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‘Significant fire season slowing’ rain set to soak Northern California

But forecasters warn the fire risk could return if the state has a hot, dry and windy fall

A firefighter battles flames from the Mosquito Fire on Thursday in Ramsey Crossing, Calif. The blaze officially became the state's biggest fire of the year and is still only 20 percent contained. (Eric Thayer/Getty Images)

An unusual September storm is set to arrive in California late this weekend, providing a much-needed pause on the state’s rapidly deteriorating wildfire season.

Forecasters welcome the incoming rain but stress it is unlikely to end the fire season, considering how hot and dry the state has been for so long. A return of hot, dry and windy conditions after this rainy spell could once again increase the fire risk.

Wildfires have turned destructive and deadly amid record-breaking September heat. But at least some help is on the way.

“A significant fire season slowing event is expected for most of the region,” beginning late Saturday and possibly lasting into midweek, according to a forecast from the Northern California Predictive Services office, a branch of the National Interagency Fire Center.

The surprise early-season storm stems from ex-Typhoon Merbok’s sprint into Alaska, which has disturbed the jet stream, causing it to dip southward along the U.S. West Coast and channel moisture into California.

How a Pacific typhoon could help extinguish California wildfires

The weather system is predicted to bring widespread and beneficial rainfall to the northern half of the state, generally ranging from a half inch to 2 inches, with higher amounts possible in the coastal ranges and the northern Sierra Nevada. The Intermountain West is also expecting a moisture influx this week, helping to further quell fire activity in Idaho and Montana, where several large wildfires are burning.

Strong winds, then rain, for California’s largest fire

Before the rain arrives in California, strong winds ahead of the storm could cause existing blazes to spread further.

Current forecasts call for strong winds across much of Northern California into Nevada on Saturday, including over the Mosquito Fire, more than 69,000 acres and the state’s largest so far this year. Peak gusts could range from 25 to 40 mph with higher gusts possible in mountainous terrain.

“Obviously, this will be a concern for any ongoing or new fires before that rain sets in,” said Edan Weishahn, senior meteorologist with the National Weather Service in Reno, Nev.

Record-dry vegetation has driven volatile fire behavior on the Mosquito Fire, including a major flare-up on Tuesday that destroyed structures and threatened the town of Foresthill.

Firefighters are bracing for significant growth on the eastern side of the fire on Saturday, before conditions improve when the core of the rain arrives later in the weekend.

“This is a slowing down of the fire season and a slowing of the fire progression, especially coming off this extreme heat and dryness,” said Eric Kurth, an incident meteorologist on the Mosquito Fire, who cautioned that the upcoming rain doesn’t necessarily mean quick and easy containment of the fire. “This is a big fire, and it’s been quite active, so we’ll see.”

That point was echoed by Weishahn.

“The fuels are still critically dry, and warm and dry weather will follow this rain,” she said.

Rain can shape the autumn fire season

The timing of autumn rain — how much falls and when it falls — is extremely important in determining whether and where big autumn wildfires occur. These destructive fires, which often strike in October, are driven by fierce winds that intensify as the state’s months-long dry season winds down.

According to Dan Cayan, a climate scientist at Scripps Institution of Oceanography in San Diego, records from Southern California show that, since 1948, there have been nearly four times as many large fires before the onset of autumn precipitation.

Recent rain from the remnants of Tropical Storm Kay brought several inches of rain to higher terrain in far Southern California, but coastal areas of Los Angeles saw much less moisture.

“This hasn’t painted the entire region with a ‘free pass’ for the fall fire season,” he said. “It doesn’t mean that the fire season is eliminated this autumn — there are still areas in California that are vulnerable.”

With vegetation drier than ever in a warmer climate, rain has become even more crucial to reining in fire risk as the season progresses. Heat waves are becoming more frequent and intense, both days and nights are warming, and mountain snowpack has declined and is melting earlier.

“The atmospheric demand for moisture goes up as the temperature goes up,” he said. “And, consequently, these western landscapes are drying.”

Given the extreme dryness this September, repeated rains, rather than a single storm, are needed to substantially reduce wildfire risk this autumn.

“These precipitation events give us a couple of weeks of indemnity — after that there could be another fire,” Cayan said.