Temperatures reached ghastly levels in southern California and wildfire carved a path close to two of the state’s iconic national parks as a historic heat dome gripped the western United States during the traditional end-of-summer Labor Day weekend.

The heat had eased somewhat by Tuesday, September 8, but increasing winds were making the fire threat even worse across sun-baked landscapes from Washington to California.

“A potentially historic fire weather event is unfolding across the West, especially over the West Coast, during the next three days,” tweeted meteorologist Nick Nauslar (National Interagency Fire Center, or NIFC) on Monday.

Offshore wind season – the period in autumn when California’s most destructive fires tend to rage – is just getting under way. New research indicates that the state’s autumn fire season has gotten more dangerous in the past 30 years, and human-induced climate change may only accentuate this trend in decades to come.

As of Tuesday morning, California’s uncontained Creek Fire had torched more than 135,000 acres just northeast of Fresno, in between Yosemite and Kings Canyon national parks. More than 200 people were rescued by military helicopter from the fast-growing fire, which surrounded the Mammoth Pool Reservoir area late Saturday. At one point, people were advised to shelter in place and jump into the reservoir if necessary. Nearly 100 more people, mainly trapped hikers, were rescued by helicopter on Tuesday morning from the Lake Edison area.

The Creek Fire helped push 2020 ahead of 2018 for the most area burned by wildfire in modern California history, with roughly 2.1 million acres affected statewide as of Monday, according to Daniel Swain (UCLA/National Center for Atmospheric Research).

Parts of Southern California endured the hottest day in their history on Saturday. Temperatures hit 121°F at Woodland Hills – the first official reading of at least 120°F ever recorded in Los Angeles, Santa Barbara, Ventura, or San Luis Obispo counties. Likewise, a scorching 121°F at Chino was an all-time high for the vast Inland Empire region of San Bernardino and Riverside counties.

As noted by the New York Times, Southern California Edison reported that electricity use soared more than 8% above the previous daily record on both Saturday and Sunday.

Climate change is worsening California’s hellish wildfires

Well beyond California, extreme heat bathed much of the western U.S. underneath a powerful dome of high pressure that extended miles above the surface. A train of energetic impulses in the jet stream, boosted by powerful Typhoons Maysak and Haishen in the Northwest Pacific, helped pump up the western heat dome. As a whole, forecast models accurately predicted the dome’s intensification and the resulting extreme heat, although that was little comfort to those enduring the blast furnace.

One index of the heat’s intensity – the height of the 500-hectopascal surface, which is roughly the midpoint of the atmosphere and which rises as the lower atmosphere warms – set an all-time record in Medford, Oregon, on Sunday evening, reaching 6,030 meters, one of the highest such readings on record for the entire nation.

On the heat dome’s eastern fringe, the High Plains town of La Junta, Colorado, hit 108°F on Saturday, setting a state record high for any September. Denver reached 101°F, also a September record. About 60 miles northwest of Denver, the Cameron Peak Fire exploded to cover more than 102,000 acres by Tuesday morning, making it the state’s fifth-largest wildfire in modern records.

In some parts of the West, sun-blocking smoke provided an ironic trimming of the intensity of the record heat.

Blast of Canadian air a double-edged sword for western fires

By Tuesday morning, a shockingly strong cold front for early September was giving Colorado relief from the heat and parched conditions. Light snow was falling on Denver with a temperature of 33°F, less than a day after Monday’s high of 93°F – a case of severe weather whiplash.

The same front easing fire concerns in Colorado was aggravating them in the Pacific states. The upper-level impulse driving cold air across the Plains pushed strong, dry winds westward across the Rockies, generating gusts to 99 mph that knocked down trees in northern Utah. As the surge continues toward the Pacific coast from Washington to California through midweek, it will warm up and dry out – a classic recipe for autumn wildfire risk.

In central Washington, the Pearl Hill/Cold Springs fire complex tore more than 50 miles southward in less than 24 hours, by Monday night burning more than 180,000 acres. Meanwhile, some 80% of homes in the eastern Washington town of Malden were destroyed by the Babb Fire, according to the county sheriff.

Other fires erupted Monday night into Tuesday in and near the Cascades of western Washington and Oregon.

Autumn fire risk is climbing as California warms

Although California’s average precipitation has held steady in recent decades, there’s trouble hidden in that reassuring-sounding statistic. The state famously lurches between wet and dry years, and temperatures are on the rise.

California has warmed close to 1 degree C (1.8°F) over the past 30 years, with most of that heating coming in late summer and early autumn. Even without any change in precipitation, long-term warming can increase fire risk by pulling more moisture out of parched landscapes. That’s especially true of California’s Mediterranean climate, where rains and mountain snows are concentrated from fall through spring and the summers are notoriously dry.

While many of the state’s largest fires occur in late summer, the deadliest and most destructive blazes tend to occur at the tail end of the dry season, from September to December. The most hazardous setups are when strong downslope/offshore winds (typically called Diablo winds in northern California and Santa Ana winds in southern California) arrive before the first heavy rains of autumn.

There’s now evidence that climate change is boosting the risk of extreme wildfire conditions in California autumns, according to an August study published in Environmental Research Letters and led by Michael Goss of Stanford University.

Examining weather data for the period from 1979 through 2018, Goss and colleagues found that the most extreme 5% of fire-weather days in autumn – as measured through an index based on temperature, humidity, winds, and precipitation – are now occurring more than twice as often as in the 1980s. In turn, these high-end days are closely associated with extreme autumn wildfires. The authors highlight two from November 2018: the Camp Fire, which decimated the town of Paradise in northern California, and the Woolsey Fire, which rampaged across parts of Los Angeles and Ventura counties.

When the authors carried out similar calculations for future climate, building on a much-used set of simulations called CMIP5, they found that extreme fire-weather days in autumn are likely to become more frequent as the century unfolds, and the most fire-prone autumns more often will tend to affect both northern and southern California at the same time. “Increased synchronicity of extreme fire danger between northern and southern California has the potential to hamper fire suppression and risk-reduction efforts, particularly as longer fire seasons increase fatigue among firefighters and evacuated residents alike,” the authors noted.

One semi-bright spot: if global greenhouse emissions peak by around 2040 (the RCP 4.5 scenario), the number of top-5% fire days will increase by only about half as much this century as would be the case in the high-end scenario (RCP 8.5), where emissions continue unabated through the century.

Other work suggests California will shift toward a wetter winter and drier shoulder seasons, including fall, although the crucial autumn timing – those periods when high winds strike before substantial moisture arrives – remains a topic of active research. Moreover, as the new study notes, fire is a natural part of the western landscape, and the explosion of wildfire in recent years is the result of a human-changed climate mingling with long-term patterns of land management, the flocking of people into the wildland-urban interface, and other trends.

In a tweet, Swain, a coauthor on the new paper, wrote “My approach is to be straightforward about effect of climate change (it’s making problem worse), while making clear that more fire on landscape is inevitable – it’s just a question of whether we manage it for good, or we alternate between ever greater suppression & firestorms.”

Website visitors can comment on “Eye on the Storm” posts (see below). Please read our Comments Policy prior to posting. (See all EOTS posts here. Sign up to receive notices of new postings here.)

Posted on September 8, 2020 (9:08pm EDT).