"The year of a forester in the western districts is divided into two parts - the forest fire season, and the rest of the year" - Elers Koch, Forty Years a Forester, 1998Not surprisingly, northern Rocky Mountain forests are most likely to catch fire at certain times of the year. The time of year with the highest fire danger is known simply as the forest fire season (e.g., Koch 1998).
Weather largely dictates fire danger. Fires are most likely to carry from a source of ignition during periods of hot, dry, windy weather (Andrews and Bradshaw 1997).
The forest fire season peaks when fire conditions are extreme and ignitions are plentiful - when both humans and lightning are likely to start fires. While people can ignite forest fires year-round, cloud-to-ground lightning strikes are generally restricted to summer and early fall when thunderstorms roll through the region (Handel et. al, Rollins et al. 2000). Although, these storms can bombard the region with hundreds of thousands of strikes each year (Handel et al.), the weather and available fuels (a.k.a. dry vegetation) limit the number and spread of forest fires from summer into fall.
The height of the forest fire season in the Northern Rockies is therefore typically midsummer, when temperatures are high, humidities are low, forest vegetation is dry, dry lightning is pervasive, and gusty winds are common (Cooper et al. 1991, Rorig and Ferguson 1999, Kipfmueller and Swetnam 2000). By midsummer, even the wettest forests may have dried to the point at which they will carry fire.
While fires usually make their most dramatic runs in August or September, the entire fire season may span from the first dry spells of summer to the onset of cool, wet Autumn weather, which can come as late as November (Arno 1980, Rollins et al. 2000). Most often, however, the forest fire season ends in September or October (e.g., NIFC 2001).
Andrews, P. L., and L. S. Bradshaw. 1997. FIRES: Fire information retrieval and evaluation system - a program for fire danger rating analysis. USDA Forest Service, Intermountain Research Station, General Technical Report, INT-GTR-367.
Arno, S. F. 1980. Forest fire history in the Northern Rockies. Journal of Forestry 78:460-465.
Cooper, S. V., K. E. Neiman, R. Steele, and D. W. Roberts. 1991 (rev.). Forest habitat types of northern Idaho: a second approximation. USDA Forest Service, Intermountain Forest and Range Experiment Station, General Technical Report, INT-236.
Handel, R, K. Mielke, and D. Bernhardt. Montana lightning climatology. Available online at http://www.wrh.noaa.gov/greatfalls/misc/lightning/ltg_climo.html.
Kipfmueller, K. F., and T. W. Swetnam. 2000. Fire-climate interactions in the Selway-Bitterroot Wilderness Area. Pp. 270-275 in D. N. Cole, S. F. McCool, W. T. Borrie, J. O'Loughlin, compilers, Wilderness science in a time of change conference - Volume 5: wilderness ecosystems, threats and management. USDA Forest Service Rocky Mountain Research Station, Proceedings RMRS-P-15-VOL-5.
Koch, E. 1998. Forty Years a Forester: 1903-1943. Mountain Press, Missoula, Montana, USA.
National Interagency Fire Center. 2001. National Interagency Coordination Center: 2001 Statistics and Summary. Available online at http://www.nifc.gov/news/2001_Stats_Summ/Intro-Intell_PredServ.pdf.
Rollins, M. and T. W. Swetnam, and P. Morgan. 2000. Twentieth century fire patterns in the Gila/Aldo Leopold Wilderness Complex New Mexico and the Selway-Bitterroot Wilderness Area Idaho/Montana. Pp. 283-287 in D. N. Cole, S. F. McCool, W. T. Borrie, J. O'Loughlin, compilers, Wilderness science in a time of change conference - Volume 5: wilderness ecosystems, threats and management. USDA Forest Service Rocky Mountain Research Station, Proceedings RMRS-P-15-VOL-5. Available online at http://www.wilderness.net/pubs/science1999/Volume5/Rollins_5-34.pdf.
Rorig, M. L., and S. A. Ferguson. 1999. Characteristics of lightning and wildland fire ignition in the Pacific Northwest. Journal of Applied Meteorology 38:1565-1576.