A series of late-season winter storms have filled reservoirs, increased snowpack and left forecasters anticipating a late start to California's wildfire season.
While the odds are also leaning toward a milder-than-normal fire season overall, that outlook could change by July, said Jonathan O'Brien, a meteorologist with the National Interagency Fire Center.
“It's too early to say at this point what the peak season months will look like,” said O'Brien, who works in predictive services for the NIFC Center in Riverside.
Currently, predictive services are forecasting below-normal significant fire activity in Southern California in May and June, and normal activity in Northern California.
The rainy season has already been wetter than normal in Southern California, and forecasts call for periods of rain and snow through April, if not early May, O'Brien said. Vegetation at lower elevations is moist and green. In higher areas, it is buried in snow.
Statewide, snowpack recently reached 104% of normal on March 5, and 95% of average on April 1, when it is typically the deepest.
The more moisture there is in a snowpack, the longer it takes to melt. This ensures vegetation dries more slowly and also helps mitigate the onset of warm weather, said Brett Lutz, BLM meteorologist for Northern California Forecast Services Operations.
“Years where we had above-average snowpack, especially in Northern California, were associated with a general tendency to burn below-average acreage,” Lutz said.
However, as climate change pushes California's fires to burn at higher elevations — places that were once too wet or cold to ignite — more snow is falling on burned areas, said Amir Agha Koshak, a professor of civil and environmental engineering at UCLA. In Irvine. He said some of these places have a small canopy of trees to protect the snow from the sun's rays, and this may cause the snow to melt more quickly.
“So it's likely that more and more fire and snow interactions will contribute to a stronger or faster change from flood risk to drought risk,” he said.
In addition, the snow line is creeping upward directly in response to the warming that has occurred over the past few decades, said Alexander Gershunov, a research meteorologist at Scripps Institution of Oceanography.
“This makes mountain ecosystems more flammable in the summer,” he said, noting that these fuel-dominated fires in forested areas have been responsible for much of the increase in wildfire activity in California over the past two decades.
In addition, forecasters are closely monitoring the state's desert areas, many of which saw higher-than-normal rainfall this year and last, leading to the growth of grasses and small shrubs. Once it physically recovers, which happens in late April through June, it could spark fires like the York Fire — the 93,000-acre blaze that burned in the Mojave National Preserve in July and was the state's largest fire last year.
Northerly and offshore winds — commonly referred to as the Diablo winds to the north and Santa Anas to the south — fanned some of the most damaging fires in the state. Forecast services meteorologists are calling for a near-to-below normal number of these wind events over the next three months.
Part of this has to do with El Niño, the warm phase of the El Niño-La Niña Southern Oscillation pattern, which is the main driver of temperature and precipitation patterns around the world.
This phenomenon typically causes the jet stream to be split into a subtropical and polar jet stream, rather than a unified jet stream, said Brent Wachter, a meteorologist with Forecast Services Operations in Northern California. “This type of energy takes away the energy needed to get as many stronger offshore wind events as possible,” he said.
While in Southern California, Santa Ana winds don't tend to fuel fire growth until later in the summer and fall, in Northern California, destructive fires that occur in May and June are typically associated with a wind event, Wachter said.
However, California is in a different place than it was at this point last year, when conditions were so wet that forecasters were confident the entire fire season would be relatively mild. They can't yet make that decision this year, and they cautioned that a lot could change between now and July, when fire season typically begins.
Some variables that can make or break a fire season are unpredictable. For example, in 2022, conditions were dry and warm, but the state benefited from some well-timed rainfall events that mitigated fire activity. In 2020, a late-summer dry lightning blast sparked dozens of fires, turning an average season into the state's worst on record.
“These wild cards can always emerge,” said John Apatzoglou, a professor of climate science at the University of California, Merced. “Things could change quickly, if the sky taps shut off too quickly, if we see a really warm summer, if we see an active streak of dry lightning coming across the state.”
Perhaps the most closely monitored variable is heat, which absorbs moisture from plants, causing them to burn. Forecast services forecast below-normal temperatures in the South and near-below in Northern California through June. But there is some uncertainty in that as well.
Other forecasts, such as those from the National Weather Service's Climate Prediction Center, favor above-normal temperatures across Northern California, and roughly equal chances of above-or-below-normal temperatures in the southern part of the state.
That's because forecasters believe we're moving from El Niño toward a potential La Niña — which refers to cooler-than-average sea surface temperatures across the tropical Pacific — by late summer or fall. The difference in temperature forecasts over the next few months is largely related to how quickly this shift occurs, Wachter said. Predictive services are counting on a faster shift, which will likely lead to cooler conditions in the spring and early summer, while the Climate Prediction Center expects the shift to occur more slowly.
“That's the big question mark, which is why the temperature forecast is more stringent right now,” Wachter said.
While researchers generally see good relationships between the extent of hot and dry fire seasons and the extent of area burns in state forests, the picture is more mixed in non-forested areas, Apatzoglou said.
Because vegetation is dry enough to sustain fire most years, other factors come into play when determining fire season severity. These include the amount of good fuel – grasses, twigs, needles and leaves – present in the landscape.
“When we have a wet year where there is enough moisture to allow the weed to grow in a somewhat uncontrolled way, which is similar to what we saw last year for a large portion of the state, that increases the weed yield,” he said. “It may not burn this year, but next year will be a very good year for fire.”
The most damaging fire seasons in Southern California actually tend to follow the rainy season when precipitation is near normal, O'Brien said. Plenty of moisture limits the spread of large fires during the summer. Too soft and too little fuel does not grow enough to be able to carry fire across the landscape.
When rainfall is enough to help grasses grow but moisture isn't present throughout the summer, it usually leads to a more active season, O'Brien said.
“By the time you get to mid- to late summer, things have dried out and you're really prepared for fire activity with a lot of dead fuel,” he said.
For example, record rainfall in the first months of 2017 boosted vegetation growth, but by that summer — the hottest summer on record in California at the time — much of it had dried out. Three of the 20 most destructive wildfires in state history broke out between October and December of that year, including the Thomas Fire.
If the state gets another exemption for extreme fire activity this year, it is not expected to last.
California wildfires burned five times more area between 1996 and 2020 than they did from 1971 to 1995, and almost all of that increase can be attributed to human-caused climate change, according to research by Apatzoglou and Agakucak published in the Proceedings of the National Academy. Science last year.
The study found that this trend is expected to continue over the long term, leading to an increase of up to 50% in area burned from 2031 to 2050.
“Within that long-term change, you can have high anomalies, low anomalies — and a lot of volatility,” Agha Koshak said. “Climate change does not mean things are going to be hotter and warmer every year. But overall, if you look at the general trend and pattern, we are seeing increasing temperatures and higher risks of drought and fires.
