Avalanche Forecast
Issued: Dec 6th, 2015 12:03PM
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
Storms will continue to cross the Northwest Sunday night and Monday. This will cause new slab or maintain persistant slab avalanche conditions along the Cascades east slopes especially in the northeast and north central zones.
Summary
Detailed Forecast
A warm front on Sunday night will be rapidly followed by another warm front on Monday. Strong southwest to west winds should be seen in the alpine with moderate to heavy rain or snow mainly Sunday night and again by Monday midday with further warming. Snowfall so far in December by Monday morning along the east slopes should range from 1-4 feet which the most in the northeast and north central zones.
More new potentially large wind and storm slab will continue to be a focus on Monday along the east slopes. Back country travel in the near or above treeline is not recommended on Monday especially in the northeast and north central zone. Visibility may be poor on Monday with slab layers increasing making it difficult to avoid avalanche terrain.
Dangerous persistent slab should still be expected on Monday. Persistent slab may be human triggered where persistent weak layers have been buried intact especially in the northeast and north central zones. Stay on low angle slopes and watch and listen for clues like shooting cracks and whumping and use caution until these layers are known to have stabilized.
Further away from the crest such as at Blewett Pass and in the southeast zone, less avalanche problems are expected due to less snow.
Snowpack Discussion
Note that the discussion and forecast text is the same along the east slopes but the avalanche danger forecast varies by zone.
Weather and Snowpack:
The northeast and north central Cascade zones have the deepest snowdepths found throughout the Cascades this season thanks to cold air pooling on the east side in otherwise warm storms for the west. Snowdepths are generally in the 1-6 foot range along the east slopes with the most in the northeast zone and the least at Blewett Pass and in the southeast zone.
We had a wet and wild November and this formed a strong crust by mid November in all areas.
A long stretch of cold weather led to widespread near surface faceting and surface hoar formation along the east slopes in late November.
The weather so far in December has gotten very active with an overall warming trend.
This weather and snowpack scenario better get everyone's attention because it is a great set up for avalanches!
Reports:
The most important report for along the east slopes comes from Mission Ridge Ski Area on Sunday where the pro patrol reported numerous and often sympathetic easily triggered slab avalanches of about 8-16 inches releasing on a rain crust from early December. One very large explosively triggered slab avalanche on the northeast slope of Windy Ridge was 5 feet deep and propagated several hundred feet which released on the mid November crust.
NWAC observer Jeff Ward was at Washington Pass on Friday and was surprised by the lack of activity on the potential PWL and considered the conditions uncertain.
So the mid November crust and late November facets and hoar frost should still be reactive to upcoming loading by rain and snow.
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 7th, 2015 12:03PM
