A U.S. Forest Service firefighter crew arrives at the scene where flames from the Caldor Fire threaten to jump Highway 50 in Meyers, Calif., on Aug. 31, 2021. 

Northern California is experiencing a lull in peak wildfire season.

A few weak storms recently delivered light rain, allowing firefighters to get the upper hand on blazes that have burned for weeks. The gargantuan Dixie Fire spread across five counties 230 miles north of San Francisco was 94% contained on Sunday, the U.S. Forest Service reported.

The Sierra Nevada and Tahoe Basin saw light widespread snow Friday, the first of the season. Snowflakes were a welcome sight over areas of the Caldor Fire, a terrifying blaze that threatened South Lake Tahoe over the summer and was 98% contained on Sunday. 

Guy Tucker photographed the Caldor Fire 3 miles due west from Fair Play, Calif., Aug. 16, 2021.

But fire officials warn that fire season is far from over, and as the region moves into the second part of its season, forecasters are predicting above-normal fire risk for much of Northern California in October and November. 

The strongest wind event so far this year is hitting the region Monday into Tuesday, with desiccating gusts knocking mountains and howling through valleys covered in vegetation that is at its most flammable point of the year because of two consecutive dry winters, lack of summer rain and plants going into dormancy. 

"It seems like we’re starting to turn the corner, but I’ll always fall back on what happens historically," said Robert Foxworthy, a Cal Fire spokesperson. "We haven’t seen a lot of those big winds yet. As soon as we get those drying winds, we’re right back into it. We’re not out of the woods until we’ve had a true transition into the wet season where we get storm after storm after storm."

🚨 🚩Red Flag Warnings 🚩🚨are now posted for the North Bay Mountains and East Bay hills and valleys as well as the Santa Cruz Mountains, Santa Lucia Mountains, and the interior mountains of San Benito and Monterey Counties.#CAwx #CAfire pic.twitter.com/Oe1NkGX58w

While Northern California's fire season is year-round and a fire can happen in any month — especially amid a changing climate when droughts are becoming more common in the state — there are generally two distinct seasons, Foxworthy said.  

In the summer months, the state usually experiences its largest fires that torch vast areas of acreage in inland areas and at higher elevations. That was the case this summer, with the Dixie Fire, the Caldor Fire, the River Complex and the Tamarack Fire — which all burned across the state's mountain ranges. In summer, inland California sees its hottest and driest months, and these fires thrive in that climate.

The most destructive fires that encroach into urban areas usually occur in the second half of the season in fall — when dry vegetation coupled with strong offshore winds and low humidity levels lead to the sorts of catastrophic fires we've seen in recent years. Take the 2017 fires in Napa and Sonoma counties that spread wildly amid an offshore wind event in October, and the Camp Fire, the state's deadliest and most destructive fire, that destroyed the entire town of Paradise in November. 

This part of the season can extend into winter, especially in Southern California, and even into spring in years where the winter was dry.

Offshore winds can occur year-round but they're significantly more common in spring and fall. They're less concerning in spring as the vegetation on the ground is still holding onto moisture and less fire prone. But in fall, the breezing conditions trigger red flag warnings from the National Weather Service, signaling high-risk fire conditions when a single spark can explode into a massive blaze in hours.

"If we got those winds in early summer when the grass is still green and everything is still wet, we can get hot temperatures and winds, but we wouldn’t get the same fire as in fall," Foxworthy said. "In fall, when we've had no rain for multiple months, everything is as dry as it’s going to get, everything is as primed to burn as it’s going to be." 

Heavy rain in October can help prevent fall fires, but wet weather at the start of fall is becoming increasingly less common due to climate change. 

"The weather starts cooling in October and we can get our first rains during this month," Craig Clements, a San Jose State University professor and director of SJSU's Wildfire Interdisciplinary Research Center. "That is happening less with climate change as some studies have shown."

The San Francisco Bay Area has yet to see a major damaging wildfire this year. While interior California saw record heat this summer, most of the coast experienced below-normal temperatures and fog. Parts of the Bay Area even received a few hundredths of an inch of rain from two weak early fall storms. But the weather on the coast is shifting into a fall pattern that's marked by less moisture-rich fog and more drying offshore winds. A single wind event can easily dry out any moisture that the fog and light drizzle provided.

"The weather is really the critical factor for us in the Bay Area," said Jonathan Cox, a Cal Fire chief who oversees fire operations at the San Mateo County Division. "When we have the normal fog and high humidities, the fire risk is lower, but that quickly changes when there’s no fog and winds pick up."

Cox noted that the CZU Lightning Complex fire in the Santa Cruz Mountains in 2020 exploded during a week without fog.

Among the most important factors impacting fire risk is the fuel moisture content of the vegetation on the ground. FMC is a measure of the ratio of moisture to combustible material in plants that indicates how prone they are to burning.

Cox said in the Bay Area, the FMCs are extremely low and the Pulgas site that Cal Fire monitors in San Mateo County is at its lowest level since monitoring began in 1977. There are signs of drought-induced die-back in the coyote brush in this area, with shrubs losing nearly all their leaves and 80% of the remaining leaves yellowing or dying, a report from Cal Fire said. 

"Until we get 2 to 3 inches of rain, we are highly concerned ... we’re not out of the woods yet," Cox said of the fire risk in the S.F. Bay Area this fall. 

Amy Graff is the news editor for SFGATE. She was born and raised in the Bay Area and got her start in news at the Daily Californian newspaper at UC Berkeley where she majored in English literature. She has been with SFGATE for more than 10 years. You can email her at agraff@sfgate.com.