Avalanche Forecast

Issued: Jan 22nd, 2018 1:20PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Very dangerous avalanche conditions are expected to develop Tuesday afternoon.  New storm and wind slabs may entrain significant amounts of recent snow, creating the potential for larger avalanches. Avalanche conditions will deteriorate further Tuesday night. 

Summary

Detailed Forecast

A frontal system will bring significant snowfall to the west slopes of the Cascades on Tuesday along with increasing winds and a warming trend forecast for late in the day and extending into Tuesday night.  The avalanche danger will quickly increase Tuesday, with very dangerous avalanche conditions developing in the afternoon and worsening overnight. Backcountry travel in avalanche terrain should be avoided Tuesday. 

Fresh storm slabs may entrain recent snowfall, quickly becoming larger and more dangerous than you expect. 

Older wind slabs may lurk beneath the most recent lower density snow. New or recent wind slabs may also produce large avalanches, entraining significant amounts of loose dry snow.

Snow immersion and tree-well hazards continue to increase and pose significant danger. Maintain constant communication with your travel partners.

Snowpack Discussion

The active weather pattern seen over the last week will continue on Tuesday. Around 2 feet of snow now sits above the most recent melt-freeze crust in the central and southwest zones. The most recent storm cycle wrapped up Sunday night with cooler temperatures, light to moderate westerly winds and snow showers adding an additional 2-6 inches Monday. 

Moderate to strong winds transported snow forming wind slabs on a variety of aspects at Snoqualmie Pass and the southwest zone including Crystal, Paradise and White Pass Sunday. Less wind transport occurred in the Stevens Pass area Sunday.  

Observations

Central

An avalanche professional in the Stevens Pass area Sunday reported 18 inches of generally right-side-up snow well bonded to the 1/16 crust. No avalanches were observed. 

An avalanche professional in the Snoqualmie Pass area Sunday reported 12 inches of snow well bonded to the 1/16 crust. They observed wind transportation of snow above 4000 feet. Several natural, small and isolated wind slab avalanches were seen in very steep northerly terrain. 

A public observation from the Snow Lake Divide area of Snoqualmie Pass reported a skier triggered wind slab avalanche that broke above the skier and was 12-18" deep. 

South

Professionals in the Crystal area Monday reported 1 to nearly 2 ft of snow above the 1/16 crust in wind sheltered terrain generally well bonded. In areas stripped of snow during Sunday's storm, less than 8" sits above the most recent crust and is poorly bonded. Loose dry avalanches were the biggest avalanche problem encountered Monday, found in steep terrain at higher elevations and easily ski triggered with the potential to run long distances. 

Problems

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

Very Likely

Expected Size

1 - 1

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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Very Likely

Expected Size

1 - 1

Valid until: Jan 23rd, 2018 1:20PM