(ABC NEWS) -- Texas Gov. Greg Abbott said he will send an additional 1,000 National Guard troops to Houston on Monday to help cope with what the National Weather Service has called "epic and catastrophic" flooding in the area in the wake of Hurricane Harvey.

Abbott made the announcement in a late-night tweet on Sunday, bringing the total number deployed national and state guard service members to more than 4,000, as officials continue to devote resources toward rescue and recovery missions in flood-torn areas across the state.

Houston, the country’s fourth largest city, has been hit with major flooding as result of Harvey, which made landfall late Friday as a Category 4 hurricane and lingered as a tropical storm over the weekend, according to the National Weather Service.

The flooding comes after the hurricane left at least three people dead after it bashed the Texas coast. Buildings and parts of communities are submerged and, in Houston, there were more than 1,000 calls for rescues and people were forced to their rooftops or higher ground.

Some 2,500 people were taking refuge in Houston's George R. Brown Convention Center Monday morning, the Houston Chronicle reported, citing the Red Cross.

More “life-threatening rainfall” expected

The Houston area has seen as much as 30 inches of rainfall in some areas since Saturday evening, according to the National Weather Service, which said additional rainfall in the coming days could be “life-threatening.”

“Additional life-threatening rainfall totals of 15 to 25 inches with isolated higher amounts are possible across the Flash Flood Watch area over the next several days,” the National Weather Service, Houston said late Sunday. “If these amounts materialize, the results could be devastating, especially if any of these rains fall where catastrophic flooding has already occurred.”

A new record daily max rainfall of 16.07 inches was set at the George Bush Intercontinental Airport in Houston on Sunday, breaking the old record of 8.32 inches that was set in 1945, according to NWS measurements. The airport was closed in the wake of Harvey.

Rivers in the surrounding area were also projected to reach historical flood levels, according the the NWS.

Much of Southeast Texas is under a Flash Flood Watch Through Wednesday night, but the NWS warned that the watch could be extended beyond Thursday evening.

Floods pressure Houston area dams

The U.S. Army Corps of Engineers said it began to release water from the Addicks and Barker dams early Monday morning to prevent uncontrollable flooding of the Houston-metropolitan area as water levels continued to rise rapidly beneath torrential rains being released by the tropical storm.

Engineers were forced to start the process earlier than previously announced because water levels in the reservoirs had “increased dramatically in the last few hours,” officials said early Monday, adding that the release would likely cause additional street flooding that could potentially spill into homes.

“If we don’t begin releasing now, the volume of uncontrolled water around the dams will be higher and have a greater impact on the surrounding communities,” Col. Lars Zetterstrom, Galveston District commander, said in a statement Monday.

Meanwhile, officials in Fort Bend County, located about 45 minutes southwest of Houston, issued mandatory evacuation orders for residents near the Brazos River levee districts as the river reached major flood stages late Sunday.

County Judge Robert Herbert, the county's top elected official, said at a news conference that the National Weather Service predicted that Brazos waters could rise to 59 feet, three feet above 2016 records and what Herbert called an "800-year flood level," according to The Associated Press.

Relief efforts

Houston Mayor Sylvester Turner said he established a Hurricane Harvey Relief Fund after "after receiving an overwhelming number of inquiries from citizens and corporations who would like to help."

The fund, administered by the Greater Houston Community Foundation, will accept tax deductible flood relief donations for victims that have been affected by Harvey.

The Coast Guard said it has performed more than 2,000 multi-person rescues in the Houston-Galveston area, including more than 200 rescues by air and more than 1,000 rescues by water as the storm sent devastating floods pouring into the area.

The Coast Guard said about 50 percent of its operational helicopters across the country are currently in Texas.

Separately, officials with the Federal Emergency Management Agency said the agency has deployed more than 1,800 staff, over 20,000 tarps and more 70 generators to the hardest hit areas of Texas.

As of early Monday morning, the agency said it had supplied more 1 million meals and over 1 million liters of water to people displaced by the deadly hurricane.

In addition, four hundred people have been deployed by the U.S. Department of Transportation in response to the danger, officials said, and 250 highway closures were established statewide.