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Archived

Avalanche Forecast

Dec 11th, 2018–Dec 12th, 2018
Alpine
4: High
The avalanche danger rating in the alpine will be high
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
4: High
The avalanche danger rating in the alpine will be high
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable

Regions: Snoqualmie Pass.

We have received significant snowfall, strong winds, and have a weak snowpack. This is a recipe for avalanches. It is not the time to travel in areas where avalanches can start, nor linger where they can run or stop.

Discussion

Avalanche Summary

We have received observations of natural and human triggered avalanches in the Snoqualmie Pass area. Some of these avalanches are failing on a layer of buried surface hoar and facets. This is an obvious sign that unstable snow is present in the mountains. Observations suggest you are most likely to find these weak layers above 4000 feet where they survived a recent warm-up and rain event.

Avalanche Problems

Wind Slabs

At higher elevations you are likely to encounter wind drifted snow, firm hollow wind slabs, and areas stripped by the wind. Firm wind slabs can be difficult to assess. Use visual clues such as drifted snow, fresh cornices, and uneven snow surfaces to identify and avoid steep wind loaded areas.

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

Storm Slabs

In areas sheltered from the wind you can find unstable snow. Snoqualmie Pass has received a significant amount of precipitation creating the potential for storm slab avalanche. Observations have shown that in some locations the new snow has fallen on weak sugary facets and/or buried surface hoar. In these areas avalanches may be triggered remotely or propagate wider than expected. If you experience shooting crack, hear whumphing, or see new avalanches, avoid avalanche terrain. 

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.

Elevations: Treeline, Below Treeline.

Likelihood: Likely

Expected Size: 1 - 1