Generally, the climate of the GYE is characterized by long, cold winters and warm, dry summers. Spring and autumn in Yellowstone tend to be short, transitional periods rather than full-fledged seasons (Dirks and Martner 1982, Despain 1987, 1991). Weather systems travel into the area from several locations. Moist air ap proaches from the Gulf of Mexico or the southern Pacific. Drier weather systems reach the area from the west, as maritime air masses shed their moisture over the steep terrain of the northwestern ranges, or when the cold polar air mass from the Canadian plains migrates to the south. The interplay of these weather systems accounts for the complex and variable nature of the climate found in this region. On both an annual and a seasonal basis, climate can be highly variable in the GYE, with summer exhibiting the most climatic consistency (Douglas and Stockton 1975). Years characterized by the frequent passage of fronts across the region can result in relatively cool, wet summers.Warm summers generally occur under anticyclonic conditions when warm upper atmospheric air masses move over the continental United States. Wet periods can occur with the advection of moisture from the southeast or southwest (Douglas and Stockton 1975).
Of particular relevance to the spread of C. ribicola is the short autumn period of September through mid-October, as this is predominantly when the final spore stages and tree infection occur in the blister rust cycle. Autumn weather is highly variable from year to year, with some years being more characteristic of summer and others more typical of winter. Dendrochronological reconstructions reveal frequent anomalous precipitation and temperature conditions in the autumn. Key departures from average conditions that favor blister rust spread would incorporate a greater frequency of cool, wet summers, with temperatures above freezing extending into October. Alternatively, extremely hot, dry summers would increase the likelihood of large stand-replacing fires. These fires would favor conversion of the forested area toward younger age classes of trees, and facilitate the successional replacement of whitebark pine by lodgepole pine and subalpine fir (Mattson and Reinhart 1989).
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