A combination of dangerous weather is fueling rapidly spreading wildfires from the Sierra Nevada to the canyons of Ventura County. This extreme weather event is occurring during the time of year when desiccating and damaging offshore winds tend to rage in parts of California.

In Northern California, these winds are known as Diablo winds, while in Southern California, they’re given a slightly less frightening name: Santa Ana winds. In both cases, they form from weather systems over and around the state that in a certain configuration can funnel air at high speeds through the narrow canyons around Los Angeles, for example, and from mountain peaks to valleys in California wine country.

As the air is compressed, it tends to heat up, leading to temperatures along the coastline well above average. Highs in the Los Angeles and San Diego metro areas on Thursday were into the 90s, and Marine Corps Air Station Miramar was the hottest spot in the Lower 48 states, with a high of 102 degrees.

Winds gusting above hurricane force have been fanning the flames of the Kincade Fire in Sonoma County, while bone-dry vegetation acts as a tinderbox of fuel. The state has an abundance of dry vegetation: Although the wet season delivered copious amounts of moisture, it was followed by a warmer-than-average summer that dried out plants.

It’s an explosive scenario that will only get worse this weekend, when an even stronger round of offshore winds is on tap, particularly in Northern California. Computer model guidance suggests a third round of perilous fire weather may be coming next week.

Friday’s setup

On Friday, the greatest fire risk will be in southern parts of the Golden State, although Northern California is not out of the woods, either. Extremely dangerous fire weather remains in the offing for southern parts of the state, with pockets of elevated and critical conditions in and around the Los Angeles metro area.

In Los Angeles, a red flag warning is in effect, meaning that conditions are “favorable for extreme fire behavior and rapid growth.” Temperatures will skyrocket into the mid- to upper 90s in many areas, with relative humidity as low as 2 to 5 percent. This will rob ground vegetation of moisture, creating more dry plant fuels. “The fuels and vegetation are critically dry,” wrote the National Weather Service in Los Angeles.

Meanwhile, sustained northeasterly winds of 25 to 40 mph, gusting up to 60 mph in the foothills and 70 mph in the mountains, will support potential fires spreading at breakneck pace.

“The expected weather will create an environment ripe for large and dangerous fire growth,” the National Weather Service in Los Angeles said in its morning forecast discussion. “We urge everyone to be extremely cautious.”

Current fires

Pacific Gas and Electric, California’s largest utility company, is working with state regulators to determine whether a broken piece of equipment on one of its transmission towers may have played a role in sparking the Kincade Fire in Sonoma County on Wednesday night. The blaze is at only 5 percent containment and has burned 21,900 acres.

In Southern California, the Tick Fire is also 5 percent contained, having already charred nearly 4,300 acres. It jumped Highway 14 about 15 miles north-northwest of Burbank, Calif., near Bee Canyon. That fire has prompted nearly 50,000 to evacuate from the Santa Clarita Valley.

Numerous additional fires remain ongoing, and more blazes are anticipated, given the combination of high winds and desert-like air.

To try to prevent wildfires from igniting, electric utilities are preemptively shutting down parts of their networks, which is also causing widespread economic impacts.

At least 189,000 people remain in the dark from PG&E’s program of preemptive blackouts, which took effect late Tuesday night into Wednesday morning in the affected areas. This is the second major blackout in two weeks, and officials are warning that there could be a bigger round of power cuts to PG&E’s network this weekend if current forecasts pan out and winds across a large swath of Northern California exceed 65 mph.

There is no word yet on when service will be restored.

Smoke: a public health concern

The strong winds responsible for fanning the flames are also carrying smoke and fine particulate matter, which are dangerous for public health as the tiny particles can get lodged in the lungs. The smoke is especially dangerous for those with respiratory illnesses.

In San Francisco, smoke from the Kincade Fire is expected to arrive late Friday morning into early afternoon. Coupled with anomalously warm temperatures in the upper 80s to lower 90s, it presents a health concern for those outdoors.

“The smoke could reduce visibilities at times today,” wrote the National Weather Service in San Francisco, “especially as northwest winds develop this afternoon and transport smoke into the Bay Area.” The visibility at Sonoma County Airport dropped to nine miles Friday morning, and smoke could cause delays at San Francisco International Airport and Oakland Airport later in the day.

The Los Angeles School District canceled all classes in the San Fernando Valley, where a temperature inversion may trap pollutants from the Tick Fire near the surface during the morning hours. The school district cited poor air quality and safety concerns, which are expected to linger through at least the rest of the day.

The next big fire weather event comes this weekend

Conditions are expected to become even more volatile this weekend, especially in Northern California, beginning Saturday night. There’s also the potential for a third round of strong winds and bone-dry air settling in toward the middle of next week.

Low pressure over the Four Corners region, with high pressure banked to the north, will slingshot a wedge of cool, continental air over the Rockies. That air will ride downhill, drying and warming as it does so, and accelerating downward as it pushes west.

By the time that air arrives in the foothills and valleys of California, dangerously low humidity will accompany the strong winds. In central and northern California, sustained winds of 45 to 55 mph are possible, with gusts up to 75 mph atop the highest peaks. It has all the makings of a long-lasting, extremely ominous event. The winds will be strongest in central and northern areas.

The National Weather Service in San Francisco is already sounding the alarm, warning that “the forecast models put this on par, strength wise, with the 2017 offshore event that resulted in the deadly and devastating North Bay/Wine Country Fires.”

High wind and fire weather watches have been issued for much of the Central Valley, Diablo Range, the North and East Bay, Sacramento Valley, and much of central California from Saturday night through Monday morning.

Meanwhile, PG&E has outlined areas where a potential public safety power shut-off is under consideration. Early estimates suggest that more than 2 million people could be affected, which would be a record, although that number will be refined as the event gets closer.

The worst fire weather will likely arrive late Saturday and stretch through at least midday Monday for central and northern California. It will be less severe in Southern California but still dangerous.

The National Weather Service in Sacramento summed it up starkly Friday morning: “Be ready to evacuate.”

Overall weather pattern favors repeat spikes in fire risk

An entrenched weather pattern involving a succession of cold weather systems, or troughs, into the mountain states east of California is what is driving California’s repeated dry events during October.

“It definitely is a little more unique this year,” said Evan Bentley, a fire weather forecaster with the Storm Prediction Center in Norman, Okla. “We’ve been stuck in this pattern, and until we get out of it, we could see continued events.”

Typical during fall and winter, these troughs leave behind a cold air mass with much higher surface pressure than exists off the California coast. The resulting pressure gradient sends desert air west, which heats and dries as it moves down the Sierra’s slopes — a process Bentley describes as a true desert weather pattern that is reversed into California.

“Every year we get these cold troughs that come into the West. If they set up further east, in the Plains states, the pressure gradient isn’t as strong,” he said. But if that cold air takes up residence closer to California — as it continues to do this month — it’s a recipe for potent downslope and offshore winds.

On Friday morning, the “extremely critical” conditions for San Diego’s inland mountains, including 30 mph sustained winds with gusts up to 75 mph and single-digit humidity, could be traced to a high-pressure center over Colorado.

On Saturday and Sunday, as another cold system moves into western Montana, “confidence is high that the strongest offshore wind event yet this season will take aim at a broad swath of Northern California,” according to the National Weather Service. Damaging northerly winds are expected to slam the Sacramento Valley, while gusty northeasterly winds could mix down to lower elevations of the Bay Area, which, when combined with “critically low humidity” and dry fuels, would mean extreme fire danger.

The multiple rounds of hot, dry and windy weather, described by Bay Area meteorologist Rob Mayeda as “an atmospheric hair dryer,” progressively dry out vegetation. “Until you get the first good wetting rains of the fall, you just continue to get drier and drier with each event,” Bentley said.

Climate change is one of the likely culprits

The ongoing wildfires come on the heels of the devastating 2017 and 2018 California fire seasons, which featured the largest, most destructive and deadliest blazes on record. The Camp Fire, for example, on Nov. 8, killing 88 people and destroying much of the town of Paradise. An investigation concluded that the fire began from a spark generated by a PG&E power line.

The current fire siege is part of a clear pattern toward larger, more frequent and destructive blazes. The fire season in California is lasting far longer than it used to, as spring snowmelt occurs earlier and dry seasons become hotter and longer-lasting. And, according to CalFire, “climate change is considered a key driver of this trend.” (It is not, however, the only driver.)

“Warmer spring and summer temperatures, reduced snowpack, and earlier spring snowmelt create longer and more intense dry seasons that increase moisture stress on vegetation and make forests more susceptible to severe wildfire,” the agency wrote. “The length of fire season is estimated to have increased by 75 days across the Sierras and seems to correspond with an increase in the extent of forest fires across the state.”

Population growth and the increase in the number of homes and businesses near lands that typically burn, known as the wildland-urban interface, are also escalating the risk of and damage from wildfires in the Golden State.

In an indication that California is suffering an uptick in large wildfires, 15 of the top 20 largest wildfires in state history have occurred since 2000.