In Northern California, this year’s windstorms have shocked forecasters because they have been so closely packed together. After being bombarded by a windstorm last Wednesday and Thursday, and a stronger “historic” blast over the weekend, the area is bracing for a third surge Tuesday and Wednesday.
“I’ve been in this business for 28 years. I’ve never seen anything like this,” said Steve Anderson, a meteorologist at the National Weather Service’s forecast office serving the San Francisco Bay area.
Meanwhile, Southern California is bracing for its second dangerous wind storm in three days on Wednesday.
How these windstorms begin
Both the Diablo, or “devil,” winds in Northern California and Santa Ana winds to the south form during similar circumstances.
The stage is set when the jet stream, the high-altitude river of air along which weather systems track, plunges south from Alaska and Canada, and 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.
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.
The winds have the critical effect of drying out the air as the air descends after passing over mountain peaks. When the ultradry air overlays parched vegetation, tinderbox conditions develop that are ripe for extreme fire growth.
This year’s jet stream: a California windstorm machine
Over the past week or so, the jet stream has been ideally situated to drop low-pressure systems into Colorado, along with trailing zones of high pressure toward the Great Basin.
The resulting pressure difference has sent winds roaring toward Northern and Southern California. In Northern California, a surge of winds first hit Wednesday and Thursday last week, when the Kincade Fire erupted in Sonoma County.
Because of its duration, the Weather Service’s Anderson called the weekend wind event “historic, without a doubt.” By comparison, he said, the wind event that powered the Tubbs Fire in 2017, which burned through parts of Santa Rosa and was blamed for 22 deaths, had a peak wind gust of 92 mph and lasted six to eight hours.
Now, Northern California braces for a third straight high-wind event, predicted between Tuesday and Wednesday, while Southern California is forecast to endure another round of extreme fire conditions Wednesday and Thursday.
The danger in having high-wind events so close together is that they progressively dry out surface vegetation, turning it into explosive fuel, said Rob Elvington, a broadcast meteorologist who tracked California fires for eight years.
“You get these events back to back, and the fuels are bone dry and ready to go,” he said. “By the third event, it’s like they’re soaked in kerosene.”
Climate conditions preceding these wind events also helped remove moisture from the vegetation. Although California had a wet winter, it was followed by brief but record-breaking heat waves over the summer and fall, and the rainy season has yet to show up.
How climate change is probably intensifying fires
The ongoing wildfires come on the heels of 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 occurred on Nov. 8, 2018, killed 88 people and destroyed much of the town of Paradise in just one day.
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.”
Population growth and the increase in the number of homes and businesses near lands that typically burn, known as the wildland-urban interface, as well as certain fire suppression policies, are also escalating the risk of and damage from wildfires in the Golden State.
This fall, John Abatzoglou, a University of Idaho atmospheric scientist who studies wildfires, said that a “confluence of factors” is increasing wildfire risks, including a delayed onset of the wet season, extremely dry vegetation and the relentless rounds of powerful offshore winds.
Climate studies, he says, suggest that this year’s delayed rainy season is no coincidence, but rather part of a gradual trend shown in climate model projections. This would make it more likely that any offshore windstorms that do occur will encounter a drier landscape that will burn more readily.
Looking into the future, Elvington said an extension of the dry season could become especially problematic if vegetation remains dried out deeper into the fall, when the intensity of wind storms tends to increase.
“If climate seasons start to cross paths, this will start to set up big, dangerous [fire] events as late summerlike fuels mix with fall-like windstorms,” he said.
At the same time, Abatzoglou said, future offshore wind events in California may become slightly less intense and less frequent as the climate continues to warm, potentially offsetting some of the effects from a longer, hotter and drier fire season. This is because climate change is expected to moderate the intensity of the cold high-pressure systems over the Great Basin, which help drive the Santa Ana and Diablo winds.
Yet when these wind events occur, they may encounter milder and drier air masses along the California coast, leading to more extreme fire behavior and canceling out any beneficial effects of any reduction in wind speeds.
“This is still an area of active research,” Abatzoglou says.