Mountain Weather Forecast

Weather Synopsis

A ridge of high pressure has gained hold across the Pacific Northwest by Monday afternoon with the ridge axis located offshore. Subsidence is warming temperatures at middle and upper elevations, primarily west of the Cascade Crest. Meanwhile, a cold pool has deepened over the east slopes of the Cascades. The battleground between these air masses is at the Cascade Crest where increasing light to moderate E winds prevent temperatures from warming below around 5000 ft. This effect has been particularly pronounced at Washington Pass where the upper station is 32F and the base station is 21F.

Monday night the ridge axis moves onshore. NE winds that were in the moderate range at higher elevations on Monday gradually ease and become light overnight. The inversion continues to strengthen east of the Cascades, so expect increased inversions. Furthermore, E flow should increase and peak around 4 AM with moderate winds possible through the mountain gaps despite the very light upper-level flow. Clouds forming beneath the inversion should thicken and should become entrenched below 3000 ft, particularly E of the Cascades, but will continue to stream into Snoqualmie Pass through Tuesday. Multiple freezing levels will be the norm Monday night and Tuesday with above-freezing air generally in the 5500 to 9500 ft range (give or take 1000 ft).

Expect sunny skies to start the day on Tuesday, with high clouds lowering and thickening first around mid-morning in the Olympics, and then starting around noon in the Cascades. These clouds and a switch to gradually increasing SW flow aloft result from a weakening system getting sheared apart as it moves into the high-pressure ridge.

This weakened system arrives Tuesday night with very light rain and snow primarily for the western slopes of the Olympics and from Snoqualmie Pass northward. The precipitation may linger into early Wednesday for the Mt Baker area. Precipitation totals should generally be around 0.1" or less above the 5000-6000 ft rain/snow line. The ridge rebounds during the day, quickly quashing the cloud cover as freezing levels hover around 6000-8000 ft. The W winds should mostly eliminate the low-level inversion and cloud cover east of the Cascades.