As violent thunderstorms lashed central Texas on Saturday night and floodwaters rose to the highest levels the state has ever seen, Austin resident Julie Shields got a frightening phone call from her sister Laura.

“We are floating in a house that is now floating down the river,” Laura said, Shields told local TV station KXAN. “Call mom and dad. I love you, and pray.”

Laura McComb, her husband and children, and another family were vacationing together in Wimberley, a quaint riverside town halfway between Austin and San Antonio, when the storms hit. As the river rose, something in the water knocked their cabin off its supports, sending the building and its nine occupants hurtling downstream.

Since that frantic phone call Saturday night, no one has heard from Laura, her children or the other family. Only Laura’s husband, Jonathan McComb, managed to escape the floodwaters. He is being treated for a broken rib, a punctured lung and other injuries at San Antonio’s Brooke Army Medical Center, his father, Joe McComb, told The Post.

“It was going to be a family, fun-filled weekend,” Joe McComb said in a phone interview Monday. “But then the rain started in the afternoon, and it got dark, and the river did what it did all in a matter of 20 or 30 minutes.”

Joe McComb and his wife, who flew back from a wedding anniversary trip in Hawaii to see their son in the hospital, said that Jonathan’s memory of what happened is hazy. As night fell, the Blanco River surged over its banks, rising past the stilts that were supposed to elevate the cabin beyond the reach of floodwaters. Authorities say the waters rose 26 feet in one hour, overwhelming the surrounding community.

Inside their Wimberley cabin, Jonathan McComb and his family were trapped.

“Then they heard something terrific hit the poles that were holding the house. Either one or two must have broken, but it was dark and nobody could really see what was going on,” Joe McComb said. “Whatever it was hit the side of the house, it broke the poles and then the water just lifted the house and started floating downriver.”

The fierce current carried the building for a few miles, then sent it crashing into a bridge. The house was torn apart and its occupants cast into the water.

“Everybody got separated from everybody,” Joe McComb said.

Buffeted by waves and bombarded by debris, Jonathan McComb lost sight of his family and was on the verge of losing consciousness. But he kept telling himself “I gotta get out,” according to his father, and was eventually carried to shore. He clambered out of the water near San Marcos, about 12 miles downstream from where he had been staying.

“He’s real beat up, but he’ll recover. It’s a miracle,” Joe McComb, a former Nueces County commissioner, said. “Now we’re just hoping that another miracle will happen and all or some of them will be found.”

The McCombs’ dog Maggie, who was brought along on the trip, was discovered caught in a tree Monday, alive and mostly unhurt. Joe McComb interprets this as a sign that the rest of his family will be found.

“Our faith in God is that the others will be able to pull through this, too,” he said.

The other people in the house — Laura McComb, their two children Andrew, 6, and Leighton, 3, family friends Randy and Michelle Charba and their son Will, 4, and Michelle Charba’s parents Ralph and Sue Carey, all from Corpus Christi — are among 12 people still missing in Central Texas.

The floods Saturday night were the worst Texas has ever endured, Gov. Greg Abbott (R) said at a news conference Monday.

“You cannot candy-coat it,” he told reporters after a flight over the Blanco River. “It’s absolutely massive. It’s a powerful message to anyone in harm’s way of the relentless, tsunami-type power this wave of water can pose to people.”

Abbott described seeing a “relentless flow of water” and trees “mowed down like grass by a lawnmower.”

“You may think it’s something you can escape. All too often it’s something you cannot escape if you don’t get out of harm’s way in advance,” he warned, according to the Austin American-Statesman.

The weekend storms wreaked havoc from the Southern Plains into the Gulf of Mexico. Five people have been reported dead in Texas and neighboring Oklahoma, and 13 people were killed by an associated tornado in Ciudad Acuna, a city in Northern Mexico, The Post reported Monday. Abbott has declared a state of emergency in 24 Texas counties.

Hays County, which includes Wimberley, was among the hardest-hit areas. County officials told the Associated Press that some 1,000 homes there were destroyed.

“We do have whole streets with maybe one or two houses left on them and the rest are just slabs,” Hays County emergency management coordinator Kharley Smith told the AP.

After years of drought, the Southern Plains, which includes part of Texas, have been bombarded with rain in recent months. But the once-welcome wet spring has now become a deadly threat. Meteorologists told Reuters that the saturated soil couldn’t absorb any more water, causing this weekend’s flash floods. And since thunderstorms are forecast through the end of the week, continued flooding remains a possibility.

“It’s still raining,” Hays County Judge Bert Cobb told the Austin American-Statesman on Monday. “This event is not over.”