Fire weather arrived in San Diego this weekend, traveling southward from Butte County via Ventura and Los Angeles.
Its journey here gave us snapshots of a grim new reality: the unbelievable speed and scale of California wildfires. A line of burned-out cars in the Sierra foothill town of Paradise, and social media accounts of desperate attempts to outrun the Camp Fire. Four one-way lanes on the Pacific Coast Highway during a mass evacuation of Malibu to escape the Woolsey Fire.
The weather pattern behind the harrowing last week – hot, dry winds blowing from the east – is not unusual. Known as Santa Ana winds  in Southern California, they form when a cold air mass settles over the intermountain Western states, and high pressure there drives air toward lower pressure off the California coast. The winds heat up and dry out as they move downslope and get gusty as they funnel through the mountains, like the Santa Ana, Santa Monica and Laguna coastal ranges.
Common during the cooler months of November, December and January, Santa Anas can be lethal if we don’t get enough autumn rain.
“Each passing week without decent rain in the fall leads to added chances of a Santa Ana that may make an ordinary fire that is contained into a fire that we talk about for decades later,” said John Abatzoglou, an associate professor of geography at the University of Idaho who studies climate and wildfire. “The water year that officially commences on Oct. 1 has been a dud so far for nearly the entire state of California. Precipitation totals for the last six weeks near the Camp Fire are 80 percent below average.”
In San Diego County, parched vegetation means potentially explosive wildfires here as well, especially after a near-record dry 2017-18 water year and one of the top five hottest summers on record.
“All fuels are at critically dry levels,” said Alex Tardy, a forecaster with the National Weather Service in Rancho Bernardo. “They are well below average even for this time of year.”
Even though San Diego did see some downpours one night in October, and ended up with a normal .57 inches of monthly rainfall, the benefit was fleeting. Four Santa Ana wind events followed those rains and dried everything out again.
“The rain is like a Band-Aid; it helps for a week or two,” Tardy said. “But it’s like washing your clothes and then putting them in the dryer. You are back to square one.”
When vegetation is this dry, a small brush fire can erupt into a conflagration, and the winds can make it nearly impossible for firefighters to control. But the winds themselves this week were not extreme.
“The winds were stronger than your average Santa Ana, but not record-setting.” said Nick Nauslar, a fire weather forecaster with the Storm Prediction Center in Norman, Oklahoma. “However, when considering the very dry fuels, very low relative humidity and strong downslope, offshore winds across Southern California, especially in the mountains, it’s a very conducive environment for large, fast-moving fires.”
The Storm Prediction Center issues fire weather outlooks  for the nation, based on a suite of factors including winds, low humidity, temperature and vegetation dryness. They warned of critical conditions ahead of the Camp Fire in Butte County. They also issued rare “extremely critical” alerts for Southern California coastal mountains throughout the last week, including the Laguna Mountains in San Diego County on Sunday through Tuesday. “‘Extremely criticals’ are the last bullet we have to say that something truly unique is possible,” said Patrick Marsh, a warning coordination meteorologist there.
Much of the risk comes from a drought-stressed landscape that is increasingly prone to very large fires.
Tom Rolinski, senior meteorologist at the U.S. Forest Service in Riverside, developed a tool to forecast those large fires in Southern California, the kind that devastate communities and steal lives. The Santa Ana Wildfire Threat Index  has shown skill in doing what it was designed to do: anticipate the days the worst fires could happen. It has worked well because, in addition to wind speed, it includes a sophisticated estimate for fuel moisture. For Dec. 7, 2017, the day the Lilac Fire started in San Diego, the index reached its highest level (purple or “extreme”). The day the Woolsey Fire roared through the Santa Monica Mountains, it was at its second highest level (red or “high”) in Los Angeles/Ventura. That level was also the forecast for San Diego County on Monday and Tuesday of this week, with the worst conditions expected in the backcountry.
“When that index lights up in the higher categories – red and purple – you should have a bag packed, have your valuables and be ready to go. It’s that bad,” Rolinski said.
After this tragic week, and an exhausting year, how much rain would it take to finally end the 2018 fire season? Rolinksi and Tardy stressed that it will take repeated rains to lift the region out of fire danger.
“It’s really more about the frequency of rainfall than the amount,” Rolinski said. “Once our grasses start greening up, that really starts to mitigate the fire threat. Grasses ignite easily and carry fire into the heavier brush.”
As the weather pattern driving our offshore winds shifts east, our winds will return to their normal flow off the ocean. There is even a hint of rain in the forecast for Thanksgiving week.
But the threat from fast-moving, uncontrollable fires will be a worsening problem as the climate warms.
“We do expect increases in the frequency of critically dry fuels in California and much of the West in the coming decades,” Abatzoglou said. “Of direct concern to Southern California is what happens in autumn, and how often there is an alignment of critically dry fuels with offshore winds.”
The last two years may offer a preview of what lies ahead. In December 2017, a very late start to the rainy season and record autumn heat, combined with the longest known Santa Ana wind event, drove the Thomas Fire in Ventura and Santa Barbara. It was the largest fire in California history, until the Mendocino Complex broke that record this summer.
Abatzoglou said that just one well-timed Santa Ana before winter rains arrive is enough to wreak havoc.
“We saw this play out last year, this year, and we should expect this will play out in many years to come. The question is how can we better prepare communities to cope with the inevitability of these fires,” he said.
For Nauslar, watching the events unfold on the ground after issuing the forecast was “just absolutely gut-wrenching,” he said. “We had the notice out there but sometimes there are certain conditions with climate, weather and fuels that we can’t overcome.”