Heavy rainfall poses extreme risks in the Western US

The western United States is facing a heightened risk of heavy rains in areas that were recently scarred by wildfires, new research warns. Heavy rainfall on burned areas can have devastating impacts, including debris flows, mudslides, and flash floods.

• In 2018, debris flows in Montecito, California, caused by a brief and intense rainfall over an area that had burned a month earlier, left 23 people dead and widespread property damage. 
• Last year, torrential rains in Colorado’s Glenwood Canyon triggered a massive mudslide in a recently burned area, stranding more than 100 people and closing a portion of the canyon for weeks. 

For this study, the research team focused on three regions – Colorado along with California and the Pacific Northwest. More than 90 percent of extreme fire events in these regions will be followed by at least three extreme rainfalls within five years.

This increased frequency came as a surprise for the research team. “Clearly we need to understand the risks better, as this creates a major threat to people and infrastructure,” said study co-author Danielle Touma.

By the end of the century, there will be a doubling or more of weather conditions that lead to the risk of extreme wildfires throughout much of the West. Some regions will experience elevated extreme wildfire risk within the next few decades. 

The researchers looked at the number of cases in which extreme rainfall is likely to fall on the same region after an extreme wildfire. They found that more than half of extreme wildfire events will be followed within a year by an extreme rainfall event across much of the West. In the Pacific Northwest, virtually all extreme wildfires will be followed within five years by extreme rainfall. 

Once every three years, drenching rains in western Colorado or much of the Pacific Northwest may be expected just three months after extreme wildfires – a scenario that was virtually unheard of in recent decades.

Climate change is altering the seasonality of these events. The study found more extreme rains occurring in the early fall in Colorado and the Pacific Northwest, close to the peak fire season of May to September.

“The gap between fire and rainfall season is becoming shorter,” said Touma. “One season of disasters is running into another.”

The study is published in the journal Science Advances. 

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By Katherine Bucko, Earth.com Staff Writer