Fast-moving blazes kick-started California’s fire season this past week, a turnaround from a deceptively quiet summer — and one that might signal a relentless autumn. The culprit: a long hot spell, followed by a transition to more turbulent weather.

It started in Southern California, where firefighters wrestled the nearly 2,000-acre Tenaja Fire, struggling to protect homes. Then, in far-northern California, thunderstorm winds propelled runaway fires, including the Walker Fire, the year’s largest at more than 44,000 acres and counting. There were evacuation orders in Butte County over the weekend and again near Santa Barbara on Monday, where brush fires closed parts of U.S. 101.

“It’s been a very slow buildup; that’s why it seems so sudden,” said Alex Tardy, a meteorologist at the National Weather Service in San Diego.

The ramp-up

After a very wet and cool May, the summer months were remarkably calm, as dips in the jet stream or “troughs” continued to stall the fire season not only in California but also in the Pacific Northwest and the northern Rockies. However, the state started heating up in late July, and August was well above normal.

Last month’s prolonged heat dried out dead plants and stressed larger live plants and trees, making them more susceptible to fire. In many areas, it’s as if the wet winter and spring never happened because vegetation is right back to where it usually is in late August and early September — at peak dryness — and in some regions, below that. And this winter’s heavy grass crop not only ignites easily but also burns more intensely, with longer flame lengths.

But the tipping point was a bout with gusty thunderstorms last week and a high-altitude weather disturbance that is now bringing cooler but dry, windy weather.

Southern California: The Tenaja Fire

The Tenaja fire, which threatened neighborhoods in Murrieta, north of San Diego, “was our first major fire here in Southern California all summer,” said Matthew Shameson, a meteorologist with the U.S. Forest Service in Riverside.

This past Wednesday, afternoon temperatures were in the low 100s when thunderstorms hit with 40-45 mph gusts. The last gasp of the monsoon season that brings summer rains to Southern California’s deserts and inland mountains, these storms traveled farther west than usual and weakened. By the time they reached the valley, there was little rain left. That’s where the Tenaja fire ignited, probably by lightning, and rushed up hillsides of dry grasses and chaparral, growing to 1,400 acres by morning.

Shameson said erratic winds made the difference in this fire. “This fire had some wind on it, so it grew quickly,” he said. “When storms popped up again the next day, there was another push; it grew another 600 acres.”

Tardy emphasized that low-elevation fuels also played a role. “The concern is areas below 5,000 feet that had all kinds of growth,” he said. “That’s why they were struggling with the Murrieta fire initially.”

Northern California fires

On Wednesday and Thursday, that same monsoonal moisture plagued Northern California, where it converged with a trough in the Pacific to produce thunderstorms that moved too quickly to leave wetting rains but did generate 1,614 cloud-to-ground lightning strikes.

“Abundant lightning over dry fuels led to multiple ignitions, at least 128 confirmed and probably many more than that,” Brent Wachter, a meteorologist at the U.S. Forest Service in Redding, said in an email. “Some of the fires responded to the gustier winds and lower surface humidity on the 5th and grew big. Some were driven by gusty outflows like the Lone Fire in Modoc County. Some were driven by both nearby storm cells and straight-line wind, like the Walker Fire.”

All of this is happening against a backdrop in which shrubs and other “ladder fuels” have become much more flammable (though still at normal levels for this time of year), allowing surface fires to move into tree canopies. That’s how the largest two fires — the Walker and Red Bank — took off.

Trouble in the fall?

While Pacific troughs helped to shorten heat waves in summer, they could be a menace in the fall because they are bringing dry northerly winds and lightning to northern California. “Now seasonal dryness, weather, plus ignitions are aligning for large fire,” Wachter said.

Although the monsoon made a brief appearance last week, it was complete no-show in August, and that could spell trouble for fall fires in the Southern California mountains.

“August was not very kind to our mountains and our foothills because they’re used to getting rain,” Tardy said. “They literally didn’t have any precipitation in August.” Further stressing vegetation, mountain temperatures ran two to four degrees above normal “every single day” of the month, he said.

Forest Service meteorologists expect continued dry and hot weather through the fall, with a chance that Southern California’s fire season could extend well into December. According to the wildfire outlook from the Riverside office, there is no sign of rain on the horizon: “barring the influence of a dissipating tropical storm, offshore winds will almost certainly arrive well before the onset of meaningful rainfall this fall.”

The dry, easterly Santa Anas — and their sister winds in northern California — are the season’s biggest threat. “We’re going to get at least a few; it’s just a matter of how strong,” Tardy said.

“We haven’t been tested by the Santa Anas yet; they will test us by late September,” he said.