Risk of damaging and deadly debris flows and flash floods following fires likely to increase in the West

New research from NCAR paints a dire picture for portions of the West that are susceptible to wildfires and extreme rainfall events.

Burn scar area in California.

If greenhouse gas emissions are not significantly reduced there may be a doubling or more of the weather conditions that lead to a risk of extreme wildfires in the western United States by 2100.

Burn scar areas are very susceptible to extreme flash flooding and debris flows during significant rainfall events, as there is no vegetation to absorb the moisture and hold back debris flows.

Following a fire, the risk of debris flows from extreme rainfall events persists for 3-5years and the risk for flash floods persists for 5-8 years following the wildfire event. The reason for the long period is that it can take years to regrow ground cover and roots and even longer for a full regrowth of vegetation.

Post-fire debris flow.

If human-caused greenhouse gas emissions continue to rise at a steady rate, the number of times an extreme wildfire event is followed by an extreme rainfall event within a year will likely increase by more than eight times in the Pacific Northwest and double in California by the end of the century.

The research team found that more than half of extreme wildfire events will be followed within a year by an extreme rainfall event in the West by 2100. Almost all extreme wildfire events will be followed by an extreme rainfall event within five years.

One of the factors that was taken into account with this prediction was the fact that the study found that more extreme rain events were occurring earlier in the fall across Colorado and the Pacific Northwest, which is getting closer to overlapping the peak of the western fire season.