Changes in Fire Season Precipitation in Idaho and Montana from 1982-2006

Fire season precipitation trends were investigated using daily rainfall data obtained from 76 Remote Automated Weather Stations (RAWS) across Idaho and Montana for the period 1982 to 2006. Station records were made temporally consistent/comparable by replacing missing/erroneous observations with data from the North American Regional Reanalysis (NARR) using methods easily reproducible by fire managers. Monthly precipitation was then analyzed during the core fire season(July-Sept) and biweekly precipitation was analyzed for the start of fire season (June). The end of the season was examined using October precipitation data and through identification of season slowing rain events (SSE). These analyses reveal significant changes in precipitation amount, timing and spatial autocorrelation between stations. While June precipitation has generally increased, core fire season is getting drier and longer. Season slowing events are occurring 15 days later, on average, than they did in 1982, while summer rainfall is decreasing at 97% of stations with clusters of significant change focused in the central Idaho mountains and in west-central Montana. The observed trends in precipitation paired with later season slowing events could result in more active fire seasons in the Northern Rockies and may help explain some of the changes in fire season that have been previously attributed to earlier spring snowmelt and warmer temperatures.