Early on 9 January 2018, slabs of the Santa Ynez Mountains hurtled down into the community of Montecito, Calif. More than a centimeter (half an inch) of rain had fallen in 5 minutes over the burn scar of the Thomas Fire, which had been raging for more than a month and had yet to be fully contained. Mudslides that morning killed 23 people and caused millions of dollars in damage.
As climate change upends temperature and precipitation patterns across the western United States, communities can expect to see more of these compound fire-rain disasters, according to new research that will be presented on 17 December at AGU’s Fall Meeting 2021.
A One-Two Punch of Fire, Then Rain
Climate change is driving more days with ideal wildfire conditions and more frequent rainstorms, as well as more of these one-two punch events in which rain falls after an extreme fire day.
“After a fire, the landscape can change,” said Danielle Touma, lead author of the research and a postdoc at the National Center for Atmospheric Research in Boulder, Colo. “You have ash buildup. You have loss of vegetation. The soil becomes hydrophobic.”
Those landscape changes over a burn scar create perfect conditions for a debris flow after it rains. According to Touma, that risk is elevated in a burned area for up to 8 years.
To better evaluate the risk, Touma and her team counted the number of joint rain-fire events using the Community Earth System Model (CESM) Large Ensemble. They focused only on statistically extreme events and considered rainstorms occurring up to 5 years after fire weather.
The proportion of fire days followed by an extreme storm within less than a year could triple by 2100 across the West, according to their findings. The risk is greatest in the Pacific Northwest, where the proportion could double by midcentury.
The results were “striking,” said Samantha Stevenson, an assistant professor at the University of California, Santa Barbara and one of study’s coauthors. “We were expecting extreme rainfall to follow wildfires. We were just not sure how bad it would be.”
Touma, Stevenson, and their coauthors are “on the frontier of research thinking about how extreme events interact and how the interaction of them is more than the sum of their parts,” said Colin Raymond, a postdoc at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who was not involved in the research but convened the session in which Touma will present the work.
According to Raymond, the study’s approach illustrates the value of climate ensembles, which contain multiple simulations, for modeling compound weather events. The CESM Large Ensemble, for instance, contains 40 simulations, which allowed the team to account for natural climate variability and calculate the average likelihood of fire-rain events across 40 possible futures.
Stevenson concurred. By using ensembles, “we can start to understand the statistics of what we expect with climate change,” she said. “That’s really important when you’re trying to understand things that are fairly rare individually.”
No Relief for Emergency Managers
For fire-prone communities, these findings are a stark reminder that the arrival of rainy weather no longer brings the relief it used to.
Kevin Taylor is fire chief of the Montecito Fire Protection District and is all too familiar with the risk posed by this kind of compound disaster. Taylor is particularly concerned about managing multiple evacuations in short order. When evacuation orders were issued in advance of the January 2018 debris flow, he said, few residents complied. According to Taylor, many residents suffered from “evacuation fatigue,” having been evacuated for several weeks already because of the Thomas Fire.
Now, evacuation orders around Montecito are more precise, covering targeted areas based on new mapping of debris flow risk.
“What the fire department learned was that we needed to do a better job of communicating the risk,” Taylor said. “Now our messaging is much more direct, i.e., ‘This may be a catastrophic event.’”
In other words, these kinds of postfire mudslides “will kill you,” said Taylor.
—Leah Campbell (@lcampbell401), Science Writer