Avalanche Forecast

Issued: Dec 18th, 2015 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Dangerous avalanche conditions should persist Saturday above treeline, though be improving. Best to be particularly cautious, especially near and above treeline, avoiding steep or wind loaded slopes, watching for sensitive storm layers and wind slab deposits.

Summary

Detailed Forecast

A transitory ridge of high pressure should move across the area Saturday to allow for cool weather, light winds and some possible sun breaks. This weather should allow for a slow improving trend as recent storm and wind slab slowly settle and strengthen. 

Avoid terrain where wind may have built isolated wind slab layers, mainly higher elevations near ridges, most likely on a wide variety of aspects..    

Avalanche problems Saturday should continue to be associated with recent storm and wind slab layers. Avalanches should be confined to the new storm snow since Thursday, but a few may step down to older storm layers or crusts in isolated locations.  

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 leaving a new rain crust up to at least 6000 feet in the north Cascades and near or above 7000 feet in the central and south. All recent observations indicate the rain event has eliminated any persistent weak layers formed in late November. 

An active and cool weather pattern over the last week has produced about 2 to 3 plus feet of new snowfall that now sits over the 12/9 crust. 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! 

Extensive surface hoar was evident Wednesday afternoon in the Stevens Pass area, near Rainy Pass. This layer may have been buried intact early Thursday with the onset of Thursday's storm, though no evidence of avalanches on a surface hoar layer has been reported through Friday.  

Further observations by Tom Curtis in the Icicle Creek drainage Thursday, failed to identify the persistent layer of buried surface hoar found prior to the rain event December 8-9. There was significant evidence that the  rain event has destroyed this layer. Coupled with other recent observations along the east slope areas, we are removing the PWl from the mentioned avalanche problems. 

Recent guide reports from the Washington Pass zone indicate strong northerly winds redistributing recent snow on lee southerly aspects above treeline. The most recent storm snow was reported to be well bonded to the underlying crust, with good ski conditions found in non-wind affected terrain.  

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

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, South, North West.

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

Possible

Expected Size

1 - 1

Valid until: Dec 19th, 2015 10:00AM