Avalanche Forecast

Issued: Dec 19th, 2015 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Dangerous avalanche conditions are expected in the above and near treeline along the east slopes on Sunday with careful snowpack evaluation and cautious route finding essential.

Summary

Detailed Forecast

The next strong cool cold front will cross the Northwest Saturday night and Sunday morning. This will renew moderate to heavy snow in the Olympics and Cascades Saturday night and Sunday morning with a minor warming trend but the snow level will stay below the passes. It looks like the heaviest snow will shift to Oregon Sunday.

Rapid loading will cause new storm and wind slab to build Saturday night and Sunday. Natural storm and wind slab should be possible and triggered storm and wind slab should be likely during the main part of the storm Sunday morning in the above treeline and near treeline. These conditions could easily last all day Sunday.

Terrain anchors are still causing significant anchoring at the lowest elevations. Use caution near creeks which are still open in some areas.

Snowpack Discussion

Heavy rain transitioned to snow around Dec 9th along the east slopes leaving a rain crust up to at least 6000 feet in the north Cascades and near or above 7000 feet in the central and south.

A series of cool storms since then has deposited increasing storm snow at relatively low freezing levels. There has been about .5-1.5 feet of snowfall at sites along the east slopes the past 2-3 days. Snow depths below treeline are increasing nicely, but many barely hidden hazards like rocks, streams or snags remain. The skiing in non-wind affected areas with enough snow cover has been good! 

NWAC pro observer Tom Curtis was at Rainy Pass near Stevens Pass on Wednesday and noted extensive surface hoar. This layer may have been buried intact Thursday though no evidence of avalanches on a surface hoar layer has been reported through Saturday.

Tom visited Mt Cashmere via the Icicle Creek drainage Thursday and did not find the persistent layer of buried surface hoar seen prior to the rain event December 8-9. There has been good evidence that the rain event destroyed this layer along the east slopes so the PWL has been removed as an avalanche problem along the east slopes. Tom did note small storm and wind slab up to near treeline but did not venture higher.

A guide report from Thursday at Washington Pass zone indicated good snow and good stability at the elevation of the highway hairpin with blowing snow along the ridge tops.  

The southeast zone should have a much shallower snowpack, more affected by recent rain and warm temperatures. However, we have no recent observations from the southeast zone.

Problems

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: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 1

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

Valid until: Dec 20th, 2015 10:00AM