One man was clinging to a tree and had to be rescued by a sheriff's helicopter. Four other people sought higher ground on an island and were rescued by crews from the Riverside Fire Department.
Rain turned to heavy snow in higher elevations, including rural eastern Nevada's Lincoln County.
Deputies there were trying to help people after 50 of 60 cars got stranded overnight in "a long stretch of absolutely nothing," according to dispatcher Shannon Miller.
No injuries were reported, Miller said, but U.S. 93 was closed south of U.S. 6 in Ely.
Snow in higher elevations in New Mexico and Arizona, including up to 4 inches around Flagstaff, Ariz., created dangerous road conditions and prompted some schools to delay opening Friday.
A flood watch was in effect until early Saturday in the Phoenix area, where several miles of the Loop 303 freeway in the western suburbs were closed due to flooding. Several cars were covered up to their windows. Forecasters said they expect ponding of water on local streets and in low-lying areas.
Authorities in Las Vegas, meanwhile, responded to hundreds of crashes as a storm dropped an inch of rain over desert dwellers.
Trooper Loy Hixson said the Nevada Highway Patrol responded to 141 collisions between 7 a.m. Thursday and 7 a.m. Friday, including 37 with minor injuries. That's four to five times what the agency sees on a normal day.
Las Vegas police reported 112 accidents on surface streets during a rainy five-hour period Thursday.
"This happens every time we get bad weather," said Las Vegas police officer Laura Meltzer. "People need to slow down and leave more space."
At least one business in thirsty southern Nevada was rejoicing over the storm system. Officials at the Las Vegas Ski and Snowboard Resort north of Sin City measured 11 1/2 inches of snow at midday Friday, with a week left until opening day.