Houston is being inundated with “unprecedented” flooding after ferocious Hurricane Harvey set in over the Texas Gulf Coast this weekend, dumping torrential rain on the city, with no end in sight.

“This event is unprecedented & all impacts are unknown & beyond anything experienced. Follow orders from officials to ensure safety,” the National Weather Service (NWS) tweeted this morning.

The flooding comes after the hurricane, which has now left at least three dead after it bashed the Texas coast, left buildings and parts of communities submerged. In Houston, there were more than 1,000 calls for rescues and people were forced to their rooftops.

A third death was confirmed Sunday by La Marque, Texas, Mayor Bobby Hocking. The body of a 52-year-old man was found at the Walmart on Interstate-45. La Marque is about 38 miles southeast from Houston on the Gulf Coast.

“At this time it is unknown if the subject passed away from health conditions or due to drowning, there was high water in the area of Walmart last night,” the La Marque Police Department said in a statement.

The second person who died in Houston was “swept away,” according to Art Acevedo, Houston’s chief of police.

“Sadly, we have lost one female member of our community who encountered floodwaters in her vehicle, got out and was swept away,” Acevedo said. “We also have reports of one other brutality but have not confirmed it.”

Officials said another person died in the coastal city of Rockport.

Upward of 340 billion gallons of rainfall

The National Hurricane Center said at least 50 inches of rain has fallen in some areas, and one meteorologist estimated that upward of 340 billion gallons of rainfall inundated the area.

Meteorologist Travis Herzog of ABC Houston station KTRK-TV estimates that 340-370 billion gallons of rainwater has fallen so far, which exceeds other major flooding events that have affected the region in recent years by over one hundred billion gallons.

Officials respond

President Trump tweeted about the storm this morning, saying “experts are calling #Harvey a once in 500 year flood,” but that the United States had “an all out effort going” in response to it.

The response included the deployment of 3,000 national and state guard service members, 500 vehicles and 14 aircraft, officials said.

Six shelters have been created so far in response to the emergency.

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.

More rain is likely coming

Texas Gov. Greg Abbott briefed the press Sunday afternoon and said that heavy rains are expected to persist, which will lead to even more flooding.

He stressed that people stay off the roads and keep on the lookout for tornado warnings, which may pop up in the region in the coming days.

Rescue efforts underway

Meanwhile, workers are scrambling to locate survivors of the deadly storm, and some police departments are even seeking help from local residents in their rescue efforts.

The League City Police Department, located about 30 miles south of Houston, posted on social media that it was looking for boats to jump in and help save lives.

“We are looking for people with flat bottom or low water boats to assist with rescue and evacuation. Please send LCPD a Facebook message with name, phone number, location of boat, length and style of boat if you can be mobile with your boat and are experienced in operation of the boat,” League City police posted on Facebook.

Images of a submerged city

Images posted on social media, drone footage and video recorded by news organizations show parts of Houston and southeastern Texas completely submerged in water.

Joe Gleason, a producer for station KTRK, showed viewers the damage done to his home after the persistent flooding that has hit the region, with his living room, kitchen and dining room completely underwater.

“It’s never gotten this bad before,” Gleason told the station, comparing the damage to previous floods in the area.

Gleason and his family are hiding out upstairs to avoid the flooded downstairs area of their home, which appeared completely unlivable in video footage he provided to the station.

Other images recorded in the region today show water reaching freeway signs and traffic lights. Water reached the windows of a tractor-trailer that was stuck in rising flood waters at an intersection.

“This storm is beyond comparison,” Acting Department of Homeland Security Secretary Elaine Duke told ABC News.

ABC News’ Brittany Borer, Lucien Bruggeman, Mary Bruce, Christopher Donato, Erin Dooley, Michael Kreisel, Jonah Lustig, Daniel Manzo and Brendan Rand contributed to this report.

Click below to watch an ABC Special Report: