The heavy downpour hit southern Arizona Sunday, prompting flash-flood warnings for parts of Maricopa and Pinal counties, The Arizona Republic reported.
The storm "didn't create a lot of wind damage, but it created a lot of flooding damage and people being plucked from cars," said meteorologist Mark O'Malley.
At least nine people had to be rescued in five municipalities, officials said.
In Apache Junction, Ariz., rescue crews utilized helicopters to save two people whose vehicles became submerged, Accuweather.com reported.
Sunday was an unusually cool day in the area with a high of 87, O'Malley said. The average high temperature for July 21 is 106, The Arizona Republic said.
"You don't get too many days like this in Phoenix for the year," O'Malley said. "This will probably be the only day we get like this."
Since the start of the region's monsoon, which began June 15, the area has received 0.98 inch of rain -- more than the 0.63 inch that usually falls by July 21, O'Malley said.