Texas flooding 2015
Mandatory evacuations have been ordered in parts of Texas as rivers rose by up to 20 feet (file photo)twdb.texas.gov/

Weather forecasters have warned that a small amount of new rain could still have shocking consequences after the record-setting rains left three dead and eroded hundreds of homes in Texas and Oklahoma.

"Only an inch or two of rainfall could quickly lead to more flash flooding concerns," the weather service told CNN.

It is estimated that thunderstorms could cause 1-3in of rain on Monday – and around 5in in isolated areas.

In Hays County, Texas, just outside of Austin, up to 400 homes have been washed away. More than 1,000 homes were ruined and waters eroded two main bridges away in Hays.

One person has been confirmed dead there in the town of San Marcos, and the bad weather has caused difficulties for the search of missing people on Sunday.

Rescuers used helicopters to help residents off rooftops of buildings surrounded by flood lakes, where normally fields are situated.

Rudy Olivo, a resident of San Marcos, told Associated Press: "This is the worst I've seen it because the water rose so fast."

The Blanco River, which flows through San Marcos, moved past its previous flood record of 33.3ft to a new level of 40.21ft late Saturday.

An area of about 400 homes around Louis Creek Dam, 200 miles of north-east of Hays County, near Houston, are being evacuated. The dam has not yet breached and workers continue to pack soil on it.

In Oklahoma, two people have died. On Saturday, a woman in Tulsa died after her car hydroplaned. In Claremore, a firefighter for swept into a storm drain while attempting a high-water rescue on Sunday.

The National Weather Service says Oklahoma City already has a new monthly rainfall record for May – at 18.19in.