"We're getting right into the heart of the most active part," said John Adair, a veteran meteorologist at the weather service office near Harry Reid International Airport in Las Vegas.
"This is turning out to be a pretty active monsoon season, compared with the last five years or so. There's plenty of more opportunities for thunderstorms to develop."
The annual weather pattern has brought a parade of storms across the US Southwest in recent weeks that lead to flooding in normally dry washes, rain measured in inches and rescue operations.
In Arizona, a driver had to be rescued from a vehicle caught in floodwaters in Apache Junction. A youth conservation crew abandoned the red truck they were riding in at Canyon de Chelly National Monument on the Navajo Nation when it got stuck in the mud and water rose around it. Mohave County sheriff's officials rescued a woman who was clinging to a stop sign earlier this week after her car was swept away.
Parts of the Hualapai Mountains in Mohave County have received up to 15cm of rain in recent days, Adair said. The National Weather Service said parts of Arizona can expect 2.5 to 5cm of rain per hour before a flood watch expired Saturday morning.
While the rain is welcome in a region gripped by drought, it creates headaches for neighbourhoods where wildfires have stripped the land of vegetation, which normally slows and partially absorbs floodwaters.
Parts of Arizona are near or above 200 per cent of normal rainfall so far during the monsoon, which started June 15 and runs through September.
While only 0.76cm of rain was registered at the Las Vegas airport late on Thursday, more than 2.5cm fell just 1.6km away at the University of Nevada, Las Vegas, Adair said.
Nearby wind gusts peaked at 114km/h and toppled trees. Pea-sized hail fell from lightning-streaked skies in suburban Henderson, where almost 2.5cm of rain fell in some areas.
Police, county and city officials and the weather service said no injuries or widespread damage was reported.
Casino patrons posted videos of water pouring from ceilings at Caesars Palace and Planet Hollywood resorts on the Las Vegas Strip and from behind a huge video display at the downtown Circa hotel-casino sports book. One video showed a man continuing to gamble at a casino slot machine while water fell around him.
Las Vegas firefighters responded to 330 calls for service and swift-water teams rescued seven people.
The surface level at the region's drought-stricken water supply - the Lake Mead reservoir behind Hoover Dam on the Colorado River - has dropped to less than 30 per cent.
While runoff from storms in the Las Vegas area will reach the lake, monsoon moisture is not likely to affect the ongoing regional drought.
