Avalanche Forecast

Issued: Jan 26th, 2018 11:37AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Avoid terrain where avalanches start, run, and stop until we exit this storm cycle. Warming temperatures, increasing winds, and additional precipitation will create conditions for very large avalanches. Avalanches may entrain significant amounts of snow and travel far into their runouts effecting traditional travel routes. Numerous hazards will challenge your ability to travel safely in the mountains Saturday. 

Summary

Detailed Forecast

Warming air temperatures, more precipitation, and increasing winds will develop very dangerous avalanche conditions Saturday morning. Expect natural avalanches to occur around periods of increased precipitation and significant warming.

Very large and destructive avalanches may occur Saturday entraining significant amounts of snow and traveling far into runout areas. Avoid traveling in areas where avalanches may start, run, and stop. Traditional travel locations may be impacted by larger avalanche starting at higher elevations. Identify and avoid traveling in areas steep open slopes overhead.

This is a time for very conservative terrain use. If you choose to travel in the backcountry utilize lower angle terrain well away from avalanche paths. Remember large terrain can produce large avalanches.

Cornices during this storm cycle have grown quite large. Give them a wide berth as they may break much farther back from the ridge than you might expect.

Snow conditions have changed significantly during this storm cycle. Tree-well and snow immersion hazards are very dangerous. Maintain communication with your partner at all times.

Snowpack Discussion

A very active storm cycle over the past nine days deposited significant amounts of snow along the west slopes of the Cascades and in the Passes. In most locations 2 to 3 feet of settled snow sits on top of the most recent crust layer.

Several subtle storm weaklayers can be found. Depth and sensitivity of theses layers varies with location. Observations demonstrate storm snow weaknesses gaining strength. Widespread loose surface snow conditions were reported in most areas.

Winds redistributed storm snow near and above treeline. This was most notable in the Crystal backcountry where much of the terrain was reshaped by strong SW winds.

Poor visibility and frequent storm has limited observations above treeline.

Observations

Central

Avalanche professionals in the Stevens Pass and Snoqualmie Pass areas Friday found very touchy 6-12 inch storm slabs in open terrain. Wind transportation of snow was localized to exposed ridges and upper elevations near and above treeline.

On Thursday morning, NWAC staff in the Alpental valley observed debris from a very large natural avalanche that released from Chair Peak. This avalanche likely occurred sometime Wednesday into early Thursday morning. The debris crossed Source Lake and extended far into the runout.  

Ski Patrollers at the closed Alpental ski area reported a large skier triggered slab avalanche 20" deep releasing in Powder Bowl. Fresh large cornices have built along ridgelines in this area.

South

NWAC Observer Jeremy Allyn was in the Crystal area Friday. He observed wind redistribution of snow forming wind slabs on lee slopes near treeline. In sheltered areas, Jeremy found soft snow conditions, with 3 feet of storm snow over the most recent crust layer.

NWAC forecasters in the Crystal backcountry on Thursday reported 1 to 2 feet of storm snow well bonded to the 1/16 crust. The new snow generally had a right-side up profile with recent storm instabilities healing. Occasional gusty winds transported recent snow near and below ridgeline.

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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Very Likely

Expected Size

1 - 2

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

Likely

Expected Size

1 - 1

Valid until: Jan 27th, 2018 11:37AM