Updated July 26, 2023
One can never predict the exact weather with any certainty, of course, especially given the lead time required to plan a trip to Las Vegas. However, the possibility of seeing a Southwestern-desert electrical storm can be narrowed down to a roughly 10-week period: July 15-Sept. 30. The worst (or best, given your point of view) of it, however, is confined to the six weeks from mid-July to the end of August.
The storms occur as a result of the North American monsoon, which has been called "perhaps the most regular and predictable weather pattern in North America." It gathers steam in late May in southern Mexico and spreads into the southwest U.S. by mid-July. It supplies up to half of the annual precipitation in Arizona and New Mexico and affects weather patterns from California to Utah, from Texas to Colorado.
The cause of the North American monsoon is a subtropical-ridge high-pressure zone that moves northward during the summer months and converges with a thermal low over the Southwest U.S. and northwest Mexico. In other words, moisture-laden currents of air blowing inland northeast from the Gulf of California and northwest from the Gulf of Mexico collide with a wall of hot dry desert air. However you define it, the monsoons are characterized by spectacular lightning activity and torrential thunderstorms.
In Las Vegas, the storms, foreshadowed by towering cumulonimbus clouds with dark flat bases boiling up over the valley, embody the worst weather that southern Nevada experiences. During the height of it, the National Weather Service's sophisticated and sensitive lightning-strike monitoring equipment can’t keep up with the onslaught of atmospheric electrical activity.
But it's the torrents of rain that cause most of the trouble in Las Vegas. Record rainfall was recorded in 1955, when three inches inundated the valley in 60 minutes. In 1975, a famous flood washed away 300 cars from the front parking lot of Caesars Palace (prompting city officials to begin planning the valley's flood-control system, which is covered in passionate detail in our book Beneath the Neon -- Life and Death in the Tunnels of Las Vegas).
Two back-to-back storm systems in August 1981 unleashed seven inches of rain in less than three hours on Moapa Valley, about 45 miles northeast of Las Vegas.
A severe thunderstorm in August 1997 dumped heavy rain near Page, Ariz., sending a 50-foot-high wall of floodwater thundering down Antelope Canyon and killing 11 hikers who were touring the narrow canyon that drains into the Colorado River.
The Regional Flood Control District's Gowan North Detention Basin can fill with 12 feet of water in 15 stormy minutes.
In short, these storms are almost otherworldly in their beauty and fury. Just make sure you're in a safe place when one hits, as the following personal stories from readers illustrate.
"I happened to be at an awards banquet at the top of the Stratosphere during a thunderstorm, and it might have been one of the neatest things I have ever experienced -- being able to see the lightning from that level and seeing it strike out across the valley was awesome. If someone really wants to see a storm, see it from the top of the Stratosphere."
"Four of us were hiking up to Robber's Roost in the Mt. Charleston area when we felt some 'hard' raindrops hit us. Not thinking anything of it, we continued on. Well, those hard raindrops became hail, then a super-bright flash of lightning and an instant boom of thunder. We made the dash down the mountain to the kiosk where we'd parked in nanoseconds, all the while thinking we would become the R-J's headline the next day: 'Stupid tourists killed by lightning at 9,000 feet.' Moral: If you feel hard raindrops ... take cover!"
"You don't want to be in Las Vegas when it's hit by a thunderstorm. I've been in Las Vegas for three. The first time, the water came so fast my rental car died. I was sitting at a main cross-street when it happened. The second time was the famous one when water destroyed hundreds of cars in the Caesars Palace parking lot. The third time was when I was staying at the Hilton. My family and I watched as the pool area was taken apart by the wind and rain. Las Vegas is no place to be in a thunderstorm."