Avalanche Forecast
Regions: Stevens Pass.
Back country early season conditions are changing fast! The forecast has been updated early Sunday morning for dangerous conditions. Back country travel is not recommended Sunday near and above treeline.Extra caution is needed in the Stevens and Snoqualmie Pass areas due to an unusual and potentially dangerous persistent weak layer.
Detailed Forecast
Strong alpine winds and moderate to heavy rain and snow will accompany a cold front across the Northwest Saturday afternoon to Sunday morning. This should bring 1-2 feet of new snow near and above treeline with a warming trend which greatly increases the likelihood of storm and wind slab layers.
Back country early season conditions are changing fast! New potentially large storm and wind slab will be the focus on Sunday especially in the Mt Baker area but also along the entire west slopes. Human triggered storm slab should be likely to very likely on varied aspects. Human triggered wind slab should be likely to very likely mainly on lee north to east slopes especially above treeline. Back country travel is not recommended Sunday near and above treeline.
The next strong system should begin to move into the Olympics and Cascades Sunday night. But be prepared to curtail your plans later Sunday if conditions deteriorate sooner than expected.
Recent observations have identified a persistent weak layers in the Stevens Pass area. We don't know anything about this layer yet in the Snoqualmie Pass area. We don't deal with PWL layers very often near and west of the crest so we need to use extra caution in the Stevens and Snoqualmie Pass areas until more information is obtained. Stay on low angle slopes and watch and listen for clues like cracking and whomping.
Early season hazards exist for much of the below treeline band throughout the Northwest, so ski and ride with caution.Â
Snowpack Discussion
Note: The snowpack discussion is for the entire west slopes of the Cascades but the avalanche danger forecast varies by zone. Â
We had a wild and wet month of November, with 12- 25 inches of water at west side NWAC stations only amounting to about a 1 meter or 3 foot snowpack in the north Cascades above 4000 feet and above 6000 feet elsewhere. Then strong high pressure led to strong temperature inversions and brought very cold air through the Cascade Passes over Thanksgiving weekend. The weather pattern has become more active for the first few days of December with a few inches of new snow over the west slopes paired with moderating temperatures in the Passes.Â
Update Sunday morning: There has now been about 1-3 feet of snow with an overall warming trend since the start of the month along the west slops with the most at Mt Baker. New NWAC pro-observer Simon Trautman was near the Mt Baker Ski Area on Saturday and reported reactive upside down P hardness wind slab over lower density layers with numerous natural storm slab avalanches and some remote ski triggered wind slab avalanches up to about 15 inches. The avalanche danger forecast for the west slopes of the Washington Cascades has been nudged up due due to this new information.
The west slopes do not have a uniform maritime snowpack! Due to the cold temperatures and clear skies near the end of November, buried surface hoar can be found in the Stevens Pass area. NWAC pro-observer Dallas Glass found this layer at 4-5000 feet at Skyline on E-NE aspects around 45 cm down. Faceting may also be found at a similar depth on solar aspects around a buried sun crust.  Stevens pass pro patrol reported a 12-14 inch (30-35 cm) natural persistent slab avalanche in closed Corona Bowl of the ski area Friday on a N-NW aspect.  Professionals also reported shooting cracks and whumpfing in the area as temperatures warmed and the slab above the PWL became cohesive.
Avalanche Problems
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
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.
Elevations: Alpine, Treeline.
Likelihood: Very Likely
Expected Size: 1 - 1
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: Very Likely
Expected Size: 1 - 1