Avalanche Forecast
Regions: Cascades - East.
Caution, areas of new wind deposited snow are possible Saturday on unusual aspects, mainly near or above treeline. 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
Cold and fair weather early last week allowed for a sun crust to form on many solar aspects and for cold low density snow, near surface faceting and/or surface hoar to become fairly widespread in the Washington Cascades.
Many of these weak snow grain types were buried by the last storm cycle to affect the east slopes of the WA Cascades from late Thursday, December 8th through Monday morning, December 12th. About 1-3 feet of snow were recorded at the NWAC stations in these zones 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 4-8 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 with some avalanches releasing on the December 8th persistent weak layer in the Washington Cascades.
An observation for the Mission Ridge via the NWAC Observations tab for Sunday in the Lake Marion area indicates a right side up snowpack, but also noted spatial variability and 10-35 cm wind slab giving shooting cracks on loaded slopes.
Pro-observer Jeff Ward was at Washington Pass on Sunday and  noted slightly increasing slab structure near ridges above about 7000 feet with some cracking and limited propagation when testing steep convex slopes.
The pro-patrol at Mission Ridge on Monday gave a very similar report to the Lake Marion report above, that of a generally shallow, but stable snowpack, also noting spatial variation and a sensitive snowpack on local steep wind loaded N-E slopes.
More critically, on Tuesday there was a skier triggered slab avalanche release on the December 8th Persist weak layer in the Highland Bowl back country next to the Steven Pass ski area. The Stevens pro-patrol reports it was on a steep SSW slope at 5400 feet with a 40 cm x 100 foot crown releasing in buried hoar frost and facets on a crust.Â
A member of the Stevens Pass pro-patrol reported wind transport on Rock Mountain and especially in the Chiwaukums on Wednesday.
The most recent reports for the Washington Pass is well stated on our recent observations page! Â
NWAC Pro observer found weak faceted snow above the ground in a still fairly shallow overall snowpack in the Blewett Pass area Thursday. No observed avalanches were noted, but snowpack tests indicate the potential. See a recent NWAC instagram post about the results of a PST test.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: Possible
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