Avalanche Forecast
Regions: Cascades - North West.
New or existing storm slab and wind slab are likely Tuesday, with new wind slab mainly forming on lee slopes ranging from N-SE facing, especially by afternoon as the next storm increases.
Detailed Forecast
Continued cool weather with snow showers and moderate winds overnight should build further storm slabs. A brief decrease is expected early Tuesday ahead of another approaching Pacific frontal system. The next system moves into the Cascades early Tuesday afternoon with renewed moderate snow and moderate westerly winds. This should begin to build new areas of wind slab on lee slopes below ridges and create more extensive storm slabs.
Dangerous avalanche conditions are expected in the above and near treeline bands Tuesday, especially later in the day.
Remember to give cornices a wide margin. Also above tree line strong winds recently have scoured snow from exposed slopes, leaving exposed rock and vegetation. Terrain anchors are still causing anchoring at the lowest elevations, but less and less with every storm passage!
The recent unconsolidated storm snow is getting very deep! There was a snow immersion fatality in a tree well in bounds at Snoqualmie on Saturday, so always ski with a partner.Â
Snowpack Discussion
A series of cool storms has deposited increasing storm snow at relatively low freezing levels since early December. The most recent brief warming period occurred Thursday, December 17th and caused a rain crust up to about 4500 feet in the north and about 6500 feet in the south. There has been about 2-4.5 feet of snowfall since the December 17th warm up as of late Monday afternoon! Â
Widespread sensitive storm slab conditions occurred over the past few days with soft slab releases from control results at ski areas and identified in test snow pits from observers.Â
A back country skier also triggered a 15 inch storm slab that ran about 500 feet on Skyline Ridge at Stevens Pass on Saturday on a northwest slope at about 5400 feet.
The pro patrols at Stevens Pass and Alpental reported reported sensitive small storm slab and loose dry avalanches on Sunday.
Patrol at Crystal Mtn reported sensitive wind slab releases from ski cuts Monday, releasing relatively shallow within the most recent storm snow early Monday.Â
NWAC observer, Matt Schonwald, found increasing wind and storm slab layers forming through the day Monday in the Crystal Mountain backcountry. Multiple test pits on the wind loaded west facing slopes at about 6800 feet showed propagation in ECT tests. Failures ranged from 16-30 inches down on or near the rain event crust formed December 17th. Wind and storm slabs were noted developing at lower elevations on cross loaded terrain features, well below ridgeline.
Avalanche Problems
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
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1