Stevens Pass Avalanche Forecast

Issued: Dec 19th, 2013 10:00AM

Thu Dec 19th Current Conditions
Alpine High Treeline High Below Treeline Considerable
Fri Dec 20th 2 Day Outlook
Alpine Considerable Treeline Considerable Below Treeline Moderate

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

Bottom Line: Strong storm building through the day Friday with heavy precipitation at warming temperatures and strong winds creating very dangerous avalanche conditions by afternoon. 

Summary

Detailed Forecast

The strongest storm in some time should begin to deposit shallow snow Thursday night and increase and become moderate to heavy through the day Friday along with significant warming and strong crest level winds.  

This storm will likely test the strength and bonding of the near surface weak layers as well as introduce new storm slab instabilities.

The storm should begin with very cold temperatures Friday morning and this should deposit a layer of low density snow over a firm crust. Temperatures should rise through the afternoon along with heavier precipitation rates. This should build increasingly unstable storm slabs through the day causing a significant increase in the danger. By midday and through the afternoon there will be an increasing likelihood of triggered or natural slides, especially near and above treeline.

The strong west to northwest crest level winds will also build unstable dense wind slab on lee slopes near ridges. 

This storm should cause very dangerous avalanche conditions near and above treeline where travel in avalanche terrain is not recommended by later Friday. Dangerous avalanche conditions should also develop below treeline by afternoon. 

 

Snowpack Discussion

Early December storms ranged from rain to high elevations followed by cooling with 1-2 feet of snow. No significant storms have occurred since early December. However there have been significant variability in temperature regimes ranging from very cold temperatures to recent high freezing levels and warm temperatures early this week.

Very warm temperatures over the weekend through this Tuesday produced some wet snow avalanches on steeper slopes. Initial news from an off-duty patroller at Crystal Mountain Thursday saw evidence of a recent wet loose snow avalanche on a south facing slope that stepped down to the ground Monday.  The slide likely stepped down on a pocket of depth-hoar possibly found near rocks and shallower snow cover. 

Significant cooling Wednesday and Thursday with some light snow has now formed a new near surface crust in most areas with some shallow trace to few inches of new snow on the surface. 

Shallow snow cover is limiting the avalanche danger at lower elevations.

Problems

Storm Slabs

An icon showing Storm Slabs

Likelihood

very likely

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:

All elevations.

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, South.

Elevations:

Alpine.

Valid until: Dec 20th, 2013 10:00AM

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