The Pacific Northwest (PNW), encompassing Oregon and Washington, experiences snow, but the reality of its occurrence is far more complex than in many other regions. Geographically, the PNW is defined by its proximity to the Pacific Ocean and the dominating presence of multiple mountain ranges. This unique geography creates an exceptionally varied climate, meaning winter conditions can range from perpetual rain at sea level to some of the heaviest annual snowfalls recorded in North America. The likelihood and amount of snow depend entirely on the specific location within this diverse landscape.
The Significant Regional Variations in Snowfall
The Cascade Range acts as a profound climatic dividing line, separating the mild, marine-influenced climate of the western lowlands from the colder, more continental climate found to the east. Snowfall totals are highly location-dependent, leading to three distinct zones of winter weather.
The coastal lowlands and major metropolitan areas, such as the Puget Sound region and the Willamette Valley, experience snow infrequently. Due to the moderating effect of the warm Pacific Ocean, winter temperatures often hover just above freezing, meaning precipitation usually falls as rain. When snow does fall in these areas, it is often a transient event with accumulation that rarely lasts more than a couple of days. This low-elevation snow is typically disruptive because the infrastructure is not built to handle it.
The mountain ranges, particularly the Cascade and Olympic Mountains, receive predictable and large amounts of snow. The Cascade volcanic peaks host permanent glaciers and snowfields above 7,000 feet, with locations like Mount Rainier’s Paradise Ranger Station averaging over 600 inches of snow annually. This reliable snowpack serves as a natural reservoir for the region’s water supply during the drier summer months. High elevation ensures temperatures remain below freezing, allowing consistent snow accumulation from late fall through spring.
East of the Cascade Mountains, the interior regions of Washington and Oregon shift to a colder, drier continental climate. While the overall moisture is less than on the Pacific side, lower average winter temperatures ensure that precipitation more frequently falls as snow. Snow cover in this zone, including areas like Spokane, is more frequent and persists longer than in the western lowlands. However, the overall seasonal accumulation is generally less than the totals found in the high-elevation mountain passes.
Key Weather Patterns Driving Snow Events
The atmospheric conditions required to bring snow down to the low elevations west of the Cascades are relatively rare, demanding a specific alignment of temperature and moisture sources. The most common mechanism for lowland snow involves an Arctic air intrusion, often called outflow, which brings frigid air south from Canada. This cold, dry air mass is channeled through mountain gaps, such as the Fraser River Valley, pushing temperatures in the lowlands down to or below the freezing mark.
This cold air mass alone does not guarantee snow, as the air is often too dry; Pacific moisture must combine with the cold air for snowfall to occur. When the cold, dense air is trapped near the surface and a warm, moist Pacific system moves overhead, the resulting interaction can generate widespread snow.
In the mountains, the process is less complicated, relying on the constant flow of moisture-laden air from the Pacific being forced upward by the terrain, a phenomenon known as orographic lifting. As this moist air rises over the mountains, it undergoes adiabatic cooling, causing the water vapor to condense and freeze.
Elevation is the biggest determinant of snow conditions in the PNW, with snow levels consistently dropping to 2,000 feet or lower in the mountains during winter storms. Even in the lowlands, dynamic cooling, caused by heavy precipitation falling through a marginal air mass, can briefly drop temperatures enough to switch rain to snow.
Seasonal Timing and Duration of Snow Cover
The onset and duration of the snow season vary dramatically across the PNW, reflecting the differences between the marine and continental climates. In the high mountain passes and ranges, the snow season is long and consistent, typically beginning in late November and lasting through April. The deep snowpack in the Cascades generally reaches its maximum depth in early spring, often in March, and can persist at the highest elevations into the summer months.
In the western lowlands, the window for snow events is much narrower, generally confined to the coldest period of winter, which is late December through mid-February. While historical records show trace amounts of snow as early as October and as late as April, the majority of significant lowland snowstorms happen within this core winter period. The occurrence of snow is highly variable year-to-year; some coastal winters may pass with no measurable snow, while others feature multiple significant events.
When snow does fall at sea level, it is often short-lived due to the proximity of the warm Pacific air mass, which quickly scours out the cold air trapped in the valleys. Consequently, lowland snow tends to be transient, often melting within a day or two as temperatures rise above the freezing point. This highly variable and often brief nature of lowland snow contrasts sharply with the reliable, months-long snow cover that dominates the region’s mountain ecosystems.