California’s rainy season is starting later in the year than it used to, extending the threat of fast-spreading wildfires into the cooler months when, in earlier decades, the fire season would have been over, according to a new study.

The findings are contained in a recently released study in the journal Geophysical Research Letters. It finds that the state’s wet season is beginning nearly a month later than it did in the 1960s and that the autumn season has become progressively drier. Both of these patterns have been tied at least in part to human-caused climate change.

“Wildfires can occur at any time in California, but fires typically burn from May through October, when the state is in its dry season,” Jelena Lukovic, a climate scientist at the University of Belgrade in Serbia and lead author of the study, said in an email. “The start of the rainy season, historically in November, ends wildfire season as plants become too moist to burn.”

Because this study looked at precipitation trends in finer detail than previous work, examining what is happening month by month, it actually found that the most pronounced and significant drying trends are occurring in November, when the storm track should shift south and bring heavier and more consistent precipitation, especially to Northern California.

“There were very few bone-dry Novembers a few decades ago,” Daniel Swain, a climate scientist at the University of California at Los Angeles, said in an interview. “When you get to Thanksgiving, it’s pretty wet in Northern California — at least it used to be.”

The 2019 and 2020 fire seasons extended well into November in Northern California; in December 2020, Southern California’s fire season was still going strong.

The state had devastating wildfires in November 2018, including the Camp Fire in Paradise — the deadliest and most destructive on record. In early December 2017, a wildfire outbreak brought Southern California one of its largest blazes on record — the Thomas Fire.

This study is the first to document an ongoing delayed start to the rainy season in the observed record, Lukovic said. It lends support to other recent research that has found a decline in autumn precipitation in California and provides solid, real-world evidence for a lengthening of the dry season that has been predicted by climate models.

The missing autumn rain also means that the state must play catch-up in the winter months to avoid falling into drought, relying on heavier or more frequent storms to make up the deficit.

That has indeed occurred: the authors show that more precipitation has been concentrated into a few core winter months, leading to a shortening or “sharpening” of California’s wet season.

According to Swain, who was not involved in the new study, the research convincingly shows that this seasonal sharpening is already happening, which a growing body of climate model studies say should be occurring as greenhouse gases build up in the air from the burning of fossil fuels for energy, among other human activities.

“It is exclusively real-world-observations-based and is essentially complementary to climate models,” he said of the study results.

Weather patterns are already shifting

Changes in atmospheric circulation patterns are behind the seasonal shifts, as a typical summer weather pattern lingers into November, followed by an enhanced storminess in winter caused by the southward migration of a low-pressure area near the Gulf of Alaska.

“There is a persistent high pressure across the Northeastern Pacific and a summer atmospheric circulation pattern remaining throughout autumn … leaving California dry in recent decades,” Lukovic said.

“We also found a deepening of the Aleutian Low in December and January, which manifests itself as a stronger westerly flow that brings more precipitation to California in these months.”

The delayed autumn rain has greater ramifications because there is no doubt that temperatures are going up in California, especially in late summer and early fall, and will continue on that track. Autumn is not only getting drier but also warmer, and dangerous fire weather days are on the rise.

Fires are therefore burning hotter, faster and more intensely as heat makes vegetation tinder dry.

“You are actually adding an extra month to the worst part of the fire season, because it’s at the end when things are at their driest,” Swain said.

There’s another dire consequence of pushing the fire season later into the year. Land-to-sea winds that have driven some of the state’s worst fires typically escalate statewide in October. And Santa Ana winds, which affect Southern California, are stronger and more frequent during the cool season between November and February.

“Historically, the peak in dryness and the peak in offshore winds were slightly mismatched,” Swain said, with the most flammable conditions occurring in August and early September, before the winds typically arrive.

Now, with the loss of gentle early-season rains, summerlike flammability is aligning with the strongest winds in October, November and even beyond. This year, for example, fires continued into December.

Although October is a variable and not particularly wet month, the month’s rain showers helped to dampen the grasses and plants that ignite easily and carry fire. The wind-driven Camp Fire would not have burned as ferociously, and may not have ignited at all, had the Paradise area received more rain leading up to that fateful November day.

A detailed report on the Camp Fire, released Monday by the National Institute of Standards and Technology, notes that it had been more than 200 days since the area had received half an inch of rain or more, allowing embers to spread the fire quickly.

The 2020 fire season continued the progression toward delayed autumn rains. The September-October 2020 period, for example, was the driest on record in California, with an average of only .09 inches of rain falling during that time. The fire season again extended into the cooler months, swinging wildly from drought and fire in December to flooding and mudslides in January, another risk of expected shorter and more intense wet seasons.

“If they had included the 2020 data point in the study, it just would’ve made the conclusions even stronger,” Swain said. “We ended up having yet another late, dry autumn and a bad fire season to boot.”

Diana Leonard is a science writer covering natural hazards. You can follow her on Twitter @HazardWriter.