Mountain Weather Forecast

Weather Synopsis

After 3 weeks of mostly dry weather, we're in store for a massive change. A strong frontal system approaches Thursday evening, spreading light rain and snow into the area as a NE-lifting warm front moves toward the coastline. Snow levels start around 2500 ft in the north Cascades, 3000 ft in the central Cascades, and 4500 ft in the southern Cascades around Mt Hood. Rain and snow intensify overnight with little change in snow levels. 

Heavy rain and snow will continue as the warm front moves through the Cascades early in the day and the primary surface low moves onshore near the northern tip of Vancouver Island. Precipitation may briefly let up mid-morning before a secondary wave re-intensifies precipitation, particularly in the southern and central Cascades in the afternoon. Subtle warming begins Friday morning in areas west of the Cascade Crest. However, an intensifying cold pool east of the Cascades Crest and light to moderate E low-level flow should insulate the passes from warming for most of the day, although we can't rule out a mix or changeover to rain at Snoqualmie Pass in the afternoon.

The trailing cold front moves through the Cascades in the late evening hours. Expect an uptick in snow levels and a likely changeover to rain at Snoqualmie Pass before rapid cooling lowers snow levels below the Pass by midnight. Moderate to heavy snow post-frontal snow showers with moderate and gusty WSW winds focusing showers and convergence on areas near and west of the Cascades. Convergence develops around the mountain loop in the late evening hours and drops southward toward Snoqualmie Pass overnight. Expect very heavy local snowfall within this band.

With cold air aloft and moving down toward the surface expect unusually low snow levels (below 1000 ft) on Saturday. This will lead to high snow-to-liquid ratios as moderate to locally heavy snow showers become increasingly convective.

should fSnow levels rise by around 500-1000 ft, but E flow will likely insolate Snoqualmie Pass from the warming through at least the morning hours. The afternoon is more uncertain. The cold frontal passage should hold off until Friday evening.

Snowfall amounts should be in the 6-18" range for the Cascades through Friday above 3000 ft.