Stevens Pass Avalanche Forecast

Issued: Dec 26th, 2013 10:00AM

Thu Dec 26th Current Conditions
Alpine Moderate Treeline Moderate Below Treeline Low
Fri Dec 27th 2 Day Outlook
Alpine Moderate Treeline Moderate Below Treeline Moderate

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

Be aware of increasing danger in the afternoon - especially if new wind and storm slab develop quicker than forecast. 

Summary

Detailed Forecast

Mild temperatures should continue overnight into Friday morning.  An approaching frontal system should spread light to moderate rain and snow first to the Olympics and north Cascades Friday morning and then further south and east in the afternoon.  Snow levels should start out rather moderate...and begin to lower later in the afternoon.   The most snowfall is likely in the near and above treeline zones from about Snoqualmie Pass and north to Mt. Baker. 

New snow may cause shallow storm slabs near and above treeline by the end of Friday. Strong SW winds around the Cascade crest level develop new wind slab in the above treeline zone on the usual lee NW thru E aspects.  

Snowpack Discussion

Warm wet weather December 13-14th followed by dry weather with high freezing levels helped to form crust layers and generally helped stabilize persistent buried weak layers from earlier in December. A series of fronts last weekend caused additional precipitation, mostly at moderate freezing levels, causing rain or wet heavy snow at mid and lower elevations while depositing 30-60 cm (1-2 ft) of storm snow over most areas along the west slopes and volcanic peaks. This storm cycle while not powerful by northwest standards, has caused some recent avalanche activity ranging from wet slabs, loose wet, storm slabs and wind slab releases. The stormy period began Friday with cold temperatures, warmed over the weekend and then was followed by a cooling trend Monday night into Tuesday morning.  Dry and generally mild weather occurred Tuesday through Thursday.  

On Monday 12/23, widespread natural and triggered wind slab releases where seen in the Mt Baker area. The slab releases ranged mostly from 40-60 cm (18-24 inches) and ran long distances with some sympathetic releases. These 1.5-2 size slides released on wind loaded northerly facing slopes below ridges and were extensive.  Also on Monday, an estimated size 2 natural web slab was observed on a NE aspect of Chair Peak near Snoqualmie Pass. 

NWAC pro-observers in the Rainier area on Sunday and in the Stevens Pass area on Tuesday still reported some concern with denser and newer snow bonding somewhat poorly with newly buried weaker layers. Snoqualmie Pass avalanche professionals on a NNE aspect of Alpental around 5000' showed a generally stable and well bonded snowpack made up of crusts and rounded melt forms except for a hard to trigger but high quality shear on a softer melt form layer about 60 cm (2 ft) down. Small wet loose avalanches were observed on solar aspects near and above 4000'. While the overall trend this week favors stabilization of the upper snowpack, these observations are worth keeping in mind as you change aspects and elevations. 

The avalanche danger diminishes rapidly at the lower elevations due to our ongoing lower than normal snowpack. Non-avalanche hazards such as exposed rocks, creeks, and icy surfaces exist and present a travel challenge.   

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, North West.

Elevations:

Alpine, Treeline.

Valid until: Dec 27th, 2013 10:00AM

Forecast Trend

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