Mountain Weather Forecast

Weather Synopsis

A slow-moving large-scale upper trough gradually shifts south and east. Remnant trapped low-level moisture maintained more cloud covered than forecast models expected on Tuesday. However, increasing NNE winds as the trough exits should help to mix the atmosphere and bring mostly clear skies by the late evening and overnight hours. With cold air already in place at all levels of the atmosphere and light winds near the surface, mountain temperatures should drop into the single digits and negative single digits in most areas. Inversions likely form as upper-level temperatures moderate very slightly, leaving the coldest air in deep valley locations to the east of the Cascade Crest.

Temperatures warm slowly on Wednesday, but an approaching disturbance should help moderate temperatures by switching low-level flow westerly in the late afternoon. This disturbance should ride down the British Columbia coastline on the backside of an offshore ridge. Some clouds reach northern Washington State by late Wednesday afternoon with the potential for light snowfall overnight. Most models keep this storm relatively dry, but with very cold temperatures in place, any moisture will be efficiently squeezed out by our mountains. At this time we think the NNW flow should favor the northern slopes of the Olympics and Mt. Baker areas for 1-5" of very-low-density snow with light amounts possible in other areas.

Snowfall may linger Thursday morning before the disturbance exits the region and we return to dry northerly flow aloft, weak offshore flow, and partly cloudy skies by the afternoon. Temperatures remain well below seasonal norms.

On Tuesday night, expect the coldest temperatures of the week with strong radiational cooling under clear skies and mostly light winds. Light to moderate E flow through the mountain gaps should continue to ease. Inversions should form overnight and linger into Wednesday morning. Most locations can expect a very cold start to the day. By Wednesday afternoon temperatures begin to moderate slightly and freezing levels rise to around 1000 ft as light easterly low-level flow switches westerly ahead of a weak disturbance dropping south on N flow aloft.