In a year of wild heat waves, wildfires, and record-setting hurricanes, why not add one more weird weather phenomenon to the list? Tropical Depression Rosa made landfall in Baja California on Monday and is crawling into the Southwest, bringing with it the threat of flash floods from southern Arizona to Idaho.
All told, 9 million people are under a flash flood watch or warning as of Tuesday morning Mountain Time as Rosa lumbers inland. The storm attained Category 4 status at its peak late last week, but has lost steam since then (at least in the wind department). It made landfall as a tropical storm in Baja California late last night and has since become Rosa’s lowly remnant low.
But the winds were never the biggest concern for the Southwest. Instead, it’s the threat of torrential rain as the storm brings tropical moisture to an arid landscape. More than half an inch of rain has already fallen in and around Phoenix, making this one of the heaviest rain events to hit the city since 1990. Phoenix is already up to its seventh-wettest October on record despite it being just Oct. 2. With the heaviest bands of rain to come in the afternoon, the storm is likely to continue climbing the record books.
All told, up to six inches of rain are expected in Arizona’s mountains. That may not sound like a lot, especially after Florence’s recent deluge in the Carolinas, but the parched soil ups the risk of runoff. The scenic slot canyons and dry washes that cut across the desert landscape are disguised highways for water, waiting for rain events like this to send torrents rushing downstream.
A recent drought has hardened soils even further, which could help accelerate the runoff process. Large portions of the Four Corners area are in exceptional or extreme drought, according to the U.S. Drought Monitor.
Rosa’s rain will certainly help alleviate that drought, but it will also exact a toll on the desert landscape and those living in it. Indeed, footage from Phoenix—itself chock full of impervious pavement—and the surrounding area shows that roads are already becoming inundated as the aforementioned dry washes fill up with frothing, muddy water. The Utah National Guard has already been activated according to CBS News, and we could see more resources mobilized as flooding continues to unfold.
It’s weird but not unprecedented for tropical systems to amble into the Southwest. Tropical Depression Storm Newton reached Arizona in 2016, the last tropical system to hit the region.
Rosa’s remnants are just the latest example of a growing problem: Heavy rain events are on the rise owing to a warmer atmosphere. We’re seeing that trend driven by named storms like hurricanes Harvey, Lane, and Florence as well as random rain events like the one that hit Kauai in April.
The Southwest has seen a five percent increase in heavy downpours since 1958 even as the region becomes drier, according to the most recent National Climate Assessment. This type of weather whiplash poses a huge infrastructure challenge as city and state planners have to figure out how to prepare communities both for downpours and water conservation in increasingly dry times.