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During the deep of California's winter, telltale plumes of smoke are rising from the hills.

Midwinter Western fires are unusual because the cooler, wet season dampens the potential for flames. Yet California's fire regime is changing, creating the opportunity for unexpected wildfires, particularly in Northern California.

In early 2021, a potent combination of dry weather and a warming climate have produced fuels — grasses, shrubs, and trees — that can ignite in winter. Though the relatively small wildfires (many less than 100 acres) aren't nearly of the scale and intensity of summer or fall fires, numerous blazes burned in fire-weary regions like Sonoma and Santa Cruz counties early this week. A robust though small fire also burned near Yosemite National Park on Tuesday, a place people can often ski and snowshoe this time of year.

Northern California should not support these fires during the winter, said Daniel Swain, a climate scientist at UCLA and the National Center for Atmospheric Research.

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"Clearly, this is not normal, and I think that might understate it a little bit," Swain emphasized.

Many of the state's trees and shrubs, after being parched by record heat waves and profound dryness in 2020, are in need of rain. Yet, much of the state has only received between 20 to 40 percent of the rainfall it typically gets for this time of the season, noted John Abatzoglou, a fire scientist at the University of California, Merced. Meanwhile, recent warm spells, including daily record temperatures in both Southern and Northern California, have further dried out fuels. This has allowed vegetation to carry fire, said Abatzoglou.

To top things off, fierce winds have boosted the potential for fires to spread. Typically these winds wouldn't pose a fire problem. But with stubbornly parched fuels in early 2021, they fan flames.

The stage is set. All that's needed is a spark, perhaps from a wind-damaged power line.

"You’ve got the perfect ingredients for wildfire in a season when we should not even be thinking about it," said Jennifer Balch, an associate professor of geography at the University of Colorado Boulder who researches fire ecology.

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California has seen a surge in wildfires this century. The 10 largest fires in state history have all occurred since 2003. Overall, a warming Western climate that parches vegetation is a significant driver of modern blazes: Fire scientists have concluded that wildfire in California has increased fivefold since the early 1970s, largely caused by drier fuels.

The climate, however, isn't the only potent factor stoking flames. Of note, decades of ill-informed fire suppression by the federal government — which manages 57 percent of California's forests — has led to grossly overcrowded woodlands (fires naturally thin these environments). So there are not just drier fuels — there's significantly more fuel to burn.

Yet a warmer climate means more opportunities for fire weather (hot, dry, gusty conditions) to line up in these fire-prone places. And as 2021 shows, it can happen when we least expect it.

"Fire weather conditions have always happened in California," said Michael Gollner, a fire scientist in the Department of Mechanical Engineering at the University of California, Berkeley. "But we're seeing more [fire conditions] more often, and over a wider range of the year."

It's unlikely that clusters of winter wildfires will become the winter norm, said the climate scientist Swain. But this burning could certainly be something that occurs during years of drought or pitiful rainfall. Climate scientists expect the Golden State's precipitation to grow increasingly volatile in the 21st century, meaning more periodic extremes in floods and drought. So, perhaps we'll see similar winter fires during some drier years.

"It’s a complicated puzzle piece in how California's future climate might look," explained Swain.

The good news is the West still has much sway over future flames. Humans (often unintentionally) start 84 percent of wildfires in the U.S. by the likes of downed power lines, cars, recreation, and even a hammer. "It's people and our infrastructure that are starting the sparks," said Balch. There are compelling, practical ideas to limit these sparks, such as ambitiously expanding solar energy so spark-prone power lines can be shut down during extreme fire weather. (Solar panels also reduce the need to burn fossil fuels for energy, which cuts the amount of potent heat-trapping greenhouse gases now relentlessly warming the climate.)

And when some fires inevitably do start, the state has bounties of opportunity to "harden" neighborhoods against fastly-spreading flames, such as by regularly clearing dry fuels from around homes. "There's a lot we can do right now," said Gollner.

Recent destructive, deadly fire seasons have made the need to prepare overwhelmingly clear. This winter brings yet another poignant reminder of the state's worsening, and at times dystopian and eerie, fire regime.

"Winter wildfires is an oxymoron," said Balch, referencing Western fires. "We shouldn't be having this conversation."

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