Remnants of a tropical storm drenched parts of the US southwest’s desert on Tuesday, trapping some drivers on swamped Phoenix streets as authorities prepared for possible flash flooding in Arizona, central Utah and elsewhere.

Rosa, a hurricane that was downgraded to a tropical storm and then to a tropical depression, reportedly killed one person in northwestern Mexico before moving north into the United States.

The National Weather Service issued a flash flood warning for the Phoenix area, saying that more than 50mm of rain fell in metro Phoenix by mid-morning and additional rain was expected.

The day had already made it one of the wettest Octobers Phoenix has seen, the agency said. The city sees sudden and heavy downpours during the summertime monsoon, but the continuously falling rain is a rarity.

The heavy showers caused a riverbed to overflow, spilling muddy waters into a north Phoenix intersection. Firefighters slogged through waist-deep water to get to people stuck in their cars.

Crews pulled at least six people, including a child, from vehicles and carried them one at a time to a fire truck. Around 10 vehicles, including a bus, were either at a standstill in the water or tried to drive through it.

The wet weather was a factor in numerous Phoenix-area freeway wrecks but no serious crashes were reported, according to police. Authorities warned drivers to avoid entering streets or washes inundated with water.

A reporter for Phoenix news station KNXV-TV posted a video on social media showing a person riding a unicycle into a flooded area. Pictures have also circled of a man riding a surfboard in the street, prompting the weather service to reiterate that all types of vehicles should stay away.

The rain also led three elementary schools and one high school to close for the day. Flash flood watches remain in effect for parts of Arizona, California, Nevada and Utah.