9 Million in the Southwest Face Flooding From Tropical Depression Rosa

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. 

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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.

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Rosa’s remnants are just the latest example of a growing problem: Heavy rain events are on the rise owing to a warmer atmosphere. 

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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.