Avalanche Forecast
Regions: Snoqualmie Pass.
The avalanche danger will increase sharply by late Sunday afternoon and evening with human triggered slides becoming likely. Choose a conservative travel plan and make sure your plan allows for a safe exit if conditions deteriorate quicker than expected.
Detailed Forecast
Light snowfall Saturday night and Sunday morning will increase in intensity Sunday afternoon along with increasing winds. A warming trend should lead to inverted storm snow by later Sunday afternoon. Snow should change to rain at lower elevations outside the cooler Cascade Passes Sunday afternoon. Loose wet avalanches will locally be an avalanche problem in these areas.Â
All of this will lead to increasing avalanche danger Sunday. Expect human triggered avalanches to be likely as the afternoon wears on. Be prepared to curtail your trip early if conditions become sensitive earlier than forecast.Â
Shallow storm slab avalanches releasing within the storm snow may step down to a variety of surfaces including newly buried surface hoar below treeline in certain areas. New wind slab should form on NW through SE aspects near and above treeline.
More significant warming and precipitation is expected Sunday night and a natural avalanche cycle is expected. An avalanche watch has been issued Sunday night through Monday. An avalanche watch means HIGH avalanche danger may develop for all three elevation bands Sunday night and Monday along the west slopes of the Cascades.Â
Snowpack Discussion
After last weekend's storm, north through east winds, strongest in the alpine, developed locally sensitive wind slab on unusual aspects for the central and south Cascades near and above treeline. High freezing levels and sunshine activated small loose wet slides on steeper south facing slopes midweek. Northerly aspects that were not wind affected had soft, right side up snowfall generally well bonded to the underlying crust. Very light snowfall accumulated along the west slopes Friday due to a weak weather system.Â
One of our biggest concerns along the west slopes of the Cascades heading into the upcoming weekend storm will be the new storm snow's ability to bond to a variety of snow surfaces; near surface facets, wind board, melt-freeze crusts, etc. Â
Also of concern will be a thin layer of surface hoar formed midweek in sheltered areas below tree-line. In the Stevens Pass zone, WSDOT avalanche forecaster Brandon Levy found 2 mm of surface hoar buried intact at Stevens Pass Saturday morning. Â
A generally strong mid and lower snowpack is expected west of the crest. One caveat is a persistent weak layer noted near a crust about 1 meter down in the Stevens Pass area last week, but no known avalanche activity has occurred on this layer.
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: Very 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: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1