Interior Alaska and the Yukon

The vegetation in this regain is predominantly spruce and aspen, with some tundra and other lesser vegetation in the north. The Yukon Basin has a warm, short summer. Continental heating has produced summertime temperatures of 100° F., but temperatures as low as 29° F. also have occurred in July. Winters are extremely cold. The high coastal mountains generally prevent the invasion of mP (maritime polar) air masses at low levels. The Brooks and other ranges block the inflow of even colder cP (continental polar) air from the north.

Annual precipitation is only about 10 to 15 inches, the maximum occurring during the summer in convective showers and with weak fronts. Precipitation is highest in the southern portion, which includes the northern extension of the Cordilleran Highlands and their parallel chains of lesser mountains. Although precipitation is maximum in summer, it is so scant that wildland fuels dry out considerably during the long, clear, dry summer days. Dry thunderstorms are not infrequent.

The usual fire season starts in May after melting of the winter snows and lasts until September.