Fire weather

Four meteorological elements are most commonly cited as driving fire weather risk: low relative humidity, strong surface wind, an unstable atmosphere, and antecedent drought or dryness. The U.S. National Fire Danger Rating System (NFDRS), developed by the U.S. Forest Service, breaks these down into components including the Spread Component (forward rate of fire spread), the Energy Release Component (potential available energy at the head of a fire), and the Ignition Component (probability that a fire will start and require suppression if a firebrand reaches fine fuels). High temperature and low humidity dry out fine, fast-drying fuels such as grass, leaf litter, and cured vegetation, making them easier to ignite, while sustained wind both supplies oxygen to combustion and physically spreads flame and embers ahead of a fire front. Lightning is also a significant natural ignition source under fire weather conditions, particularly "dry lightning," cloud-to-ground strikes accompanied by less than 0.10 inches (2.5 mm) of precipitation, which can ignite dry fuels without the rainfall that would otherwise help suppress a resulting fire.

In the United States, fire weather is forecast at two timescales. The NWS Storm Prediction Center (SPC) issues Day 1 and Day 2 categorical Fire Weather Outlooks (each covering a 12:00-12:00 UTC period), rating areas as Elevated, Critical, or Extremely Critical based on sustained wind speed, minimum relative humidity, temperature, and fuel dryness, each sustained for at least three hours; thresholds are relaxed somewhat for Florida. Extremely Critical, the highest category, is reserved for conditions that deviate significantly from climatological norms, or for borderline conditions occurring during exceptional drought. SPC also issues extended, non-categorical Day 3-8 fire weather outlooks.

On a shorter, tactical timescale, local NWS forecast offices, working with other agencies, issue Red Flag Warnings when high-to-extreme fire danger (per the National Fire Danger Rating System) is present. Required conditions are set by locally calibrated thresholds for sustained wind, relative humidity, and temperature that vary by region to reflect local fuels and climate. For example, the Detroit/Pontiac, Michigan forecast area requires sustained wind of at least 15 mph, relative humidity at or below 25%, and temperature above 75°F, while Boulder, Colorado requires wind gusts of 25 mph or greater with relative humidity at or below 15%. A related but less urgent Fire Weather Watch may be issued to advise of conditions that could result in extensive wildland fire occurrence or extreme fire behavior expected to develop in the next 18 to 96 hours.

Several indices translate weather and fuel-moisture observations into a numerical fire-danger rating. In the United States, the National Fire Danger Rating System produces the Spread Component, Energy Release Component, and Ignition Component described above. The Fosberg Fire Weather Index (FFWI), developed by USDA Forest Service researcher Michael A. Fosberg and presented in 1978, combines temperature, relative humidity, and wind speed into a fast-response 0-100 rating tuned to fine, quickly drying fuels such as grass and litter, rather than long-term drought. Canada, France, Australia, and other countries use the Canadian Forest Fire Weather Index (FWI) System, first issued in 1970 by the Canadian Forestry Service after several years of research, and now one component of the broader Canadian Forest Fire Danger Rating System.

The Haines Index was developed by Donald Haines, a USDA Forest Service research meteorologist, and published in 1988 as a lower-atmosphere severity index measuring the potential for dry, unstable air to produce large or erratic fire behavior. Haines himself described the index as preliminary, writing in his original paper that it was "a first effort ... [that] will undoubtedly require further refinement and/or additional components," and later told Potter he had envisioned an iterative refinement process incorporating feedback from fire practitioners. Citing new science indicating the information was better assessed through other forecast fields, the National Weather Service discontinued Haines Index and the related Lightning Activity Level (LAL) index as elements of its Fire Weather Forecast, spot forecast, and fire weather matrix products effective on or about February 1, 2025, replacing them with mixing height, thunder probability, and quantitative precipitation forecast information. The NWCG Fire Weather Subcommittee separately recommended the same discontinuation for its own training and curricula, citing research indicating the Haines Index was unsupported as a predictor of large fire growth and was not, properly speaking, a metric of atmospheric stability.

Sufficiently intense wildfires can generate their own convective weather, a phenomenon known as pyroconvection. The heat, smoke, and moisture released by a large fire form a rising plume that, given a sufficiently unstable atmosphere, can develop into a pyrocumulus cloud and, with enough moisture, intensify further into a full pyrocumulonimbus (pyroCb) thunderstorm. These thunderstorms are formally classified by the World Meteorological Organization as cumulonimbus flammagenitus. A worldwide record of pyroCb events from 2013-2021 indicates the phenomenon is "neither new nor rare," though instances of fire-generated storms reaching the stratosphere have increased since 2013. Counterintuitively, pyroCb thunderstorms are not typically associated with significant precipitation reaching the ground. They can, however, produce lightning, hail, damaging downdraft winds, and tornadoes. These storms can drive smoke plumes to altitudes of 10 to 15 kilometers, penetrating the stratosphere at heights comparable to a moderate volcanic eruption. Lightning generated by a pyroCb can strike with little or no accompanying rainfall. This phenomenon is called dry lightning. It is dry because raindrops evaporate in the hot, dry air beneath the storm (virga). Dry lightning can ignite new, separate fires nearby.

The 2002 North American fire season alone produced 17 separate pyroCb events, including those generated by Colorado's Hayman Fire and Arizona's Rodeo-Chediski Fire, at the time the largest fires recorded in either state's history, both human-caused. The Rodeo-Chediski pyroCb, on June 20, 2002, registered the maximum Haines index value of 6 and an estimated cloud-top altitude of about 10.2 km.

In late July 2026, wildfires in southwestern France and Spain forced roughly 325,000 people to evacuate; experts said the fires generated pyrocumulonimbus storms for what was reportedly the first time observed in France, with fire intensity described by Víctor Resco de Dios, a forestry engineering and global change researcher at the University of Lleida, as comparable to "several atomic bombs."

Fire weather most directly affects how fast and how far a wildfire spreads once ignited. In the 2018 Camp Fire, downslope winds reaching 50-60 knots (roughly 57-69 mph) at exposed locations on the western slopes of the Sierra Nevada, peaking in the hours around sunrise, drove the fire from its ignition point near Pulga, California, to the outskirts of the town of Paradise within about 90 minutes; the fire went on to burn roughly 70,000 acres (28,000 ha) in its first 24 hours, ultimately killing 85 people and destroying nearly 20,000 structures, at the time the deadliest and most destructive wildfire in California history. The National Weather Service's Sacramento office had issued a Red Flag Warning for the region two days ahead of the fire's ignition. Pacific Gas & Electric had warned on November 6 and 7 of possible preemptive power shutoffs across nine Northern California counties, but ultimately did not carry them out, having judged that wind forecasts fell short of the utility's own shutoff criteria (relative humidity below 20%, sustained winds above 25 mph, and gusts exceeding roughly 45 mph) even though observed winds at some exposed sites went on to exceed those thresholds.

A 2026 study in the Bulletin of the American Meteorological Society, based on 19 focus groups with 85 participants across all six NWS regions, found that fire management agencies consistently value Red Flag Warnings but use them in a supportive rather than an initiating role: participants described using the warnings to confirm existing suppression strategies, build situational awareness for less experienced personnel, and lend credibility to public messaging and resource requests, rather than as the first signal prompting a change in strategy. This is consistent with formal interagency dispatch protocol, under which local dispatch centers incorporate red flag warning notification into resource-ordering procedures, including requests for additional personnel, equipment, and aircraft.

Utilities in California and Oregon have also responded to forecast fire weather by proactively cutting power to reduce ignition risk from electrical equipment, so-called Public Safety Power Shutoffs. One such shutoff by Pacific Gas & Electric in October 2019 was estimated to have cost California's economy up to $2.5 billion.

Great Fire of 1910 (U.S.)

The Great Fire of 1910, also known as the Big Burn, followed an anomalously warm spring and dry summer across much of the western United States. On August 20-21, 1910, strong winds associated with a frontal disturbance caused numerous fires in northern Idaho and western Montana to spread rapidly and merge. About 3 million acres (1.2 million ha) burned and 87 people were killed, including 78 firefighters.

Yellowstone fires (1988)

An unusually wet spring in Yellowstone National Park gave way to the driest summer in the park's recorded history, with essentially no measurable rain falling for nearly three months. Lightning from a series of dry storm fronts started many of the season's fires, while the fronts also brought sustained high winds that rapidly spread fires already burning. On August 20, 1988, later called "Black Saturday" (not to be confused with the 2009 Victoria, Australia bushfires), wind-driven growth roughly doubled the park's burned area in a single day, to more than 480,000 acres (190,000 ha). By the time snow halted the fires' advance in mid-September, about 793,880 acres (36 percent of the park) had burned; some 63 percent of that total resulted from fires that started outside the park boundary and burned in, including the North Fork Fire, the largest single fire at over 410,000 acres. Extreme fire behavior associated with the event included convection reaching the stratosphere as pyrocumulonimbus.

Black Saturday bushfires (Victoria, Australia, 2009)

On February 7, 2009, record heat, very low humidity, and strong winds produced extreme fire-weather conditions across Victoria, Australia. Temperatures exceeded 45 °C across much of northern and central Victoria, and a meteorological study later characterized the conditions as the worst fire weather on record. Strong northwesterly winds initially drove the fires before a fast-moving southwesterly change crossed the state, turning long fire flanks into broad fire fronts. Predicted Forest Fire Danger Index values exceeded 100 in many areas, above the upper end of the scale then in use. Following the fires, a new "Catastrophic" fire-danger category was introduced for values above the previous scale.

Yarnell Hill Fire (Arizona, U.S., 2013)

The Yarnell Hill Fire was ignited by dry lightning on June 28, 2013, after below-average precipitation and seasonal dryness had left vegetation in central Arizona extremely dry. On the afternoon of June 30, thunderstorms near the fire produced an outflow boundary that caused an abrupt change in wind direction and speed. Wind gusts in the region reached nearly 44 mph (71 km/h); as the outflow reached the fire, it turned south and began spreading at about three times its previous rate. Nineteen members of the Granite Mountain Hotshots were overtaken and killed.

2016 Fort McMurray wildfire (Alberta, Canada)

An unusually hot, dry air mass over northern Alberta brought temperatures of 32.8°C (91°F) and relative humidity as low as 12% on May 3, 2016, followed by 31.9°C (89°F) and winds gusting to 72 km/h (45 mph) on May 4. Low winter snowpack and unusually warm, dry spring conditions contributed to the extreme fire weather. The resulting fire forced the evacuation of more than 88,000 residents, the largest wildfire evacuation in Alberta's history, and caused an estimated C$8.9 billion in damage, the costliest disaster in Canadian history at the time. The fire itself generated pyrocumulus clouds and began producing its own lightning as it grew, and is the subject of John Vaillant's 2023 book Fire Weather.

2018 Camp Fire (California, U.S.)

See the Effects/impacts section above for a detailed account of the fire weather conditions driving the Camp Fire.

January 2025 Southern California wildfires

Five days ahead of the first ignition, the NWS began warning of extreme Santa Ana wind and fire conditions, escalating from a Fire Weather Watch to a Red Flag Warning and, on January 6, to a "Particularly Dangerous Situation" designation. The Palisades Fire ignited the following morning, January 7, and spread rapidly; the Eaton Fire broke out shortly after, expanding past 200 acres within an hour. That evening, the NWS reported a wind gust of 99 mph near Altadena. Particularly Dangerous Situation Red Flag Warnings recurred through the fires' first week and a half, remaining in effect as late as January 15. The Palisades and Eaton fires became among the most destructive in California history since records began in 1932, forcing evacuation orders affecting roughly 88,000 people, destroying more than 12,000 structures, and killing at least 25.

July 2026 Gironde wildfires (France)

Pyroconvection had been observed previously in France, but the wildfires in Gironde near Bordeaux in late July 2026 repeatedly generated pyrocumulonimbus thunderstorms. Jean-Baptiste Filippi, a wildfire researcher with the French National Centre for Scientific Research (CNRS), described the frequency and intensity of the events as unprecedented. The storms produced lightning and strong fire-generated winds, and burning material carried out of the convection columns started spot fires.