How California's firestorm spread so mind-bogglingly fast: From 'Diablo' winds to climate trends.

Source: DailyClimate 45b5 bcab%2fthumb%2f00001

The firestorm that engulfed large parts of Napa and Sonoma Counties in California on Monday will go down in history as one of the worst such events ever recorded in the Golden State. 

By the end of the day on Monday, at least 15 people had been killed, 1,500 or more structures destroyed, and hundreds injured by flames that moved so quickly one hospital had to evacuate patients using nurses’ own cars, rather than wait for ambulances to arrive. 

The fires show yet again how cruel nature is when the right combination of ingredients come together. Five months of unusually hot and dry weather following the state’s record wet winter ensured a ready supply of combustible vegetation. 

On top of these background conditions, there was a unique combination of weather conditions in place on Sunday night and Monday that ensured that virtually any fire that started would spread rapidly and unpredictably. 

Coffey Park, Santa Rosa California before the fire

Coffey Park, Santa Rosa California before the fire

Image: Google maps

Coffey Park, Santa Rosa California October 9th, 2017

Coffey Park, Santa Rosa California October 9th, 2017


In addition, operating just behind the scenes, like a puppeteer hiding in the shadows, is climate change, which is tilting the odds in favor of extreme heat events and larger fires, even as other factors — such as the buildup of sprawling suburbs close to forested areas — make us more vulnerable to damaging fires. 

The fires took thousands by surprise, giving many only enough time to grab their keys, pick up a beloved pet, jump in the car, and flee toward safety. The unlucky ones never made it out in time, and the number of fatalities is expected to rise. 

‘Diablo winds’

The weather conditions in place across California on Sunday night and Monday were well-known to forecasters as being associated with some of California’s worst wildfires. 

Hot, dry, and powerful winds were blowing down hillsides, from inland areas to the coast. Such air currents are known as “Diablo” winds in Northern California, and Santa Ana winds in southern parts of the state. 

On Sunday night, the Diablo winds reached as high as 79 miles per hour, which meant that any fires that started quickly raced ahead with the gusts. 

“Fire literally exploded and raced along the landscape,” the National Weather Service’s San Francisco office wrote in a forecast discussion on Monday. 

As the air, forced by the circulation around an inland area of high pressure, rushed down hillsides and accelerated through canyons, air temperatures increased, and humidity dropped. One weather station reported a temperature of 91 degrees Fahrenheit at 4:30 a.m. ET on Monday, though the NWS suspects that reading may have been due to a nearby fire.

A burnt out car sits in front of a home destroyed by a wildfire in Santa Rosa, California, on Oct. 9, 2017.

A burnt out car sits in front of a home destroyed by a wildfire in Santa Rosa, California, on Oct. 9, 2017.

Image: JOHN G. MABANGLO/EPA-EFE/REX/Shutterstock

“Clearly, the biggest single factor in creating yesterday’s firestorm were the powerful and incredibly dry east-to-west offshore winds…” said Daniel Swain, a climate scientist at UCLA. “These kinds of winds have a long history of fueling many of California’s most devastating and fast-moving wildfires over the years.

“This includes the 1991 Oakland Hills event, which (until now) had been the benchmark urban-interface wildfire in the Bay Area,” he said. 

The fires were so intense that descriptions of them are apocalyptic. According to a New York Times dispatch from a neighborhood in Santa Rosa, California, the fire burned virtually everything it touched:

Evidence of the fire’s intensity was everywhere in Coffey Park, which residents described as an apocalyptic scene. The aluminum wheels on cars melted and dripped down driveways like tiny rivers of mercury before hardening. A pile of bottles melded together into a tangle so contorted it looked like a Picasso. Plastic garbage bins were reduced to mere stains on the pavement.

Such extreme fire behavior isn’t random, or the result of a single factor. Instead, many ingredients must combine to create such monstrous conflagrations. 

“These winds were a necessary condition for the fires’ rapid spread, but the broader climate context certainly set the stage for the event to be as bad as it was,” Swain said.

Wettest winter, hottest summer

The fires capped off an epic case of weather, or in this case, climate whiplash. 

The firestorm is occurring on the heels of the warmest summer in California’s history, which featured numerous record-shattering heatwaves, including one that set a new all-time heat record for downtown San Francisco, at 106 degrees Fahrenheit. The hot, largely dry summer helped to dry out the vegetation that bloomed in the wake of Northern California’s wettest winter. 

Summer temperature departures from average for California, showing that 2017 had the hottest summer on record.

Summer temperature departures from average for California, showing that 2017 had the hottest summer on record.

Image: ncei

The wet winter, in turn, followed the state’s worst drought in modern history, which lasted for 5 years. 

Such weather whiplash, from cool, wet conditions, to hot and dry weather, is becoming a telltale sign of the new normal, as global warming reshapes the long-term climate and raises the odds of both extreme precipitation events and heat waves. 

Heat waves are one of the clearest, best understood indicators of human-caused global warming. Such extreme events are becoming more severe, frequent, and longer lasting as the overall climate warms, and this past summer, it was the West’s turn to roast. 

In July, normally hot Death Valley, California managed to reach a dubious milestone for the planet. With an average monthly temperature of 107.4 degrees Fahrenheit, Death Valley saw the warmest month ever recorded for any location in the world, according to the Washington Post’s Capital Weather Gang blog

The heat, particularly the relentless heat waves in September, played a crucial role in setting the stage for the devastating fires. 

“[The] record heat in early September capped an already extremely warm and dry summer to date, which acted to dry out vegetation even more than would typically be the case this time of year,” Swain said. He added that the heat, following the wet winter, causes seasonal grasses and brush to grow, which is known to firefighters as “fine fuels.” 

“Finally, the longer-term context of record multi-year drought just a year or so ago meant that some of the region’s forests remain residually stressed. The net effect: ‘fuel moistures’ (the amount of water in vegetation) were at or near record-low values when these wind-driven fires ignited, which almost certainly contributed to the extreme rate of spread and the likelihood of spot fire ignition during this firestorm,” Swain said.  

A horse runs from the flames from a massive wildfire, in Napa, Calif. on Oct. 9, 2017.

A horse runs from the flames from a massive wildfire, in Napa, Calif. on Oct. 9, 2017.

Image: AP/REX/Shutterstock

According to Noah Diffenbaugh, a climate scientist at Stanford University, global warming has increased fire risk across the West, and raised the odds that hot and dry conditions will coincide. 

“I am careful not to make any attribution statements without having run formal analysis,” Diffenbaugh said in an email. “That being said, we know that hot, dry conditions increase fire risk. Even though we had a wet winter this year, we know that the protracted drought drastically stressed vegetation in California (including killing tens of millions of trees), and that the summer and early fall have been extremely hot in California and the Bay Area.” 

“We also know from previously published work that [human-caused] global warming has increased fire risk in the western United States, and my published work has shown that global warming has increased the odds of the co-occurring warm and dry conditions that caused the California drought and the odds of extremely warm conditions in California,” he added.

“So, although we don’t yet have formal analyses of this particular event, we do have a lot of evidence that global warming has influenced the conditions in which the event is occurring.”

Given the property damage involved, these fires could rank as another billion dollar event in a year that’s turning into an extremely costly one — both in dollars and lives — for the United States. Given climate change projections for a hotter, drier future in the West, perhaps we’d better get used to it.

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