BRAZORIA COUNTY -- Hurricane Harvey is expected to send catastrophic flooding in the coming days along the Brazos and San Bernard rivers, County Judge Matt Sebesta said.

A voluntary evacuation has been issued for residents living along the San Bernard River. The river is expected to climb to upwards of 31.4 feet, according to projections by the National Weather Service, shattering the 1998 record of 23.9 feet. Sebesta said the county is working on identifying what should be the hardest hit areas along the river and a mandatory evacuation will likely be issued Saturday.

“We’re going to see catastrophic flooding from this rain event over the next few days,” Sebesta said. “I’m encouraging everyone along the San Bernard River to get out, and if you’re ready, get out now.”

Sebesta estimated between 3,000 to 5,000 people live along the river, though he said he didn’t have exact numbers in front of him as of Friday night.

The county is expecting between 15 to 30 inches with isolated areas seeing 40 inches over the course of the next four to five days as Hurricane Harvey makes landfall and creeps up the Texas coast. Harvey strengthened to a Category 4 storm Friday night as it crept to the Texas coast near Corpus Christi.

The Brazos River, in areas from Lake Jackson and Rosenberg, will also see significant flooding. The river will climb to 52.5 feet, according to projections from the National Weather Service. If that holds, that would nearly match the significant flooding along the county faced in 2016 when the Brazos River reached 52.56 feet. The county is preparing to bus residents to Belton, some 200 miles northwest, if mandatory evacuations are ordered.

Sebesta did concede that all the flooding is based on projections and that the “water is not on the ground.” But, he added, the National Weather Service based its projections on rainfall intensities and what’s being seen on the radar.

Harvey is the first Category 4 hurricane to hit Texas since 1964 when Hurricane Carla made landfall. Harvey is the strongest hurricane to hit the United States since Hurricane Charley hit Southwest Florida in 2004.