Hurricane Harvey, that is described as the fiercest storm to hit the United States in last 10 years, swept through Texas on Saturday.
The storm killed at least three people in Texas and more than 1,000 people were rescued overnight.
The torrential rains left thousands of people stranded in the Houston area while rescuers took to the air and water in an attempt to pluck them to safety.
According to officials, more than 56,000 calls were made to 911 in the wake of the weekend storm.
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.
"This event is unprecedented & all impacts are unknown & beyond anything experienced," the National Weather Service (NWS) tweeted.
President Trump tweeted on Sunday night, saying "Spirit of the people is incredible. Thanks! "
Rescue operations were underway with "deployment of 3,000 national and state guard service members, 500 vehicles and 14 aircraft", officials said.
Texas Gov. Greg Abbott said at a news conference that as the heavy rains are expected to persist, which will lead to even more flooding, people are urged to stay off the roads.
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