Texans on Monday morning were about halfway through the worst of Hurricane Harvey’s rains, and as the storm moves back out onto the Gulf of Mexico, they braced for even more devastating flooding in much of the southeast section of the state.

Houston has already received more than two feet of rain, turning many roads into rivers as the city of 2.3 million people prepares for another two feet to fall in the coming days.

Harvey arrived on the shores of Texas as a hurricane Friday night, packing sustained wind speeds as high as 130 mph. As of Monday morning, it was classified a tropical storm with maximum winds of 40 mph, and it has parked itself over the southeastern half of the state.

Thousands of residents, many in the towns of Port Aransas, Port O’Connor, and Corpus Christi, where the hurricane first made landfall, evacuated their homes, and five deaths have so far been reported. Tens of thousands could be driven into shelters, and hundreds of thousands could seek some sort of disaster assistance, officials said.

“This is a landmark event for Texas,” FEMA Administrator Brock Long said. “Texas has never seen an event like this.”

The rain hasn’t stopped, and forecasters don’t expect it to let up until at least Tuesday, with total rainfall reaching 15 to 25 inches across much of Texas by Tuesday, reaching up to 50 inches in isolated areas.

The storm is moving at 3 mph, crawling its way across the southeast flank of the state, leaving flooding and destruction in its wake. At 7 a.m. CDT on Monday, the center of the storm was sitting in Matagorda Bay.

The National Hurricane Center is still warning of “catastrophic and life-threatening flooding,” and the National Weather Service called the rainfall event “unprecedented.”