Avalanche Forecast
Regions: Olympics.
Recent or new wind slab is expected to be the main avalanche problem on Friday. But there is a lot of uncertainty regarding the December 17th PWL and skiing or riding on lower angle slopes is the safest bet until there is more certainty that this layer is no longer a problem.
Detailed Forecast
A weak front will move across the Northwest Cascades late Thursday. This should cause southwest winds and a cooling trend. Snowfall should be pretty light except with 5-10 inches looking likely in the Mt Baker area. By Friday a large digging trough offshore should cause much lighter winds and but with renewed snow mainly over the south Cascades with low snow levels.
Wind slab should be the primary problem Friday. Southwest winds in the last storm cycle and for the late Thursday system make this most likely on north to southeast slopes. Watch for firmer wind transported snow mainly north to southeast slopes.
The latest tests of the December 17th PWL in the Cascades don't seem to indicate a regionally reactive layer. There is a lot of uncertainty regarding this layer and there still may be a lot of variability from area to area and location to location. Snow pits valid for slopes you intent to ski or ride may give some indication of the presence and reactivity of this layer. But skiing or riding on lower angle slopes is the safest bet until there is more certainty that this layer is no longer a problem. While triggering this layer seems unlikely  remember that PWL's generally cause larger avalanches.
Mostly light new snow amounts and the cooling trends in the Olympics and Washington Cascades make a significant new storm slab problem seem unlikely on Friday.
The surface crust formed in some areas following the storms early this week is reported to be strong and hard enough in some areas of the west slopes to present an out of control fall danger. Avoid steep hard slopes where there will be fall consequences if you are not confident you can manage this problem by walking or using ski or boot crampons.
Snowpack Discussion
Weather and Snowpack
Clear and cold weather from Wednesday, December 14th to Friday, December 17th allowed widespread surface hoar and near surface faceted snow to develop in the Olympics and Cascades. Thin sun crusts formed on steeper solar slopes during sunny periods. In many areas, these persistent grain types were buried intact by December 18th during a period of light snow and light winds.
Strong westerly flow directed two Pacific frontal systems across the Northwest Sunday night and again Monday night with generally up to an inch of water accumulating at the Hurricane Ridge station through early Tuesday morning. Storm snow totals varied quite a bit due to a mix of precipitation types during the storm cycle. Many areas in the Cascades experienced a natural avalanche cycle involving either shallow, loose wet or storm slabs Monday night or early Tuesday morning.  A strengthening rain crust was noted in many areas by later in the day Tuesday with the arrival of colder air. Â
Westerly winds were especially strong with the 2nd system late Monday night and into Tuesday with gusty winds mixing down into usually more wind sheltered terrain.
Cool, dry weather with sunshine Wednesday helped settle recent storm snow and possibly create a thin sun crust on steep directly south facing slopes. Â
Recent Observations
The road to Hurricane Ridge is closed midweek and no observations have been received since last weekend.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: North, North East, East, South East, North West.
Elevations: All elevations.
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