Weather in the North Cascades varies greatly depending season and location. From autumn to spring much of the park is buried under a thick layer of snow. The most accommodating weather is from mid-June to late-September, when all but the highest trails are generally clear of snow. The west slopes and high peaks catch the brunt of wet systems blowing in from the Pacific Ocean creating a more lush and temperate evergreen forest, while the shielded lower east slopes tend to be warm and dry throughout the summer.
The difference between west and east is so great that the western slopes receive an average of 76 more inches of precipitation and 407 more inches of snowfall annually. So much snow falls in the mountains that State Route 20—the only road that traverses the park from east to west—is closed every winter for four or more months. During an average year heavy snow and continuous avalanches bury the highway beyond Ross Lake Trailhead from mid-November to April. Only once (1976-77) in its more than 30-year history has the route remained open through the entire winter. No matter what time of year, visitors entering the North Cascades should remember that mountain weather is unpredictable and prone to sudden changes. Even in the summer storms are common. Especially those visitors who plan and traveling into the backcountry should be prepared for adverse conditions. Warm, waterproof clothing is all but required year-round ... just in case.