Flash floods are common in the small polygamous town of Hildale, Utah, but the volume and pace of Monday's rain was a "100-year event,” said Brian McInerney, hydrologist with the National Weather Service in Salt Lake City.
The torrent of water killed 12 people in this small town that straddles the state’s border with Arizona, as floods swept them away in their cars.
"It was terrifying," said resident Virginia Black, who watched from her home as a van of 16 women and children was swept away by the water and tumbled over an embankment into a ravine, reports the Associated Press.
"They were getting washed away, and there was nothing I could do about it," she added.
Only three children and one adult in the van survived.
In Zion National Park, about 20 miles to the north, four people died and three more are still missing after being swept away by the floods that coursed through a narrow slot canyon, according to officials.
"It was an act from God," Hildale Mayor Phillip Barlow told the the Deseret News. "This is something we can't control ... It happened too fast."
The storm climax lasted about a half hour, pouring 1-1/2 inches of rain into a desert-like landscape with little vegetation and steep slopes.
The Associated Press described the flooding as “like a bucket of water being poured onto a rock — it slid right off and began running downstream, picking up sediment to create the forceful, muddy mess that rushed through the city.”
Hildale has fewer than 3,000 residents who are followers of the polygamous Fundamentalist Church of Jesus Christ of Latter-day Saints, which is not affiliated with the Church of Jesus Christ of Latter-day Saints based in Salt Lake City.
The search for the missing hikers had been suspended because of flooding concerns, but searches are expected to resume Wednesday.
This report contains material from Reuters and the Associated Press.