Northern Rocky Mountains

Heavy pine, fir, and spruce stands dominate the Northern Rocky Mountain region. Many mountain peaks extend above timberline. The portion of this region in Canada includes the Cordilleran Highlands with numerous mountain ranges and dissecting river courses, in addition to the Rocky Mountains. Winter temperatures are quite low, and summer temperatures are moderate.

Annual precipitation ranges from 10 to 20 inches in the valleys to 40 to 60 inches locally in the mountains. Most of the precipitation falls in the winter and spring in the southern portion of this region, while in the northern portion it is fairly well distributed throughout the year, in most years. Winter precipitation is in the form of snow. In the southern portion, there often is widespread rainfall until June, followed by generally light precipitation during the summer.

There is a gradual drying out of forest fuels during July and August with increasing fire danger. Frequent thunderstorms may occur then but little or no precipitation reaches the surface, so that frequent and severe lightning fires occur in both the Canadian and United States portions of the region. Also, extremely low humidities can result from large-scale subsidence of air from very high levels in the atmosphere. Occasional chinook winds on the east slope of the Rockies produce moderate temperatures and are effective in bringing subsiding air to the surface. The fire season usually extends from June or July through September.

The synoptic weather types producing high fire danger are similar to those described for the Great Basin region. Particularly important are the ridge-aloft pattern which produces warm, dry weather and the patterns producing high-level thunderstorms.