9 million people in the southwest face a flood caused by the tropical depression



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Clouds spread over Tucson.
Photo: Ken Bosma (Flickr)

In this year of heat waves, forest fires and record hurricanes, why not add one more weather phenomenon to the list? The tropical depression Rosa landed in Baja California on Monday and crawls in the southwest, bringing with it the threat of sudden floods from southern Arizona to Idaho.

In all, 9 million people are under surveillance or flood warnings as of Tuesday morning Mountain Time, while Rosa flaps inland. The storm reached Category 4 status at its peak at the end of last week, but has since lost steam (at least in the wind turbine department). He made landfall as a tropical storm in Baja California late last night and has since become the lowest of Rosa's remains.

But the winds have never been the major concern of the Southwest. Instead, it is the threat of torrential rain as the storm brings tropical moisture into an arid landscape. More than half an inch of rain has already fallen on and around Phoenix, making it one of the most violent events hit the city since 1990. Phoenix is ​​already up October the wettest The record was recorded while we are only on October 2nd. With the most abundant rain coming in the afternoon, the storm should continue to climb to record highs.

In all, up to six inches of rain are expected in the mountains of Arizona. This may not seem like much, especially after Florence's recent flood in the Carolinas, but dry soil increases the risk of runoff. The picturesque canyons and dry lava zones that cross the desert landscape are roads disguised as water, waiting for rains like this to send torrents rushing downstream.

A recent drought has further hardened soils, which could help speed up the runoff process. According to the US Drought Monitor, large parts of the Four Corners region experience exceptional or extreme drought.

The Rosa rains will certainly help to alleviate this drought, but they will also affect the desert landscape and its inhabitants. Indeed, images of Phoenix – itself filled with impervious sidewalks – and the surrounding area show that the roads are already flooded when the aforementioned dry washes fill with muddy and foamy water. According to CBS News, the Utah National Guard has already been activated and we could see more resources mobilized as the floods unfold.

It's weird, but not unprecedented, that tropical systems are permeating the Southwest. Tropical Depression Storm Newton reached Arizona in 2016, the last tropical system to hit the region.

The remains of Rosa are just the latest example of a growing problem: heavy rains are increasing due to a warmer atmosphere. We are seeing this trend in the wake of storms such as hurricanes Harvey, Lane and Florence, as well as random rain events like Kauai in April.

According to the latest national climate assessment, the southwest has seen a 5% increase in heavy showers since 1958, even as the region dries up. This type of meteorological whipping poses a huge infrastructure problem, as municipal and state planners need to find a way to prepare communities for showers. and water conservation in times of increasing drought.

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