The National Weather Service warned that “life-threatening flash flooding of low-water crossings, creeks, normally dry washes and roads” was likely.
Monday’s downpour in Death Valley came about two weeks after it set the world record for the hottest day recorded, with an average temperature of 118.1 degrees.
Featuring an exceptionally dry climate, Death Valley averages about 2.2 inches of rainfall annually, with only 0.1 typically falling during July. February is the wettest month, with an average of 0.52 inches. Monday was Death Valley’s second-wettest July day on record, falling just a hundredth of an inch behind the 0.75 coincidentally recorded on the same date in 1954.
“Out east when it rains, the primary concern is going to be rainfall rates. Usually, out east when you get an inch of rain, the soil is quick to absorb,” said Jenn Varian, a meteorologist at the National Weather Service office in Las Vegas. “You’ll notice this if you have a potted plant.”
The desert, however, doesn’t absorb much.
“Death Valley is very hydrophobic; it hardly absorbs anything,” Varian said. “If you’re talking an inch of rain over a large area falling over the same wash, now it’ll flow. If it falls, it flows. You could also have a larger storm falling into several washes that converges. Some of these cross over roadways.”
According to Brian Brettschneider, a researcher and climatologist based in Alaska, that 0.74 inches corresponds to more than the total rainfall observed this July in Portland, Ore., San Diego, Los Angeles, Honolulu, San Francisco, Spokane, Wash., and Denver. Much of the West remains in a severe or exceptional drought, with little rain to dampen the parched landscape.
Rapid runoff from Monday’s desert deluge brought significant flooding near the California-Nevada border along State Line Road, near Death Valley Junction. Video posted to social media shows torrents of swiftly moving water gushing across the ordinarily dry terrain.
“There are tons of photos circling around Twitter in the Desert SW that show cars being swept away by seemingly shallow flooding. Do not risk it,” tweeted the Weather Service in Las Vegas. Basketball-size rocks were left behind by the floodwaters on some roadways.
In Pahrump, Nev., an unincorporated community in Nye County about 65 miles east-southeast of Death Valley, public offices were closed at 1 p.m. Monday because of flooding reported at several buildings. Highway 160 was shut down, too. Photos on social media showed vehicles on roadways submerged up to their fenders.
The same severe thunderstorm blew the roofs off multiple structures in Pahrump and snapped trees three to four inches thick in diameter.
In Cedar City, Utah, 250 miles south of Salt Lake City, numerous cars were swamped and basements were inundated by floodwater.
What’s going on now “seems a lot more substantial because the last couple seasons have been ‘nonsoons,’ ” Varian said, referring to a lack of monsoon rains in the summers of 2019 and 2020. “You’ll see pretty similar damage and patterns in 2018. Our monsoon season is usually the second week of July, and it lasts through September.”
Farther west, Los Angeles tapped into some unusual July rainfall, too. The city picked up 0.12 inches, which may not sound like much but was a record for July 26. Los Angeles averages only 0.04 inches during July and ordinarily sees no measurable rain in August. The city has pronounced and separated wet and dry seasons.
Instigating the storminess was a combination of rich monsoonal moisture and a pocket of aloft cold air associated with a wave of low pressure drifting westward.
Monsoon activity is forecast to ease until later this week, when it may surge north, bringing moisture into drought-stricken areas of the Intermountain West.