Avalanche Forecast
Issued: Dec 9th, 2015 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs, Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Persistent weak layers are likely to linger in the northeast zone with a transition to the north central and southeast zone. Persistent weak layers are dangerous and cause large, rapidly propagating avalanches.
Summary
Detailed Forecast
Cascades East Slopes Forecast
An occluded front should move south to north over the Northwest Wednesday night and Thursday. South to southeast alpine winds Thursday morning should generally shift to southwest Thursday afternoon. Look for mostly light to moderate amounts of snow by later Thursday.
Along the east slopes watch for possible new wind slab on lee slopes and new storm slab in calmer less exposed areas. The light to moderate amounts of snow and the cooling should somewhat limit the extent of new wind and storm slab. Remember that the avalanche danger will be greater if you encounter stronger winds and heavier snowfall. Evaluate snow and terrain carefully Thursday. Change your plans if you find yourself in unexpectedly prolonged stormy weather.
Note that persistent weak layers are likely to linger in the northeast zone with a transition to the north central and southeast zone. Do not assume that these layers have stabilized especially in the northeast zone.
Less snow below tree line should limit the avalanche danger there.
Snowpack Discussion
Cascades East Slopes Weather:
Snowfall over the east slopes the first 7 days of December was about 1-4 feet with an overall warming trend. Then an atmospheric river arrived the past couple days (Main Fork of the Pineapple Express). This looks like it caused about another 14 inches of snowfall in the northeast zone, rain in the southeast zone, with a transition in between in the central east zone.
But snowfall has resumed on Wednesday with a cooling trend.
Cascade East Slopes Snowpack and Reports:
The snow and warming trend the first week of December followed by the heavy rain, building on crust layers and on faceted snow and surface hoar caused instability and avalanches mainly Saturday to Tuesday such as seen at Mission Ridge.
It seems we now likely have varied conditions along the Cascade east slopes.
In the southeast zone there should be a shallower more consolidated and more stabilized snowpack.
Beware that persistent weak crusts and faceted snow and buried surface hoar layers are likely lingering in the northeast zone. These potentially dangerous layers will have been loaded by the additional snowfall in the northeast zone the past couple days! This layer may still be reactive and it should still be best to proceed with very great caution until there are more definitive observations.
A transition is likely in the central east zone between the conditions of the northeast and southeast zones.
NWAC observer Tom Curtis was at Blewett Pass in the central east zone on Wednesday and found sudden collapse test results in faceted snow at the November crust 15 cm from the ground. Persistent weak layer presence and distribution there is still uncertain.
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
Expected Size
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
Expected Size
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: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 10th, 2015 10:00AM
