Mountain Weather Forecast

Weather Synopsis

Rounds of increasing moisture continue to impact the Pacific Northwest as a weakening cutoff low ejects northward along the Pacific Northwest coastline through Tuesday evening. The SSW upper-level flow associated with this weather pattern has largely been splitting the Cascades as bands of showers increase across the region.

Banded precipitation should increase Monday night with the low passing off the Oregon Coast. Bands may have locally heavier precipitation, but any such precipitation would be short-lived. Light to moderate ridgeline winds increase and moderate low-level E flow below 4500 ft increases through the night. This increasing E flow combined with subtle upper-level cooling should enable precipitation that initially starts as mixed precipitation in the central Cascade passes to most likely turn over to snow at times. The wintry precipitation should continue for the east slopes and pass through Tuesday morning before a transition to rain and freezing rain everywhere except the Methow Valley and Washington Pass. The system ends warm in most areas. Rain and high-elevation snow predominate west of the Cascade Crest with snow levels generally above 5500 ft.

The steadiest precipitation arrives Tuesday morning with a round of moderate SSW winds that focus locally heavy precipitation on Mt St Helens, Mt Adams, and Mt Rainier before the band lifts into the northern Cascades and activity becomes drippy or showery further south. Showers linger in the northern Cascades Tuesday night on a continued SW flow but gradually trail off. A new cutoff low moves into position off of California's coast, resulting in fair weather and very light winds over the region on Wednesday. Freezing levels rise to around 5500-8500 ft across the region with light to moderate E flow keeping the passes cool.

Cloudy skies cover the Pacific Northwest on Monday morning. A weak disturbance passed east of the Cascades early this morning, bringing very light rain to Wenatchee, but mountain locations have remained dry thus far. A second wave of moisture moves currently over southern Oregon moves northward into the region by the afternoon and through the northern portions of Washington state during the evening hours. Another band of light to moderate precipitation moves northward in the early morning hours on Tuesday through the end of the day.

The devil is in the details for the east slopes of the Cascades and mountain gaps. For places like Snoqualmie Pass and Stevens Passes, precipitation likely starts as freezing rain or sleet, may mix with snow through late morning on Tuesday, and then changes back to rain. Snow accumulations are more likely for Stevens pass with 1-4" possible. Washington Pass should remain mostly snow. The system ends warm as E winds ease somewhat and SSW flow continues.