Avalanche Forecast

Issued: Dec 29th, 2013 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Summary

Detailed Forecast

Clouds should increase overnight ahead of an approaching weak frontal system.  Light rain and snow should impact mainly the north Cascades on Monday, with the bulk of the new loading holding off until Monday night or Tuesday morning.  Shallow new wind slab may develop by the end of the day in the north Cascades on lee N thru E aspects near and above treeline. 

Snowpack Discussion

Most areas have a mix bag of surface snow conditions given the lack of recent storm activity as well as periodic warm temperatures, sunshine, rain or shallow new snowfall. The snowpack can be characterized by a mixture of supportable or breakable crusts with sun and wind protected areas at higher elevations maintaining some shallow soft snow over a firm underlying and well consolidated older snowpack. 

The last avalanche activity of note occurred following a series of frontal passages and warming temperatures from December 20-23. This caused 1-2 feet of heavy snow in the alpine and some very wet snow and rain near and below treeline. The Mt Baker area professional patrol reported widespread 4-14 inch sensitive ski triggered wind slabs on December 20-21 while a professional guide reported a large wet slab avalanche running from Chair Peak near Snoqualmie Pass December 22, along with other signs of wet snow instability.

Recent observations over the past few days include stable surface crusts, a few minor wet-loose triggered avalanches on solar aspects near and above treeline as well as some settled powder near treeline on non-solar aspects. Snow profiles and tests at Alpental Thursday showed generally stable snow well bonded snow but a few high quality but hard to trigger shears were noted about 60 cm below the surface above old crust layers within softer melt-form crystals.

Very light rain and snow and moderately strong alpine winds out of the NW from about Snoqualmie Pass and north on Sunday should have had little impact on the regional avalanche danger. 

The avalanche danger will continue to be low 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

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.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Dec 30th, 2013 10:00AM