A few areas could pick up a quick 2 to 4 inches of rainfall through Saturday evening. Some storms could contain rain falling at rates of 2 inches per hour. These rates can easily cause flooding, given already wet soils that don’t absorb much rain.
“Considering we’re in a pretty extensive drought, the fact that we have this moist air mass capable of very high rainfall rates could lead to some significant flooding,” said Grant LaChat, a meteorologist at the National Weather Service in Phoenix.
The abrupt arrival of rains marks an active period in the southwest monsoon, a staple of the summertime in the Desert Southwest and adjacent parts of Mexico.
The term monsoon describes a seasonal wind shift, which pumps in more moisture from the south. The moisture typically originates from both the Gulf of Mexico and the Pacific, and it is sufficient to brew rounds of isolated to widely scattered slow-moving downpours.
“Given the particularly wet preconditions, the risk of numerous flash flood events remains,” wrote the National Weather Service, which hoisted a level 3 out of 4 moderate risk of excessive rainfall for much of Arizona and west central New Mexico. “The footprint for heavy precipitation is quite large.”
Heavy storms will break out across eastern Arizona and western New Mexico along and east of Interstate 17 during the midafternoon hours Friday, spreading westward and congealing into one or more clusters. Redevelopment of storms is likely on Saturday morning, with the coverage becoming more widespread during the afternoon and evening.
The Flagstaff to Phoenix corridor, including the Mogollon Rim, could see the greatest totals, partly thanks to an assist from orographic lift, where moisture accumulates traveling upward in elevation. Storms may even linger into Sunday in northwest Arizona as the upper-level system slings a lobe of moisture toward Las Vegas.
Any places that receive heavy rainfall could quickly flood because of the impermeability of the sandy soil, which results in rapid runoff that inundates arroyos (dry river beds) and ordinarily empty canyons. Debris flows are also possible where heavy rains pour down on burn scars left behind from recent wildfires.
The monsoon typically lasts for much of July through September.
In Phoenix, June tends to be the driest month of the year; temperatures skyrocket before the stabilizing effects of the monsoon arrive. July and August, conversely, are the two wettest months, combined averaging just under 2 inches of rain in the city. The actual amount of precipitation observed during that window varies wildly year-to-year because of the significant variability and randomness of monsoon thunderstorms.
Last year featured a virtual “nonsoon,” but rains have proven more robust this year. In July 2020, Phoenix only saw a tenth of an inch; rain this month has already outperformed that number fivefold.
“I can tell you now just being here, it’s a lot different this year,” said LaChat. “Last year it was a lot of heat, no real rain chances. That high [pressure system] wasn’t willing to move.”
This year, he noted, the “monsoon high,” a summertime high pressure system that helps direct moisture northwestward toward Arizona, is in a more favorable position.
“Last year it was more so overhead,” he said. “It kind of kept us cooking in the heat. This year it’s been shifting toward the Great Basin, or near the Four Corners. That allows the winds to have a more easterly component. When storms form over the high terrain, it allows them to push west toward the Phoenix metro.”
Monsoon-induced storms aren’t exactly like thunderstorms elsewhere in the U.S. They have their own special character.
Monsoon thunderstorms are often isolated cells that quickly pulse up and down in intensity. Dry air near the surface makes for extraordinarily high cloud bases, allowing for picturesque views of rain shafts cascading to earth as crackling lightning flickers and leaps.
Sometimes the surface, baked by the sun, steams when the first raindrops splatter down. The waters bring life to the desert.
The thunderstorms can also bring other hazards, like damaging microburst winds and large hail. Tornadoes aren’t common, but they do happen periodically. Some also bring dust storms, often called haboobs.
Above-average precipitation is likely to continue for much of the Intermountain West in the coming weeks.