9 2018, post-fire debris flow in Montecito, California.

Decades of rising temperatures and more intense wildfires have turned parts of the American West into a nearly impervious landscape where even one downpour can trigger deadly, post-fire debris flows, commonly called mudslides.

USA TODAY spent five months examining the growing phenomenon of post-fire debris flows as part of a broader investigation into the impacts of climate change on precipitation patterns.

These prolonged droughts that help fuel devastating wildfires, followed by intense bursts of rain, can create the perfect conditions for post-fire debris flows.

But because other natural disasters like fires and floods have been more widespread and inflict more damage, the phenomenon of post-fire debris flows has been overshadowed.

They figured out ways to predict what rainfall intensity will trigger a post-fire debris flow, the crucial piece to effectively warn people of the danger.

“We're always behind the problem,” said Jason Kean, a research hydrologist and part of a team of USGS scientists who develop debris flow hazard maps after a fire.  .

“We know fires are going to happen,” he said.

That meant the threat of post-fire debris flows was high. .

His parents, he said, were there visiting his grandfather’s cabin – an annual summer tradition that included hiking among the canyon’s tall pine and aspen trees and eating fish his dad caught from the river.

Even places that typically never burn – like the Sonoran Desert – are now experiencing wildfires, said Ann Youberg, a senior research scientist at the Arizona Geological Survey.

Climate change has already increased the area burned in the West and the severity of the fires, said Daniel Swain, a climate scientist at UCLA and the National Center for Atmospheric Research.

It doesn’t take a big storm or a lot of rain to cause a catastrophic debris flow, said Nina Oakley, a research meteorologist with the University of California, San Diego.

“You just need that high intensity rainfall,” Oakley said, adding that the more intense the rain, the higher the probability of a larger flow.

Research into post-fire debris flows is still relatively new compared to other natural disasters and is largely based on data from a few geographic areas. .

The USGS work began in earnest 25 years ago, after torrential rains triggered post-fire debris flows that trapped dozens of vehicles along a stretch of Interstate 70 in Colorado.

USGS research geologist Sue Cannon, now retired, developed some of the earliest models to predict why and when they happen and helped launch the agency’s post-fire debris flow project.

If the rainfall expected to trigger debris flows is too high, people could die.

How long the land takes to recover to lower or eliminate the risk of the post-fire flows also may vary by location.

In drier regions, the post-fire debris hazard generally drops year to year as the land recovers.

In fact, we're still concerned, because we just don’t have a good handle on it yet,” Bill Burns, a state engineering geologist in Oregon, said about the recent rash of fires and the expanding debris flow risk.

“Other parts of the country, they've studied this post-fire debris flow phenomenon for decades.”.

Even in those parts of the country better prepared for the risk, post-fire debris flows can take people by surprise.

It was an extreme downpour several times higher than what’s needed to trigger a post-fire debris flow in the area. .

And, for those already concerned, it delivered a call to double down on their work and be better prepared the next time, said Kean, the USGS hydrologist.

USGS maps the post-fire debris flow hazards so officials can prepare and warn the public when a storm might trigger one.

Montecito resident Susan Moe said she never heard the term “post-fire debris flow” or knew she was at risk before January 2018. 

Making those predictions before a fire would buy time to figure out where a flow will go and better identify who and what is at risk, Kean said

Routinely applying the inundation models after a fire is still further off and making it happen broadly before a fire even starts still could be years away, Kean said

“My big regret about our work (in Montecito) is we didn’t provide maps of where the flows were going to go,” he said

Insurance companies, too, must prepare for the spreading risk of post-fire debris flows and the growing numbers of claims from policyholders after an impact. 

In California, that changed after Montecito debris flows, she said.  

State legislators also passed a law that year that similarly said coverage should extend to post-fire damage caused by a blaze. 

But when a disaster like Montecito happens – causing a huge loss of life and big hit to some of the most expensive real estate in the country – every insurer takes notice, she said. 

But models take time to build, she said, and she doesn’t know of one yet to measure financial loss from a post-fire debris flow. 

Right now, insurance companies also aren't required to pay those claims except in certain situations, like after the Montecito debris flow

Dan Bond, who lived in the area most of his life, talked to those scientists about what made the spot vulnerable to the hazard – one he said he hadn’t heard about before the deadly event that forever changed his neighborhood. 

A full-time resident of Black Hollow Road, the volunteer firefighter said he pulled on rubber boots the evening of the debris flow and headed outside where the nearby creek raged and homes and rocks the size of small cars washed down the mud- and log-filled river.  

Officials say it could happen again, a risk that could last years, he said