In the wake of the state’s worst wildfire season on record, these rains carry with them a high risk of debris flows in the vicinity of burn scars. They also showcase a feature of California’s changing climate, with a seesaw between drought, heat and fires, and sudden winter storm-related flooding.
The culprit for the heavy precipitation is a combination of strong storm systems embedded within the jet stream as well as a potentially significant atmospheric river that begins its arrival on Tuesday.
It’s quite a turnaround from California’s many months of heat, drought and fire, which lasted well into January.
“This week’s forecast is highly anticipated and highly welcome,” Cindy Matthews, senior service hydrologist with the National Weather Service in Sacramento, said in an email. “In most locations, the storms will bring beneficial rain and snow to the dry landscape.”
“We are forecasting a band of intense heavy precipitation in the early morning hours of Wednesday, that will increase the threat of debris flows,” she said, for burned areas between the North Bay and Central California (LNU Complex, CZU Complex, CSU Complex, Dolan, River and Carmel fires).
The wet pattern could begin to reverse the state’s steadily deepening drought, but it brings additional risks.
Life-threatening debris flows — commonly known as “mudslides” — could happen if the right conditions line up: Intense rain over badly burned mountainous land could send a slurry of ash, mud, boulders and even trees racing downhill, eventually spreading out to hit populated areas at lower elevations.
Those factors may well align this week given the scope of the impending storms and the severe nature of the 2020 fires, which left damaged vegetation and exposed soils. The state suffered five of its largest six wildfires on record during 2020, and many of them exhibited extreme behavior, exploding in size and severity within short periods.
“When you have high burn severity, the heat is so high that it creates a material on the surface that, rather than absorbs water, repels water,” said Steve Bohlen, head of the California Geological Survey. “That causes those of us who study [debris flows] a lot of concern.”
Atmospheric river to bring intense rain
The system that will bring heavy snow to Northern California and the Sierra will draw in tropical moisture and deliver a conveyor belt of rain to the California coast between Tuesday and Friday.
This landfalling atmospheric river will track south into the Bay Area on Tuesday afternoon and may stall along the Central Coast on Wednesday morning. “Models are currently highlighting the potential development of a narrow cold frontal rainband, which could produce high-intensity, short duration, precipitation, a driver of post-fire debris flows,” according to the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography.
The Bay Area National Weather Service office has posted flash flood watches for burn scars in its forecast area from Tuesday through Thursday, including those in the wine country region.
But the greatest concern remains the Santa Cruz Mountains and the Central Coast, which is likely to see heavy rain Tuesday night into Wednesday, with 6 to 8 inches of rainfall expected in these areas, possibly exceeding 10 inches in some spots.
The system will migrate into Southern California later in the week, although the exact timing is uncertain. “Stalling/pivoting moisture plumes with atmospheric river events are always tricky and we may not know more details until the event begins to unfold,” the Weather Service wrote in a technical forecast discussion.
High debris-flow risk for burned areas
California has a history of deadly debris flows from heavy rain falling across recently burned, steep slopes. For example, a January 2018 post-fire debris flow in Montecito, in Santa Barbara County, killed 23 people and destroyed more than 100 homes.
“Given the NWS flash flood watches that are in effect now, we are concerned about severely burned areas in Sonoma, Napa, Santa Cruz and Monterey Counties,” Bohlen said.
That risk could extend to Southern California as heavy precipitation moves south later in the week.
Dennis Staley, a research scientist with the U.S. Geological Survey in Golden, Colo., and his team have modeled and mapped the debris flow risk for all of the major Western wildfires during 2020, 31 of which are in California.
Their maps show the likelihood that a debris flow will reach the base of a mountain if certain rainfall rates materialize. For many areas within last year’s fires, those chances are as high as 80 to 100 percent.
“In order to have a debris flow initiated, you need rainfall that is sufficiently intense,” Staley said in an interview. While a 15-minute rainfall at a rate of one inch per hour is generally considered a concern, the required rain intensity actually varies widely by location, depending on the steepness of the terrain, the fire severity and other factors.
A group of agencies, including the USGS, the California Geological Survey, Cal Fire and the National Weather Service, are assisting local authorities in planning their emergency response and potential evacuation zones ahead of the debris-flow threat.
Because it’s impossible to outrun a debris flow, high-risk areas are typically evacuated ahead of a storm. In Santa Cruz County, emergency managers issued evacuation warnings on Sunday night, recommending that people pack a go-bag and prepare to evacuate.
“Our process is to issue 48-hour warnings and 24-hour evacuation orders prior to rainfall expected to meet certain thresholds,” said Jason Hoppin, a spokesman for Santa Cruz County. “Preventative evacuations are the only way to keep people safe in these scenarios.”
“There is a history of debris flows and landslides in the Santa Cruz Mountains even in unburned areas in the past,” Amy East, a research geologist with the USGS in Santa Cruz, said in an email.
During a 1982 rainstorm, a landslide killed 10 people in the Santa Cruz town of Ben Lomond. “We’re particularly focused on hazards here this week given that we now have the potential for a compounded problem from the storm rainfall on burned terrain,” East said.
Warming climate may mean more debris flows
With this atmospheric river event, California is lurching from one extreme, in the form of drought, heat and wildfires, to another.
According to recent research, this sort of wild whiplash from dry to wet extremes is projected to occur more often and severely with climate change, both year-to-year and within the same year, with longer dry seasons and wetter core winter months. Another study found that California is likely to see more of its annual rainfall delivered in richer, more intense atmospheric rivers, which raises flooding concerns.
That means longer fire seasons that bump up against potentially extreme winter storms, which would be a perfect setup for debris flows.
“We do know that in a warming climate the chance of fire and of extreme rainfall increase,” East said. “Over the long term, [the] increased likelihood of fire and extreme rain in California together mean the likelihood of debris flows here is higher.”
“After such a busy fire season, we were hoping for slow, soaking rainfall this winter,” Staley said. “But it doesn’t look like that’s going to happen, at least not this week.”