Avalanche Forecast
Regions: Snoqualmie Pass.
Dangerous avalanche conditions should persist Saturday, 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.
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.Â
Avoid terrain where wind may have built isolated wind slab layers, mainly higher elevations near ridges, most likely on a wide variety of aspects.. Â Â Â
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. Â
Terrain anchors are still causing significant anchoring at the lowest elevations. Use caution near creeks which are still open in some areas.
Snowpack Discussion
A series of cool storms over the past 10 days following the December 7-8 major rain event, have deposited increasing storm snow at relatively low freezing levels.
The dominant avalanche problems currently remain from storm slabs and wind slabs. Periods of heavy snowfall Thursday and early Friday quickly built unstable storm snow conditions, especially in the Snoqualmie and Stevens Pass area, both Thursday and Friday. Strong winds also built wind slab layers on a variety of aspects due to terrain channeling and redirecting local winds.   Â
These storm and wind slab problems caused extensive avalanches, both natural and triggered. The great majority of the reported recent soft slab avalanches have released on storm snow interfaces mainly in the upper 8-12 inches of snow. In the Snoqualmie Pass area a brief freezing rain crust was deposited Thursday morning and many slides have released to this layer. A few larger slides up to 2 feet were seen with larger explosives at Alpental Friday, mainly initiating in recent layers and breaking to older storm snow layers. No slides have been reported releasing to the December 9 crust interface which remains strongly bonded.
The Cascade pass areas were protected from the warm up Thursday afternoon by cool east flow and this maintained precipitation as snowfall to the valley bottoms. However, other areas experienced a warm up and rain up to about 6200 feet in the Crystal and Paradise area, lower in the north and to near or above 7000 feet in the south. This is forming a new crust layer with shallower storm snow above. Â
There was a party of two caught and injured in the Alpental Valley Thursday near midday at the height of the increased storm slab instabilities. Fortunately, no major injuries or worse occurred.
A good time to review just what the North American Public Avalanche Danger Scale definitions are during CONSIDERABLE danger, especially during a noted increasing danger trend.  Â
Avalanche Problems
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, West, North West.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
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