Stevens Pass Avalanche Forecast

Issued: Dec 13th, 2013 10:00AM

Fri Dec 13th Current Conditions
Alpine Considerable Treeline Moderate Below Treeline Moderate
Sat Dec 14th 2 Day Outlook
Alpine Moderate Treeline Moderate Below Treeline Moderate

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

Slowly stabilizing shallow storm or wind slab layers require close attention, especially in the alpine regions.

Summary

Detailed Forecast

High pressure should rebuild along the coast Saturday producing warming and generally light crest level winds. This weather should allow for wind or storm slabs to gradually settle in place as the weak underlying snow further consolidates. This should lead to an overall slowly decreasing danger. However, pay close attention to local conditions, especially if the area you are traveling in received more significant new snow. Watch for evidence of wind transported snow or potentially unstable pillows overlying the likely weak snow formed over the past week.

Snowpack Discussion

The most recent storm cycle occurred in early December with most NWAC stations receiving copious amounts of precipitation, mainly in the form of rain at mid and lower elevations followed by a sharp cooling trend and generally 1-2 feet of snow for the west slopes at high and mid elevations. This storm cycle created a strong and generally thick rain crust with cohesionless new snow above the crust and produced little avalanche activity. 

A dry and very cold period followed with two bouts of wind, first strong easterly ridge top winds last Friday and Saturday and then winds shifted to strong west to northwesterly on Sunday and Monday, transporting loose surface snow and building small and isolated wind slabs on a variety of lee slopes.  A shallow wind slab was triggered on East Peak near Crystal Mountain last Sunday. 

The dry and cold stretch produced widespread reports of thick surface hoar growth, near surface faceted snow and of the strong crust breaking down due to strong temperature gradients within the upper snowpack.

On Thursday, several hours of very strong westerly winds hit the Cascade crest, especially as recorded by NWAC weather stations at the top of the Mt Baker ski area. This wind transported loose surface snow and did build pockets of wind slab that produced some ski triggered but relatively shallow slab releases, mainly at the Mt Baker area.

A weak front has now moved across the region as of Friday and it turned out to be weaker than expected. None-the-less even small new snow amounts of 3 inches at the Mt Baker area coupled with ridgetop winds and Thursdays wind event did build unstable wind slabs on many slopes near ridgetops, especially northeast facing slopes. reports from Mt Baker patrol indicated that there were a few 8-12 inch triggered slab releases by public skiers in the backcountry. Patrol also ski cut several small slabs as well.  These shallow wind slabs were releasing on weak near surface faceted snow formed under the recent cold temperature regime.

Elsewhere, most areas only received about 2-5 inches of snow from the front Thursday night and early Friday and there have been few other reports of activity Friday. In general, it is likely that shallow storm or wind slab layers have formed over weak low density snow, surface hoar or exposed crust layers. While these potential unstable layers might be sensitive in areas they should remain relatively shallow in most areas.

Problems

Wind Slabs

An icon showing Wind Slabs

Likelihood

likely

Expected Size

1 - 1

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.

Elevations:

Alpine, Treeline.

Storm Slabs

An icon showing Storm Slabs

Likelihood

possible

Expected Size

1 - 1

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:

Alpine, Treeline.

Valid until: Dec 14th, 2013 10:00AM

Forecast Trend

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