."

Some areas of southeast Texas have already seen more than 30 inches of rain, said Louis Uccellini, director of the National Weather Service. A wide swath of the region has been hit with 15-20 inches of rain, and that much more could be coming, he said.

In an unprecedented move, the U.S. Army Corp of Engineers 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 it put 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.”

With the floodwaters rising and no order from headquarters, Marty Lancton, a Houston firefighter with Station 51, took matters into his own hands. He connected with his immediate boss, Capt. Chris Sillman, and the two hopped into Sillman’s personal flat-bottom fishing boat and began searching for stranded residents.

All day Sunday, Lancton and Sillman motored around Brays Bayou in the western part of the city, ferrying residents from flooded homes to Interstate 610, where they were taken to shelters. The pair went back and forth, over and over again, taking more than 20 trips and rescuing close to 100 people.

The scene in the flooded neighborhood was a hodgepodge of firefighters on private boats, drivers in high-water vehicles, helicopters rescuing residents from rooftops and scores of private citizens motoring around in private boats, said Lancton, who is also president of the Houston Professional Firefighters Association.

“You’re talking about hundreds and hundreds of people, if not thousands, being rescued from helicopters and everybody and anybody who could get to them,” he said.

As of Monday, Houston firefighters still hadn’t been recalled – or officially called to service – so the firefighters on the bayous were acting largely on their own, he said. Better planning would have helped, Lancton said.

“Planning is a really good thing,” he said, “especially when you have these catastrophic emergencies like this.”

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Harvey, spinning near Port O'Connor, Texas, was forecast to move back into the Gulf of Mexico before making a second landfall along the Texas/Louisiana border, likely on Wednesday, the National Hurricane Center said. Isolated storm totals may reach 50 inches in some areas around Houston.

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

President Trump, who was scheduled to visit Texas on Tuesday, issued a federal disaster declaration Monday for many parishes in southwestern Louisiana. Gov. John Bel Edwards sought the declaration, saying he expects "significant damage" in his state.

In flood-gorged Houston, a break in the rain wasn't enough to stop the deliberate, alarming march of floodwaters toward the home of Marissa Nunez. Nunez, 19, and her family escaped to the second floor of a neighbor's home before being rescued Monday by volunteers with boats.

"Our entire house was under water," Nunez said.

In Houston, the George R. Brown Convention Center was being outfitted to handle 5,000 evacuees, city officials said. More than a dozen smaller shelters have been opened across Harris County.

Desiree Mallard, who carried her toddler son to the center, said she initially heard on the news that residents were advised not to leave the city, and so she didn’t. She said she finally fled her apartment by floating her son on an air mattress through floodwaters.

“I could have (left), if I would have known it was going to be this bad, but I didn’t know,” Mallard said. “And then when it got bad, they said, ‘It’s too late to evacuate.’”

A Kroeger Supermarket on Westpark Tollway in West Houston reopened noon Monday. It quickly drew a long line of shoppers, who waited in the pelting rain outside and then endured 1-hour checkout lines for a chance to stock up on food and supplies.

Shopper Donna Balin, 45, said her house in nearby Seven Meadows didn’t flood, but she was worried that plans to release water from reservoirs could push the flooding closer to home.

“There wasn’t a lot of warning, like in previous storms,” she said, adding “They should’ve evacuated.”

Outside of Houston, communities scrambled to house the hordes of displaced residents fleeing the rising waters and heeding the increased calls for mandatory evacuations of flood-prone areas.

In Richmond, 30 miles south of Houston, the Red Cross was moving its shelter from a church community center to a bigger high school in nearby Rosenberg located further away from the floods. There were around 79 people at the shelter with another 10 on the way, officials there said.

Hector Marez, 34, had planned a 58th birthday party for his mother, who had driven last week to his Richmond home from her house in the Rio Grande Valley. But as the water starting rising, Marez was forced to leave his mother behind — along with a sister, brother-in-law, 1-year-old niece and 2-year-old goddaughter — wade down his street in waist-high water and look for help.

He arrived at a Richmond shelter Sunday night without them.

“I’m worried for them,” he said. “This has been crazy.”

Other communities helped in other ways.

In Brevard County, Fla., where residents experienced fear after last year's Hurricane Matthew approached their area before veering east, government officials issued a directive allowing employees to get paid leave to help with Harvey relief efforts.

County employees with special skill sets that would help in a serious disaster may take off work as long as serious needs persist, according to the directive.

Contributing: Doyle Rice, USA TODAY; Jason Pohl, the Arizona Republic; The Daily Advertiser in Lafayette, La.; Florida Today; The Associated Press