With heavy rain and snow, will California get a break from wildfires? The science is tricky - San Francisco Chronicle

As Bay Area residents ride canoes through neighborhoods to cope with flooded rivers, and as the Sierra sets records for snowfall, historical data show the wet winter could bode well for a state desperate for a mild fire season after two years of catastrophic damage.

Since 1970, a comparison of California precipitation totals to acres burned in the subsequent fire season reveal a promising pattern.

“The trend shows that the drier years are worse years for fires,” said meteorologist Jan Null, who has been analyzing the data for more than a decade.

It may seem obvious that dry years would create more dangerous fire seasons, but Null and other experts warn there is no perfect science to predict how a California summer will act.

For instance, in the rainy season that ended in early 2017, the state saw 159 percent of its average annual rainfall, a deluge that prompted the Oroville Dam spillway to fail. Yet California saw one of its most devastating fire seasons, with 1.26 million acres burned and dozens of deaths in a series of wildfires in Wine Country and beyond.

Despite the rain and snow this winter, Cal Fire is already warning that the wet weather will create an abundance of grass fuels that could burn easy and fast. Conversely, during dry winters, the agency often warns of arid brush and forests that can lead to dangerous fires.

“In some ways, Cal Fire can always be right and always be wrong,” Null said.

California’s Mediterranean climate means there’s no rain over the summer months, so the risk of fires always climbs through the summer and fall. And anticipating the threat based on the winter weather is tricky, said Steve Leach, a Bureau of Land Management meteorologist who helps create predictive modeling that Cal Fire leans on.

“There are rules of thumb. A lot of the story is yet to be written,” Leach said. “There’s no exact way to tell. Now you just look at the potential.”

His agency will release its next report Friday predicting “greater than normal” chances of significant fires in June in lower elevations.

“We have the potential to see a faster start of the season in June due to the robust fine fuel,” Leach said. However, he said, the higher elevations might have a delayed fire season thanks to the wet weather.

Last year, the state’s deadliest and most destructive fire season in history burned 1.8 million acres across the state. That followed a winter that saw 75 percent of normal rainfall.

This winter has been especially wet, with many Bay Area rainfall totals ahead of average (San Francisco was at 104 percent, San Jose at 107 percent, as of Wednesday). Five central Sierra tracking stations had precipitation at 116 percent of normal, and the state broke snowfall records in February.

Amy Head, a Cal Fire spokeswoman, said that while the rain and snow are encouraging, what really dictates fire-season behavior is summer weather.

“It’s those hot, dry and windy conditions that are a problem,” she said.

Other wild cards in predicting the intensity of fire seasons include the variety of climates and landscapes in Northern and Southern California. And, of course, it’s all dependent on the arrival of potential ignition sources — whether arsonists or dry lightning events or windstorms that bring down power lines.

One wet winter, Null said, can have a direct impact the following summer when it comes to fires fueled by brush and manzanita, such as the deadly Woolsey Fire that leveled hundreds of homes in Los Angeles and Ventura counties in November 2018.

However, forest blazes are less impacted by year-to-year precipitation. Trees dry out or die — or gain moisture — over a longer span of time, Null said. The Sierra is still vulnerable due to dead trees from the recent extended drought.

One encouraging sign: Weather data show that in 1997, after the last catastrophic Russian River floods, only 110,000 acres burned in California, the seventh lowest total since 1970.

Matthias Gafni is a San Francisco Chronicle staff writer. Email: matthias.gafni@sfchronicle.com Twitter: @mgafni

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