Tropical Storm Rosa neared Mexico's Baja California on Monday, spreading heavy rains that were projected to extend into a drenching of the Southwest U.S. Forecasters said the rain may cause "life-threatening flash flooding".
Tuesday's rain differed from a monsoon in that it did not bring widespread wind and lightning, but rather a steady, prolonged rainfall in many areas.
The path of Tropical Storm Rosa and its heavy moisture was unclear for several days as it moved up from the southwest, but ultimately, it split over Utah and southern Colorado, he said.
Rain from the remnants of Tropical Storm Rosa slowed the Phoenix area's morning commute traffic as authorities prepared for potential flooding across the desert Southwest.
A nearby riverbed that is normally dry overflowed, spilling muddy water onto the street. Firefighters slogged through waist-deep water to get to people stuck in their cars.
Around 10 vehicles, including a bus, were either at a standstill in the water or tried to drive through it.
However, he cautioned that even a little rain could produce more flooding. "It just runs off rather than being absorbed by the ground".
According to the National Weather Service, by the time Phoenix had reached 2.36 inches of rainfall at Phoenix Sky Harbor International Airport on Tuesday evening, Phoenix had marked the not only the rainiest Oct. 2 in its weather history, but the wettest October day ever.
Several cities posted on social media that they were distributing sandbags.
Maricopa County Community Colleges are also closed due to weather.
Flood watches are in effect across portions of California, Arizona, Nevada and Utah, including the cities of Phoenix, Las Vegas, Flagstaff and Salt Lake City.
The Utah National Guard was activated Monday to assist in flood mitigation efforts in Utah County.
The soldiers planned to build flood berms and erect barriers and sand bags, officials said. Heavy rain Monday flooded streets in the city of Yuma at the U.S. -Mexico border and caused power outages.
Flash flooding began and lasted most of the day throughout areas of the Valley near washes and drains.
Rosa will make landfall late Monday along the west coast of Baja California, Mexico.
Another storm, Tropical Storm Sergio, is about 790 miles south of the tip of the Baja Peninsula and slowly moving west across the Pacific Ocean.
The center of Rosa, which was a hurricane until late Sunday, was expected to hit Baja California and Sonora by early Tuesday, bringing 3 to 6 inches of rain, the U.S. National Hurricane Center said.
Rosa's maximum sustained winds had decreased to 40 miles per hour as of Monday afternoon, and the storm's center was heading north-northeast at 10 miles per hour.