With at least 18 people dead and two still missing, Monday’s flash flood on the Utah-Arizona border was the deadliest flood in Utah state history, and was likely the deadliest weather event anywhere in the U.S. so far this year.

Preliminary data from the National Weather Service suggests flash flooding in Texas, including the Blanco River floods, was the deadliest weather event prior to Monday, having killed 13 people on May 23 and 24 in Hays, Blanco and Medina counties. Flooding was widespread as a low pressure system swept through Texas around Memorial Day, and it’s possible the death toll for that event will be adjusted. The National Weather Service told the Washington Post they know of no other weather events that caused more than 10 deaths since May.

Monday’s flood on the Short Creek near the Utah-Arizona border was caused by a cluster of torrential afternoon downpours falling on dry, mountainous terrain. The Short Creek river gauge at Colorado City recorded a rise over 3 feet in just 19 minutes at 3:18 p.m. on Monday, which likely caused the initial flooding that attracted residents to watch, and some to wait nearby for the water to recede.

Then around 5 p.m., a second “wall of water” swept down the creek. The river gauge rose over 5 feet in about 15 minutes. The gauge stopped reporting after 5:47 p.m.

Three hours before the disaster in Utah, the National Weather Service issued a strongly-worded flash flood warning for the area. “Move to higher ground now. Act quickly to protect your life,” it said.

In a white van and an SUV, three woman and 13 of their children were among those swept away by the torrent as they were waiting for the water to recede so they could get home, reports the AP:

In an instant, floodwaters engulfed them, and the two vehicles were sucked downstream, bobbing in the turbulent water before they tumbled over an embankment. Only three children survived. Twelve are dead. One is missing.
Virginia Black watched in horror from her house as she made a video of the once-in-a-century flash flood. “There goes the van!” Black says in a high-pitched voice. “It went over the thing. Oh, dear.”
Downstream, people rushed to where the vehicles came to a stop. One witness described a gruesome scene of body parts, twisted metal and a young boy who somehow survived.
“The little boy was standing there,” Yvonne Holm recalled. “He said, `Are you guys going to help me?'”

The rain also caused flash flooding at Zion National Park, killing four people while three others remain missing. The Associated Press reports that the group from California and Nevada in their 40s and 50s began their hike before the canyons were closed by officials due to the rain.

“Park Ranger Therese Picard said Wednesday that while rangers ask detailed questions and give several weather warnings before handing out permits, they don’t judge visitors’ technical ability and let them decide whether to go,” reports the AP. “She says the group was told Monday morning that flash-flood danger was probable, a warning that rangers give almost every day during monsoon season.”

Monday’s rainfall was a 100-year event in Hildale, according to Brian McInerney the National Weather Service in Salt Lake City, meaning that on average, rainfall of this magnitude only occurs once in 100 years.

The previous most-deadly flood events  in Utah occurred in 1965, when seven family members people died while camping in Sheep Creek Canyon. Seven people also died in a 1923 flash flood near Farmington, Utah. “Farming sections were largely destroyed, with houses, barns, orchards, and highways covered with several feet of mud. Lagoon resort was flooded and patrons were rescued from trees,” says the flood report from 1923.