Snow covers the ground more than six months out of the year, even at lower elevations, and State Route 20, the only highway through the park, is generally closed from late November until late April. Numerous avalanche chutes, as many as traverse any state or federal highway, and including some over long, cross the highway and make snow removal during that period infeasible. Heavy snow and frequent avalanches are common, especially on the western slopes, from autumn to spring. The high elevation trails are generally open by mid-summer and the majority of tourism is between mid-June and late September. The east side of the park is generally warmer and drier, especially in the summer with highs reaching . Since the 1950s, there has been a five-degree Fahrenheit (2.77°C) mean winter minimum temperature increase at elevations above . This has led to a reduced winter snowpack as the mean winter freezing level is now higher.