Avalanche Forecast

Issued: Dec 12th, 2013 11:07AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Summary

Detailed Forecast

The most significant loading from new storm and wind slab will occur late Thursday afternoon through Friday morning.   The new storm snow and wind deposited snow is expected to bond poorly to the variety of existing snow surfaces.  Natural avalanche activity will likely be limited by the amount of loading on the newly buried weak layers.  In areas that receive more wind transport and loading, such as the Mt. Baker area, expect a higher likelihood of natural avalanches.  Continued light showers and a warming trend during the day Friday will continue to stress the underlying snow. 

The snowpack is expected to be sensitive on Friday.  If venturing out into the backcounty on Friday, conservative trip planning and decision making in the field will be essential for safe travel. No reports of the quality or sensitivity of the bonding between new to old snow surfaces were available at the time of this forecast. 

Snowpack Discussion

The most recent storm cycle occurred in early December with most NWAC stations receiving copious amounts of precipitation, mainly in the form of rain at mid and lower elevations followed by a sharp cooling trend and generally 1-2 feet of snow for the west slopes at high and mid elevations. This storm cycle created a strong and generally thick rain crust with cohesionless new snow above the crust and produced little avalanche activity. 

A dry and very cold period followed with two bouts of wind, first strong easterly ridge top winds last Friday and Saturday and then winds shifted to strong west to northwesterly on Sunday and Monday, transporting loose surface snow and building small and isolated wind slabs on a variety of lee slopes.  A shallow wind slab was triggered on East Peak near Crystal Mountain on Sunday. 

The dry and cold stretch produced widespread reports of thick surface hoar growth, near surface faceted snow and of the strong crust breaking down due to strong temperature gradients within the upper snowpack.

Heading into the Thursday afternoon/night weather system, a variety of snow surfaces are present including isolated areas of shallow wind slab and scoured or icy surfaces at higher elevations near ridges, hoar frost near and below treeline, and near surface facets or powder on sheltered slopes and at lower elevations. Thin sun crusts have also been reported on steeper solar aspects. 

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.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 1

Storm Slabs

An icon showing Storm Slabs

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.

Likelihood

Likely

Expected Size

1 - 1

Valid until: Dec 13th, 2013 11:07AM