Avalanche Forecast

Issued: Nov 30th, 2017 10:30AM

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

Wind slab is the most likely avalanche problem to encounter on Friday. Watch for firmer wind-transported snow on lee slopes near ridges. Early season terrain hazards still exist, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

Summary

Detailed Forecast

On Friday, a frontal system should stall over the Olympics and north Cascades before finally sagging south in the late afternoon and evening. Alpine winds generally out of the SW will increase late Thursday night and stay elevated through Friday. New and generally shallow storm snow instabilities may develop in the Mt. Baker zone on Friday. However, fresh wind slab may build at higher elevations in all zones during the day. 

Wind slab is the most likely avalanche problem to encounter on Friday, with deeper and potentially more sensitive slabs possible in the Mt. Baker area. Watch for firmer wind-transported snow on lee slopes near ridges, generally northwest through southeast aspects, but expect wind deposited snow on a variety of aspects in areas of more complex terrain.

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

Snowpack Discussion

Avalanche and Weather Summary

The great start to the 2017/18 PNW Winter lasted through mid November. Warm, wet weather before Thanksgiving caused wet snow and glide avalanches and snowpack consolidation with total snow depths decreasing by 50% or more from their mid-November peak depths. The old wet snow continues to slowly refreeze and from a crust of variable strength along the west slopes of the Cascades including the Passes.

A series of fast moving frontal systems have brought several inches of snow to the west slopes of the Cascades this week. Mt. Baker has received the most snow since Monday, about 2 feet. 

This graph shows new snowfall received at 5 different NWAC stations during the week. 24 hr snowboards are manually cleared by ski areas/DOT operations each morning. 

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'. 

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. 

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. 

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 1st, 2017 10:30AM