Avalanche Forecast

Issued: Dec 28th, 2015 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Wind slabs should be more difficult to trigger, but not impossible Tuesday. Be wary of wind slabs on NW through SE aspects, mainly near and above treeline. Remember MODERATE danger means human triggered avalanches are possible. Recent unconsolidated storm snow is still very deep in places with possible large tree wells lurking, so stick near your partner in the trees. 

Summary

Detailed Forecast

Wind slab will most likely be found on NW to SE aspects near and above tree line. NW winds may redistribute new snowfall to lee SE slopes near and above treeline Tuesday. 

Any triggered wind slab avalanches should be relatively shallow and remain within recent storm layers, but colder temperatures along the east slopes of the Cascades will help slow bonding of newly formed wind slabs to the weaker layers underneath. Test for inverted strong over weak storm snow and give cornices a wide margin.

Dry loose avalanches have been removed as an avalanche problem from the forecast as recent deep snow continues to settle making these less likely. However, continue to watch for the possibility of loose dry avalanches in steep non-wind affected terrain that could have unintended  such as knocking you off your feet and into a terrain trap.

There have been snow immersion fatalities in tree wells already this season at Snoqualmie and in Canada. Recent unconsolidated storm snow is still very deep in places with possible large tree wells lurking, so stick near your partner in the trees and maintain visual contact.  

Snowpack Discussion

For the week ending on Dec. 24th, a snowy and cool storm cycle brought 2-4 feet of snowfall to the east slopes. Only light additional accumulations over the last few days have allowed the snowpack to gradually settle, but it remains deep out there with Washington Pass area snowdepths over 2 meters and fine riding conditions reported!  The upper snowpack should generally be right side up with increasing hardness with depth. Moderate southerly transport winds above treeline Sunday may have built shallow new wind slab on NW through E aspects. 

An observation posted to the NWAC site Sunday described a shallow triggered wind slab on an SSE facing slope at 4500 feet, well below treeline, on a wind loaded feature near Merritt Lake off Hwy 2. 

Storm instabilities reported last week have likely settled out, but the North Cascades Mountain Guides reported on Christmas Day, a buried surface hoar layer in the Washington Pass area around 40 cm down that gave sudden planar (pop!) results in compression tests, but was not found to be involved in any natural or skier triggered avalanches. This layer probably needs more of a cohesive slab above it before it becomes apparent whether this is an PWL issue or not. 

The southeast zone has a relatively shallower snowpack versus areas further north, but after a snowy few weeks, has more than enough snow to warrant an avalanche risk. No snowpack observations have been received from this zone. 

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 29th, 2015 10:00AM