The soldiers planned to build flood berms and erect barriers and sand bags, officials said. Rainfall numbers for Monday and Tuesday had already combined to make for the third-rainiest October in Phoenix history, with 2.68 inches beating out the 2.66 inches recorded in October 1957.
Deserts aren't supposed to get much rain, but Tropical Depression Rosa is flipping the script.
In southern Arizona, heavy rain Monday flooded streets in Yuma and caused power outages in parts of the city. Parts of Colorado and northwestern New Mexico could also be affected.
Moisture associated with the storm arrived ahead of it, bringing rain Sunday night and Monday morning.
Hurricane Sergio has grown to major hurricane status in the Pacific, though it poses no immediate threat to land. Crews pulled at least six people, including a child, from vehicles and carried them one at a time to a firetruck. Firefighters slogged through waist-deep water to get to people stuck in their cars.
The heavy showers caused a riverbed to overflow, spilling muddy waters into a north Phoenix intersection.
Around 10 vehicles, including a bus, were either at a standstill in the water or tried to drive through it.
Some streets and schools were closed in the metro Phoenix area after more than 2 inches (5 centimeters) fell.
There was also flooding reported around Tucson. Several school systems in Mexico's Baja Peninsula were closed Monday as the storm approached the region.
It will remain a tropical rainstorm through midweek as it moves through the southwestern U.S.
One area of Arizona received 3.64 inches, the weather service said.
Flash flooding is possible near steep terrain, burn scars, and urban areas with poor drainage, the NWS said.
Forecasters say that even if the rain dwindles later in the day, some roads and washes will likely keep flowing.
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.
According to the Arizona Department of Public Safety, troopers on Tuesday morning responded to more than twice the number of rush-hour commute crashes.
Utah Gov. Gary Herbert declared a state of emergency Tuesday evening in advance of potential flooding from the remnants of Rosa.
It has sustained winds of 115 miles per hour (185 kph), making it a Category 3 storm, U.S. National Hurricane Center said.
Central and southern Arizona, including parts of the West Valley, can expect 2 to 4 inches of rain from Rosa.