Stevens Pass Avalanche Forecast

Issued: Dec 14th, 2013 8:50AM

Sat Dec 14th Current Conditions
Alpine Moderate Treeline Moderate Below Treeline Low
Sun Dec 15th 2 Day Outlook
Alpine Moderate Treeline Moderate Below Treeline Low

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.

Continuing stabilization of shallow storm or wind slab layers. Be extra cautious in alpine regions where more recent snow may have accumulated, especially where wind loading has occurred. 

Summary

Detailed Forecast

A very weak front should move through the high pressure ridge Sunday. Only very light amounts of precipitation are expected overnight Saturday and Sunday at initially high freezing levels but with a cooling trend. This weather should further allow for any unstable layers to bond and settle in place as warm dense surface snow helps to consolidate older weak snow from last week, causing 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 any wind transported snow at higher elevations where colder surface snow may be available to transport to lee slopes. 

Snowpack Discussion

The last significant snow fell over the first few days of December along. This storm produced heavy rains with a significant cooling trend that deposited 1-2 feet of low density snow over most sites in this region. Following this storm cycle, a strong and generally thick rain crust formed below the gradually lower density surface snow and also formed a relatively good bond to this crust. With the cooling profile and lack of significant wind, little avalanche activity occurred shortly after the storm.  

Dry and very cold weather followed with periods of both strong easterly ridge top winds a week ago, then strong west to northwesterly winds last Sunday and Monday. This transporting loose surface snow and built small and isolated wind slabs on a variety of lee slopes. and produced some isolated triggered soft slabs on higher steep lee terrain.  

The dry and cold stretch produced widespread and thick surface hoar growth, near surface faceted snow and began to break down the strong crust in some areas due to re-crystallization from 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.

The surface snow conditions have all but changed as of this Saturday as a result of significant warming and a little light rain or heavy dense snow Thursday night and early Friday. Precipitation amounts remained light but some areas of unstable wind or storm slabs did produce ski triggered slabs Friday in the Mt Baker area ranging from 10-30 cm (4-12 in.) 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 or light rain at mid and lower elevations by early Friday. Other than the Mt Baker area there have been few other reports of activity as of Saturday afternoon. Given a full day of warm temperatures, some previous light rain or heavy wet snow late Thursday, this will likely settle the previous low density snow making triggered releases much more difficult from any remaining shallow wind or storm slab layers.  These potential unstable layers may remain more sensitive on some steep shaded northeast facing terrain at the highest elevations, though remaining relatively shallow in most areas.

Problems

Wind Slabs

An icon showing Wind Slabs

Likelihood

possible

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 15th, 2013 8:50AM

Forecast Trend

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