Tropical Storm Harvey unleashed the worst flooding in Houston history, dumping as much as 29 inches of rain in some areas over two days, sending bayous over their banks and leaving neighborhoods across the Houston area deep in floodwaters.

Thousands of people abandoned their inundated homes and apartments by boat, helicopter and by foot, carrying children, pets and plastic bags full of belongings. At least six people are believed to have died in the storm, according to the National Weather Service, and more than a dozen were treated for injuries.

And the worst may be yet to come.

“It’s catastrophic, unprecedented, epic — whatever adjective you want to use,” said Patrick Blood, a National Weather Service meteorologist.

Houston braced for another round of potentially heavy rains overnight and into Monday, with the likelihood of strong storms continuing until at least Thursday. Harvey has already surpassed the rainfall totals of Tropical Storm Allison, which devastated the area in 2001.

Even more water will begin flowing into the bulging Buffalo Bayou early Monday when the U.S. Army Corps of Engineers begins a controlled release from dams at the Addicks and Barker reservoirs, where water levels prompted officials to begin a controlled release of water from both dams early Monday, sending additional waters into the bulging Buffalo Bayou.

Residents near the dams could see water seeping into their homes before the release, and those living along the bayou are being asked to evacuate voluntarily, said Col. Lars N. Zetterstrom with the U.S. Army Corps of Engineers.

Guard called out

Gov. Greg Abbott said 3,000 National Guard members will join the Texas Guard in areas ravaged by Harvey, which came ashore near Corpus Christi as a Category 4 storm. Hundreds will be deployed in Houston.

The federal government declared Harris County a disaster area Sunday morning, as thousands of rescues were underway across the region. By Sunday afternoon, people were lifted off the roofs of their homes in southwest Houston by helicopter.

In Fort Bend and Brazoria counties south of Houston, officials issued mandatory evacuation orders along the Brazos and San Bernard rivers, which are continuing to rise above flood levels. The Brazos could crest as early as Monday evening, officials said.

“This is pretty much like Day

2. There will be Day 3 there will be Day 4 and there may be Day 5,” Mayor Sylvester Turner said late Sunday. “ It’s important for Houstonians to be very patient. Let’s not get storm fatigue.”

6th fatality recorded

The National Weather Service reported five unconfirmed deaths early Sunday, before the body of man was found in a Walmart parking lot in La Marque.

City and county officials reported independently that a woman and child were found dead inside a submerged vehicle along Interstate 10 near Lathrop Street in East Houston and that a woman died trying to escape her vehicle after driving into high water at Warrenton and South Gessner.

Harvey’s first victim was believed to be in Rockport, where a man died during the storm in a structure fire.

Other victims are likely to be found, officials said.

“We have heard anecdotally about people in cars not being seen again, but that doesn’t mean that it’s a confirmed fatality,” said Jason Spencer, a spokesman for the Harris County Sheriff’s Office. “We fully expect that as the water recedes that there would be some grim discoveries.”

Officials opened up the George R. Brown Convention Center downtown on Sunday for up to 1,000 people displaced by the storm.

The White House announced that President Donald Trump will come to Texas on Tuesday. On Sunday, the president praised the government response in a tweet, writing: “Wow — Now experts are calling #Harvey a once in 500 year floodff We have an all-out effort going, and going wellff”

The storm brought much of Houston to a halt. Both major airports shut down, stranding hundreds of passengers.

Ben Taub Hospital started evacuating patients Monday after flooding caused power outages, with critical patients among the first to be moved. The Texas Medical Center was impassible in many places.

Lyndon B. Johnson Hospital also stopped accepting patients Sunday because it had essentially become an island, Harris County Judge Ed Emmett said.

Houston Independent School District officials and several other districts canceled school until after Labor Day.

City officials tweeted early Sunday that the 911 system was at capacity, urging people to shelter in place if they can. “Only call if in imminent danger.”

“Difficult to get to everyone right away,” Sheriff Ed Gonzalez tweeted. “Hang tight.”

On Twitter, people begged for help.

“Parents and 4 children need rescuingff” someone tweeted from the I-45 and Edgebrook area. “Blowing up mattress for last case scenario.”

“We’re all under water out here,” said Jonell Soto, the wife of South Houston Mayor Joe Soto. The couple was in an 18-wheeler on Sunday morning trying to reach the mayor’s 87-year-old mother, who lives on Indiana Street in South Houston.

“It’s just crazy,” she said.

Unrelenting rains

Harvey — the strongest hurricane to come ashore since Carla in 1961 — weakened into a tropical storm after it moved ashore late Friday near Corpus Christi.

But it continued to send bands of thunderstorms over Houston Friday and Saturday night, dumping rain on streets that quickly became saturated.

Five spots received more than 2 feet of rain between Friday and Sunday nights, according to National Weather Service monitoring stations — Dixie Farm Road in Brazoria County; an area near First Colony in Fort Bend County; Santa Fe in Galveston County; northeast areas of Pearland in Harris County; and Dayton, in Liberty County, which had the most rain at more than 28 inches.

“I know for a fact this is the worst flood Houston has ever experienced,” Blood said. “Worse than Allison. It’s so widespread.”

Emergency workers were overwhelmed with calls for water rescues, having responded to an estimated 2,500 by noon Sunday. Houston police officials also evacuated two apartment complexes in the flood-prone Greenspoint area, rescuing more than 50 children from rising flood waters overnight.

Turner said late Sunday that the police and fire officials received nearly 6,000 calls and had rescued more than 1,000 people. Many of the high-water rescues involved people trapped on their roofs or in their attics in southest Houston.

“It breaks your heart,” Police Chief Art Acevedo said via livestream on Twitter, as he stood in waist-high water in north Houston. “But, it’s Texas; we’ll get through it.”

Northeast and eastern Harris County were particularly hard hit, said Chief David DeLeon, with the department’s Marine Unit.

“We’re seeing 2-3 feet of water in their homes,” he said. “Farther south, it gets even higher — we’re even rescuing guys driving 18-wheelers.”

The area around Hermann Park was a scattershot of submerged streets as water flowed from swollen Brays Bayou and the city’s inundated drainage systems. The waters ebbed and flowed through the day as rainfall diminished midday Sunday.

The Highway 288 exchange at MacGregor Way intersection was underwater, with the tops of trucks poking out of the mostly calm water.

More than 350 people arrived at the Greenspoint area’s M.O. Campbell Educational Center shelter between 10 p.m. Saturday and midday Sunday, Red Cross shelter manager Jerry Fennell said. He said the shelter was stretched thin and was having trouble getting supplies in because of the rising water.

“We’re an island here,” Fennell said.

Houston TranStar listed 292 high-water locations on area roadways, which were lined with stalled and stranded motorists. Metropolitan Transit Authority suspended all service. The Harris County Toll Road Authority ceased tolling, so those forced to avoid high water could use the tollways.

Freeways in some parts were so deluged, water was lapping at overhead freeway signs.

CenterPoint said more than 67,000 people in their Houston coverage area were without power.

Nearly 300 personnel were staffing the Harris County emergency operations center to answer calls and assist law enforcement and rescue operations. The message remained the same from authorities: Stay home.

“What surprised me was that we saw homes that have never taken water begin to flood,” said Jeff Lindner with Harris County Flood Control District.

Late Sunday, floodwaters spilled the streets of Canyon Gate at Cinco Ranch off South Mason near the Westpark Tollway. Residents were advised to evacuate.

“We’re all trapped in here. The entire front of the subdivision is completely flooded,” said Eli Magaña, spokesman for SEIU Texas who lives in the subdivision. “There’s no way to get out, and there’s no way to get to a shelter.”

Trapped on third floor

In west Houston, water from Buffalo Bayou flooded into homes in the Farther Point community early Sunday, rising past the first and then the second story of Ra-min Almassi’s home.

He and his wife, mother-in-law and teenage son huddled on the third floor and called 911. They got no answer for about two hours. He tried to keep things calm.

“I said, ‘C’mon guys, this is fun,’” he said. “But I was worried.”

After a flurry of text messages among neighbors, a good Samaritan with a boat rescued them, along with Snicker, their chocolate lab, he said.

Parts of League City Parkway, between Interstate 45 and Dickinson Avenue, were almost completely underwater. Dozens of trucks and cars sat abandoned on the side of the road, and even some emergency vehicles struggled at points to make it through.

One neighborhood was submerged in water nearly 2 feet deep. During a brief respite from the rains, residents walked out and peered at the water lapping at their doorways.

Albert Flores, who had lived in the neighborhood for 11 years, said he has never seen anything like it. His home — built about a foot higher than his neighbors’ — was the only one spared.

“The rest have like 6 inches of water inside,” the 56-year-old said.

In Galveston County, thousands of residents were stranded Sunday morning after a night of rain and swirling winds pelted the county and its barrier island.

“Dickinson is hit really, really, really hard,” said Brittany Viegas, the county’s Office of Emergency Management spokeswoman.. “It seems that everybody in Dickinson needs to be rescued.”

Some parts of the county were pounded with as much as 19 inches of rain overnight, and the National Guard arrived Sunday to help with high-water vehicles.

“We’re prioritizing life-threatening situations and elderly and young children first,” Viegas said. “We have people calling saying they’re stuck on attics and roofs.”

But the storm-prone island itself seemed to be faring relatively well, with some flooding downtown and on the west end. Overnight, a fire ravaged one canal-side home, but by Sunday morning the Galveston Fire Department was tied up with a pair of high-water rescues, with more expected.

‘Not going to move much’

Late Sunday, a major threat was still building: The Brazos River was rising, expected to crest Monday evening.

The forecast for the next few days remains dire, with computer models showing continued rounds of thunderstorms spawned by Harvey, Blood said.

Harvey remains unpredictable. Space City Weather, a blog run by meteorologists, reported that the highest-resolution, shortest-term model put the heaviest rain prediction for late Sunday between Baytown and Beaumont. Another model had heavy rain moving from west to east across the city. Others indicated heavy concentrations over the Inner Loop and west of the city.

Blood said the computer models show no signs of the rain letting up soon.

“Harvey is not going to move much over the next few days,” he said.

Lomi Kriel, Emily Foxhall, Rebecca

Elliott, Gabrielle Banks, Katherine

Blunt, Mike Morris, Jacob Carpenter, Matt Dempsey, Robert Morast,

Andrew Kragie, Keri Blakinger, Dug Begley, St. John Barned-Smith and

Lydia DePillis contributed to this report. susan.carroll@chron.com john.harden@chron.com dug.begley@chron.com