Avalanche Forecast
Regions: Cascades - South East.
The avalanche danger will be decreasing on Wednesday. But the rate of this decrease is less certain so you will need assess conditions and be able to make wise terrain choices if you are out on Wednesday.
Detailed Forecast
Light snow showers along the east slopes should end by Wednesday morning with decreasing or lighter NW-NNW alpine winds and much cooler temperatures. Sunshine is likely by Wednesday afternoon.
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 or December 18th interfaces. 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.
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.
Snowpack Discussion
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 especially below about 5-6000 feet east of the crest.
West flow began to carry a series of fronts across the Northwest starting about December 16th. Some initial warmer temperatures and light rain may have helped wet or destroy some of the surface hoar and near surface faceted snow, but likely not in all locations especially below about 5-6000 feet east of the crest.
The last front in the series is crossing the Northwest on Tuesday. Storm snowfall totals Wednesday morning will be in the 2-3 foot range in the northeast Cascades and probably in the 6-12 inch range in the central east and southeast Cascades. Very strong southwest crest level winds have been seen Tuesday. Temperatures rose a bit to the upper 20's or near freezing Tuesday depending on elevation and will begin cooling on Tuesday night.
 East-side snowdepths are highest in the Washington Pass area and lowest in the southeast zone.
Observations
North
The NCMG passed on information to the NWAC for the Washington Pass and Holden area for Tuesday. In summary several large natural avalanches were seen and skiers were triggering very touchy storm slab which in some cases was stepping down to about 60 cm to a rain crust from Monday December 18. It was uncertain if the December 16 buried surface hoar or faceted snow was present.
On Sunday, a NCMG group found surface hoar buried intact up to 6000' in the Washington Pass area by 10-15 cm of recent snowfall. No signs of recent avalanche activity were noted.Â
Central
The Mission pro patrol on Tuesday reported new 6-10 inch wind slab on specific lee east slopes above about 6400 feet. No avalanches were triggered but the wind slab was not bonding well to previous snow. Other windward slopes were scoured.
South
No recent observations.
Avalanche Problems
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1
Wind Slabs
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1