Why only a few wildfires become extreme
The costliest event of 2025 by far was a series of cataclysmic wildfires that ravaged more than 57,000 acres (89 square miles) across the Los Angeles area in early January.
Hot and dry conditions have become synonymous with the risk of extreme wildfires. But a new paper argues that such conditions are not by themselves sufficient for blazes, and most warm years do not result in the burning of exceptionally large areas.
The reason is that extreme fires — the few percent of fires that grow rapidly, keep spreading, and end up accounting for nearly all of the area burned — need brief-lived windows of opportunity: an unusual confluence in which winds, terrain, dry vegetation, and ignition coincide. Warm, dry conditions can prime broad regions for burning, but only certain weather setups allow a fire to grow rapidly and keep spreading. These conditions vary from one region to another and may last only a few days.
Understanding such windows of opportunity is critical, according to the paper. Wildfire activity is dominated by rare events. Only a small fraction of fires grow large, and yet that small fraction accounts for nearly all of the area burned.
“Wildfire risk is not about one condition alone, such as heat or dryness,” said Janice Coen, the author of the new paper and a scientist at the U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR). “Instead, extreme fires depend on three things lining up: conditions that make vegetation ready to burn, a short-lived weather pattern that can support rapid fire spread, and an ignition in the right place at the wrong time. Each of those conditions can happen fairly often. Their overlap is much rarer.”
One reason that highly localized factors can be overlooked when forecasting fire risk is they are harder to discern than large areas of temperature and humidity. The winds that drive rapid fire growth can be brief, shaped by the underlying terrain, or generated by the fire itself, making them difficult to anticipate and observe.
Focusing on those factors and the vulnerabilities of a specific location can shift the emphasis of fire forecasts, Coen wrote. Instead of just signaling fire risk over one or several weather forecast zones, future forecasts that incorporate such fine-scale conditions could help land managers and emergency planners better pinpoint where and when an ignition has the greatest chance of outrunning early suppression efforts and how a fire in that window can be expected to behave and spread.
For her analysis, Coen synthesized statistical studies, fire weather classifications, and computer models that bring together weather conditions and wildfire behavior. Her research, published in the journal Fire, was funded by the California Energy Commission, NASA, the U.S. Department of Agriculture’s National Institute of Food and Agriculture, and NSF.
Identifying windows of opportunity
Since 1983, the years that rank in the top 10% for area burned in the contiguous United States have occurred almost exclusively in warmer-than-average years. However, Coen noted, approximately 73% of warm years have not resulted in exceptionally large areas being burned.
In fact, fire activity remained relatively moderate in some warm years in which conditions appeared favorable for extreme fires. In 2021, for example, despite unusually warm conditions across much of the Western U.S. along with widespread midsummer lightning outbreaks and other ignitions, the national area burned remained well below historical extremes. In contrast, other warm years, including 2017 and 2020, experienced sustained large-scale fire growth and extreme burned area outcomes.
The reason for this, Coen concluded, is that warm seasons only open the door. Extreme fires happen when an ignition occurs during a short-lived weather window that allows rapid spread.
In Southern California, that window often results from Santa Ana winds, in which atmospheric pressure differences between the Great Basin and the coast send strong, dry winds downslope toward coastal communities. In other regions, extreme fires may be driven by strong ambient winds, or they can develop under hot, dry conditions with relatively weak winds, when a fire plume builds, draws air inward, and strengthens fire-induced winds near the flames.
Although broad fire weather patterns can often be forecast days in advance and trigger Red Flag Warnings, the local details that determine whether a fire will spread rapidly are harder to anticipate. The strongest winds may occur in narrow bands or brief pulses, rushing through mountain gaps, accelerating downslope, or forming near the fire itself. These features can unfold over minutes and hundreds of yards, which is less than the resolution of most forecast systems.
Fires themselves can add another layer of complexity by generating their own inflow, changing nearby winds and sometimes building deep plumes whose behavior can be difficult to predict without specialized fire-atmosphere models.
Still, Coen said, forecasts could improve by looking more directly for these windows of opportunity, helping firefighters, emergency managers, and residents recognize when a local ignition has the greatest chance of becoming extreme.
“Right now, fire weather warnings often tell us when broad conditions are dangerous,” Coen said. “Those warnings are important, but we can also ask a more specific question: under this weather pattern, where would terrain and winds allow a fire to grow rapidly if an ignition occurred? That could help firefighters and emergency managers focus attention on the places where a small fire has the greatest chance of becoming extreme.”