When last month's deadly wildfires chased Jonathan Umholtz and his family from their Sonoma County home for 16 days, the disruption seemed endless. It turns out, that was just the beginning.
Umholtz, his wife and two kids were told a week after they returned home that they needed to leave again, this time for at least four months. A badly burned hillside above their three-bedroom house was on the verge of breaking loose, with the mud and all manner of debris threatening to come barreling down.
"It was like, what else can possibly happen?" said Umholtz, who as a supervising ranger for Sonoma County Regional Parks lives in park housing beneath Hood Mountain near Kenwood, and is now staying with generous friends. "It was heartbreaking."
With hundreds of square miles littered with trees and stripped of vegetation in the wake of the Northern California fires, mudslides are a near certainty as winter rains arrive. What makes the situation in the North Bay particularly dire is the large number of roads, buildings and even neighborhoods that sit in the potential path of the looming dirt and debris.
A huge effort among federal, state and local agencies is working to tame the threat. It includes the National Oceanic and Atmospheric Administration, which is fast-tracking a new North Bay radar system to help detect slide-inducing storms. The U.S. Geological Survey has mapped areas where damaging flows are most likely to strike. And hundreds of workers, donning rain suits and armed with shovels and straw bales, are hustling to secure the slopes before they unravel.
"Just pay attention," advised Cordel Stillman, an engineer and director of programs for Sonoma Clean Power, who is coordinating much of the work in the fire-ravaged watersheds. "Things may get worse than we thought."
Crews appeared to stay in front of last week's storms, the biggest since the fires ignited Oct. 8. Santa Rosa's Fountaingrove neighborhood had a small scare when a sinkhole opened up, the result of a melted underground drain that left the surface unstable. Residents there and in a few other hilly areas have been told to plan for evacuations, should soils continue to subside as is common in the aftermath of a wildfire.
Last winter, storms forced the closure of much of Big Sur when vast tracts of mountains washed out after the Soberanes Fire. The prior year, small but steady landslides threatened the rural Lake County communities that burned in the Valley Fire. But the worst damage has been in Southern California, where the toll after big blazes includes dozens of homes besieged by mudslides following the 2009 Station Fire in La Cañada Flintridge (Los Angeles County).
New computer models by the Geological Survey suggest that most burned parts of Sonoma, Napa and Mendocino counties face less slide risk than scorched spots in other regions, mostly because they're less mountainous and rugged.
"The terrain is not as steep or as high relief," said Dennis Staley, a research scientist with the Geological Survey, who mapped the likelihood of mud and debris flows across Northern California after the fires. "But, that being said, there are places within each of the burn areas that have high hazard levels."
Among the most vulnerable spots are Sonoma County's Mark West Creek area, the ridge between Sonoma Valley and Napa Valley, particularly Mount Veeder, Hogback Mountain and Hood Mountain, and parts of Napa County around Atlas Peak and Mount George.
Some of these places face an 80-to-100-percent chance of a mud or debris flow with just a short, intense burst of rain, defined as about a quarter inch in 15 minutes, according to the Geological Survey.
While mudslides can manifest in different ways after wildfires, the problem starts with a loss of protective vegetation to soak up water and stabilize a slope, coupled with soil that is both less porous and less cohesive after burning. In the most severely charred spots, the soil can develop a slick coating near its surface as a result of gases given off during the fire, making it impenetrable to water.
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When the rain comes, the ground is ripe for erosion, with the potential for runoff to snowball as it picks up dirt, branches and other debris. Slides can occur long after the rain, too, when a saturated chunk of hill collapses under its own weight.
On a recent weekday afternoon, Cal Fire Capt. Jim Mahoney worked with the Eel River Camp inmate fire crew from southern Humboldt County to clear trees with chain saws in Hood Mountain Regional Park to reduce damage should a landslide occur.
"Without these measures, all these trees could end up in the road," he said, pointing to a hillside of burned oaks and firs.
Amid the hum of saws and the heavy breathing that comes with lugging large branches, Mahoney said the work was nothing compared with the crew's last task: digging trenches for wattles, the straw rolls that are designed to soak up water and are now ubiquitous across Sonoma County.
Not far from the park, NOAA scientists and the Sonoma County Water Agency are looking to install new weather radar to help foresee flooding and mudslides. The equipment is part of a $19 million X-band radar system that eventually will ring the Bay Area with the aim of providing more localized forecasts.
Equipment for the top of Sonoma Mountain wasn't scheduled to be in place for at least several months, but federal officials moved the launch to January in light of the fires.
"That part of Sonoma County is in a real radar hole," said Rob Cifelli, an NOAA meteorologist and team leader at the Earth System Research Laboratory in Boulder, Colo. "We want to get something in there sooner than later to help emergency managers and the National Weather Service."
Long-term outlooks for this winter have offered little clue to whether the coming months will be wetter or drier than usual.
Umholtz, the ranger driven from his home, is hoping for fewer soakers this rain season, at least in the canyon behind his house.
When he recently stopped by the place, he said he was nervous about the hillside coming down in the midst of his visit. He still hasn't removed all of his family's belongings, just priority items like their dog, cat, two rabbits and two parakeets.
"It's almost the same feeling when we were evacuated from the fire," he said. "There's imminent danger, and you don't know how long you have."
While Umholtz said he and his family are lucky to still have a house, with so many others burned in the fires, he can't help thinking about the prospect of losing it after 12 years there.
"The hardest part is not knowing," he said. "Are we going to have a home to come back to?"