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A ‘high-end and dangerous’ Santa Ana wind event will dramatically escalate California’s fire risk starting Tuesday night

Historic windstorms on the west coast have ignited several wildfires in California in 2019. Here’s what you need to know about the Santa Ana and Diablo winds. (Video: Amber Ferguson/The Washington Post)

Just as California’s fire weather appeared to be abating somewhat, newly issued forecasts show that for tens of millions, the worst may be yet to come.

On Tuesday and Tuesday night, the focus remains on the San Francisco Bay region, where red flag warnings are in effect as the area suffers through its third offshore wind event in a week. Critical wildfire danger exists in the North Bay region, where the Kincade Fire is burning.

At the same time, however, residents of Southern California, particularly the hilly, heavily populated stretch from Ventura County southward to the San Diego area, are gearing up for a record-strong Santa Ana wind event slated to begin late Tuesday night and last through at least Thursday morning.

This Santa Ana episode has the potential to be the worst of the season and, coming on the heels of other offshore windstorms, it means that any ignition sources, like a stray cigarette or a sparking power line, would find extremely flammable vegetation that’s ready to burn.

Graphic: How Santa Ana winds spread wildfires in California

“You get these events back to back, and the fuels are bone dry and ready to go,” said Rob Elvington, a broadcast meteorologist with years of experience tracking California wildfires. “By the third event, it’s like they’re soaked in kerosene."

In sum, the number of people living in areas designated as being in either critical or extreme fire danger, which are the two worst categories, on Tuesday is about 20 million, according to the Storm Prediction Center. On Wednesday, the area enveloped by “extremely critical” fire risk, the most severe category, will expand to encompass 4.5 million, up from about 3.3 million Tuesday.

Residents of the Los Angeles area find shelter and supplies at the Westwood Recreation Center while the Getty and Tick wildfires continue to spread on Oct. 28. (Video: Peter Alton, John Farrell/The Washington Post)

According to the National Weather Service, the upcoming Santa Ana event is likely to involve a record-setting difference in air pressure for this time of year between coastal California and areas well inland. It’s these air pressure differences that are the engine that drives the Santa Ana winds.

Given that many Santa Ana wind events have occurred throughout history in late October and early November, the likelihood of a record-strong pressure gradient is especially concerning. In general, the greater the difference in air pressure over short distances, the stronger the winds will blow.

The ominous forecast details for Southern California

As of early Tuesday afternoon, the Weather Service forecast office in Los Angeles was predicting winds to reach as high as 70 mph in valley locations between Tuesday night and Thursday morning, with the peak risk coming early Wednesday morning into the afternoon. At higher elevations, winds could reach or exceed 80 mph, the agency said.

The cool, downsloping winds rushing in from the Great Basin region will compress the air as they descend from higher terrain, causing the air mass to become bone dry. Relative humidity levels in Southern California during the upcoming event are likely to be between 3 and 10 percent, though in some places they could get down to just 1 to 2 percent, the Weather Service predicts.

Even without any wildfires to worry about, such winds could topple trees and power lines and lead to coastal flooding on Catalina Island. In an online forecast discussion, the NWS called the upcoming event “a high-end and dangerous event.”

An “Extreme" Red Flag Warning

Any wildfires already burning, plus any that could ignite during this Santa Ana event, would be nearly impossible to control in such conditions. To emphasize the danger, the Weather Service forecast office in Los Angeles issued an “Extreme Red Flag Warning,” which technically isn’t in the agency’s lexicon (only a red flag warning is) but is intended to communicate the heightened risk.

“An extreme Red Flag Warning means that conditions are as dangerous for fire growth and behavior as we have seen in recent memory, due to the combination of strong winds, very low humidity, long duration, and very dry fuels,” the warning states.

Similar circumstances brought about the first-ever “tornado emergency” in Moore, Oklahoma in 1999. With a mile-wide tornado bearing down on the city, forecasters knew an ordinary tornado warning might not cut it.

“I think it’s that we don’t have any precedent for something like that,” said Ryan Kittell, a meteorologist at the L.A. office behind the “extreme” warning, referring to the forecast.

“Our forecasters used that language because this is a rare Santa Ana event,” he said. “It’s not your run of the mill event.” Kittell said that “everything’s primed and ready” if a fire gets started.

“A lot of people use the logic of ‘I’ve gotten through the last few events no problem,’” Kittell said. “Hopefully this will show folks this one’s not like previous events.”

“Be prepared to evacuate,” said Kittell. “I hope you don’t have to, but be ready.” In some cases, that might even mean having your car packed and ready to go, he said.

What powers these offshore winds

Both the Diablo winds in Northern California and the Santa Ana winds have the same origins. They typically occur when the jet stream, the high-altitude ribbon of powerful winds along which weather systems track, plunges south from Alaska and Canada into the Mountain West.

Once low-pressure areas riding along the jet stream land in Colorado, high-pressure zones build in quickly to their north and west over northern Nevada and Utah, a region known as the Great Basin. The difference in pressure over this relatively short distance starts generating these winds.

What’s driving the historic California high-wind events, and worsening the wildfires

The winds are then drawn westward toward California, where air pressure readings are lower (air flows from high to low pressure), and often, they gain speed passing through channels in the high terrain. Illustrating how powerful this weather phenomenon can be, wind gusts well above hurricane force were recorded over the weekend in the North Bay, near the Kincade Fire.

Over the past week or so, the jet stream has been sharply contorted, with a massive ridge of high pressure parked over the Gulf of Alaska and a sharp and deep trough to the east, which is ideally situated to drop low-pressure systems into Colorado, along with trailing zones of high pressure toward the Great Basin.

Here’s how a Weather Service forecaster in Los Angeles described the upcoming situation: “All three elements of a strong Santa Ana (Offshore flow, Upper level support, Cold air advection) are in place. Additionally most of these elements are also very strong and all of this will add up to be high end dangerous event. If everything comes together as forecast there will be 60 mph to 70 mph gusts not only in the mountains but in some of the valleys as well.”

The ongoing wildfires follow the devastating 2017 and 2018 California fire seasons, which featured the worst blazes in state history in terms of size, destruction and death toll. The Camp Fire, for example, which started Nov. 8, 2018, killed 88 people and destroyed much of the town of Paradise in a few hours.

The current fire siege is part of a clear pattern toward larger, more frequent and destructive blazes in the state. Fifteen of the top 20 largest wildfires in state history have occurred since 2000.

Although there are multiple causes, the flare-up in fire activity over the past decade or so has coincided with an observed trend toward hotter, drier and longer-lasting fire seasons. According to Cal Fire, “climate change is considered a key driver of this trend.”

Matthew Cappucci contributed to this story.