Donald Trump has declared "there's never been anything like" flooding across Texas in the wake of Storm Harvey.

The US president's heading to the state today, where 30,000 people could be forced into shelters.

Half a million people are expected to need emergency help, with the worst of the weather yet to recede.

Families and pets in Houston, Texas are transported to higher ground as Acadiana residents respond to rescue flood victims of Storm Harvey | Image: Lee Celano/The Advertiser via USA TODAY NETWORK

Mr Trump says he is predicting a "very expensive" recovery operation.

"It's the biggest ever, they are saying it is the biggest, it's historic.

"It is a historic amount of water, there's never been anything like it."

The storm is set to bring more downpours to Houston, the state's biggest city, with the life-threatening flooding predicted to peak on Wednesday and Thursday.

Up to 20 more inches of rain could fall in the coming days, on top of the 30 inches some areas have experienced so far.