Avalanche Forecast
Regions: Cascades - North West.
Dangerous avalanche conditions exist, especially above treeline. Caution, areas of new wind deposited snow are likely Saturday on unusual aspects. Watch for wind stiffened surface snow on a variety of aspects and avoid steep terrain with evidence of wind loading.
Detailed Forecast
A weak disturbance will move across the region Saturday, causing moderate to strong NW ridge level winds may build new fresh wind slabs on lee slopes, near and especially above treeline.Â
Light snow showers Saturday may bury intact a variety of weak surface snow types formed this week under fair cold weather.
Local wind slabs from the recent E-NE winds are likely Saturday. This should be mainly on westerly to southeasterly aspects near ridges.
Watch for firmer wind transported snow on all aspects, mainly near and especially above treeline.
The December 8th layer continues to round and gain strength and should be less sensitive to trigger.  Do head for lower angle slopes and ratchet back your plans if you experience collapsing or whumpfing or have evidence of this layer such as from snowpits.
Even though the lower part of the below treeline band is filling in, watch for early season travel hazards such as barely covered rocks and open creeks.
Snowpack Discussion
Weather and Snowpack
The last storm cycle to affect NW, Washington Cascades was from late Thursday, December 8th through Monday morning, December 12th. About 3 inches of water equivalent and over 3 feet of snow were recorded at the NWAC station at Mt Baker ending Monday morning Dec. 12th.Â
Fair and cold weather this week has been interspersed with periods of moderate N-NE-EÂ winds both Wednesday and again Friday. These winds were strong enough to build fresh wind slabs on more atypical SW-NW facing terrain, mainly near and especially above tree line.
There has been about 12 inches of snowpack settlement since the storm cycle ended, allowing underlying or storm snow related weak layers to stabilize.Â
Several clear nights with light winds this week have allowed for weak surface snow development. On wind and sun sheltered terrain, extensive near surface faceted snow or surface hoar have formed this week and this may be an important weak layer if buried intact by the expected return of light snowfall Saturday.
A thin sun crust is likely to have formed on steeper southerly facing slopes, which also may be a future interface for avalanches when loaded with sufficient wind transported or future storm snow.
Recent Observations
A regional avalanche cycle was seen December 8th-11th especially along the west slopes with some avalanches releasing on the December 8th persistent weak layer in the Washington Cascades.Â
The Mt Baker pro-patrol reported decreasing avalanche control results on Monday with only local new 7 inch storm slab on sheltered slopes.
Skiers and riders have been accessing steep terrain over the past few days with no reported avalanche incidents in the Mt Baker area.Â
NWAC observer was in the Yellow Aster Butte area near Twin Lakes Friday. Strong NE ridge level winds were transporting significant surface snow and building sensitive fresh wind slabs on atypical South - West facing terrain. These new wind slabs changed to intended objectives, opting for safer lower elevation terrain. Even so, where small areas of wind slab were encountered, they were sensitive to trigger. These new wind slabs are the primary concern in this zone.
In the same area, the December 8th persistent weak layer is becoming more difficult to find and gaining strength through gradual rounding. Snowpack tests on this layer Friday in that area were unreactive.
There are a variety of current snow surface types awaiting our next storm snow, including surface hoar, near surface faceted snow, sun crusts, wind sculpted snow, and low density snow in protected areas. Â Â
Avalanche Problems
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
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: Unlikely
Expected Size: 1 - 1