Avalanche Forecast

Light snow showers along the west slopes should end on Wednesday morning with decreasing or lighter NW-NNW alpine winds and much cooler temperatures. Sunshine is likely by Wednesday afternoon especially in the northwest Cascades.

This means the avalanche danger will be decreasing on Wednesday. But the rate of this decrease is less certain so you will need assess conditions if you are out on Wednesday.

Storm slab will linger on Wednesday in areas that had at least several hours of rapidly accumulating snowfall and that developed weak storm layers on Tuesday.

Given the strong winds, small to large wind slabs are still likely near and above treeline, mainly on NW through SE aspects near ridges, but are possible on other aspects. Avoid ridges or terrain features with firmer wind transported snow.

Slab avalanches would propagate farther if they step down the December 16th interface. Snow pits will help determine if this layer is present in your area.

Loose wet avalanches will not be listed as an additional problem due to the low sun and cool temperatures. But start to watch for loose wet conditions on steep solar slopes if you see significant rollerballs or find surface wet snow deeper than a few inches.

Remain near your partner and keep them in sight and direct communication at all times due to tree wells and and the threat of snow immersion. If your partner falls into tree well, you have to be very nearby to help them in time to save them.

Remember that beneath snow received during this storm cycle, early season terrain hazards exist, especially at lower elevations. Expect terrain hazards with poorly covered rocks, vegetation and creeks, particularly on south-facing terrain, at lower elevations, and on exposed ridges where wind events have stripped much of the season's snowcover.

Fair weather and inversions dominated most of the first half of December. This allowed snow to consolidate and created varied snow surfaces including surface hoar and near surface faceted snow.

West flow began to carry a series of fronts across the Northwest starting about December 16th. Some warmer temperatures and light rain likely helped wet or destroy surface hoar and near surface faceted snow at lower elevations, including Snoqualmie Pass, but given recent observations, potentially not in all locations such as in the Stevens Pass area.

The last front in the series is crossing the Northwest on Tuesday. Storm snowfall totals Wednesday morning along the Cascade west slope including the passes will be in the 20 inch to 4 foot range with the most at the top of Alpental, Stevens Pass and Mt Baker. Very strong southwest crest level winds have been seen Tuesday. Light east winds much of Tuesday are changing to west in the Cascade Passes Tuesday late afternoon. Temperatures rose a bit to the upper 20's or near freezing Tuesday depending on elevation and will begin cooling on Tuesday night.

Observations

North

On Tuesday morning the Mt Baker pro patrol reported widespread triggered storm slab, up to about 12 inches, some very large, and continual natural avalanches to below treeline. 

Central

On Tuesday the Alpental pro patrol also reported widespread natural and very touchy 8-12 inch storm slab at all elevations due in part to a weak storm layer buried about at 8 inches. This included very long distance sympathetic releases and at least one release possibly due to vibration of the chair lift!

Touchy storm slab at Alpental on Tuesday December 19th. Photo John Stimberis

The pro patrol at Stevens Pass Resort on Monday afternoon reported no new activity on slopes that hadn't previously been controlled and the new snow was not slabby. Earlier in the morning, the patrol reported widespread ski triggered and some natural soft slabs, 18-24" deep mostly on lee aspects near ridgetops, running on the December 16th weak layers. Some skier triggered shallow slabs on lower mountain entraining wet snow to create large avalanches. 

Two professional observations from Skyline ridge came in from observations on Monday afternoon. One found a sudden collapse at 45 cm on the December 16th interface, likely due to surface hoar or near surface faceted snow. The other observer did not find this problem. The recent 1-2 feet of new snow was right-side-up snow, increasing with elevation, and lay on top of a minor crust from rain on December 17. Thanksgiving crust was down 3.5-4 feet.

Additional observations and test results on the December 16th potential weak layer are available via the NWAC observations page.

South

No recent observations.