Avalanche Forecast

Issued: Dec 1st, 2017 8:45PM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs and Storm Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Isolated and stubborn pockets of wind slabs may be encountered on Saturday near ridges.  Storm slabs are unlikely to be encountered but due to recent light snowfalls, we cannot completely rule them out.  Early season terrain hazards still exist, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

Summary

Detailed Forecast

Generally light and scattered showers Saturday morning should transition to more widespread shower activity in the afternoon. Showers are expected to be heaviest in the Paradise area Saturday afternoon and evening. See the NWAC mountain weather forecast tab for more details. 

Isolated and stubborn pockets of wind slabs may be encountered on Saturday near ridges.  Storm slabs are unlikely to be encountered but due to recent light snowfalls, we cannot completely rule them out.  

Early season terrain hazards still exist, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

Snowpack Discussion

Warm, wet weather before Thanksgiving caused wet snow and glide avalanches as well as significant snowpack consolidation. There are no snowpack concerns below the Thanksgiving crust and in general new snow received post-Thanksgiving has reportedly bonded well throughout the range. 

During the week, a series of frontal systems have generally produced light amounts of the new snow for the west slopes of the Cascades. The exception is in the Mt. Baker area who received about 3 feet of snow this week including roughly a foot during the day on Friday. No significant avalanche activity has been observed outside of the Mt. Baker zone. 

Observations

North

On Thursday, Mt. Baker pro-patrol triggered isolated pockets of wind slab about 20 cm (8") deep during control work.  Also on Thursday, guides in the Mt. Baker area reported about 85 cm of recent storm snow above the Thanksgiving crust bonding well to the crust and within recent storm layers. However, active wind loading was occurring on north facing aspects around 5000'. 

By Friday, steady snowfall led to widespread and very touchy wind and storm slabs on all aspects and elevation bands on terrain ~35 degrees or steeper. NWAC observers in the Mt. Baker backcountry reported average slab depths of 30 cm in sheltered areas and about 1 meter of snow above the Thanksgiving Day crust. Winds were gusty and distributing snowfall onto all aspects. 

Central

Wednesday in the Alpental backcountry, NWAC observer Ian Nicholson found areas of unreactive wind slab on some east facing terrain features near ridges. Daytime solar warming had allowed surface snow to consolidate and form shallow storm slabs, but no avalanches were observed or triggered. 

On Thursday, NWAC observer Jeff Ward found about 40 cm of settled snow well bonded to the Thanksgiving Day crust in the Stevens Pass backcountry. With good visibility, no avalanche activity was noted in the surrounding terrain.  

South

On Thursday in the Crystal backcountry, NWAC pro-observer Dallas Glass found 25 cm (10") of this week's storm snow bonding well to the stout and fully supportable Thanksgiving rain crust. Shallow wind slab averaging 10-15 cm (4-6 cm) thick was found below ridges on W-N-E aspects, but was isolated in distribution. Total snow depth in this area peaked at about 1 meter at 6000' and above.  NWAC pro-observer Jeremy Allyn found similar conditions on Friday and noted the lack of cornice formation along ridges. 

Problems

Wind Slabs

An icon showing 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: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Storm Slabs

An icon showing 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: All elevations.

Likelihood

Unlikely

Expected Size

1 - 1

Valid until: Dec 2nd, 2017 8:45PM