Avalanche Forecast
Regions: Cascades - West.
A potent winter storm will bring heavy snowfall and strong winds Wednesday night and Thursday. Slab avalanches may become easily triggered and widespread on high elevation slopes. Give the storm snow time to stabilize, and seek out low-angle, supported terrain away from overhead hazard.
Discussion
So far this season, avalanche activity has been minor and infrequent due to low snow. Over the next 24 hours, this storm will bring rapid change and create dangerous avalanche conditions at upper elevations. Snow levels will fluctuate between 3500-4000ft, and 1-2ft of new snow may accumulate above 4500ft by late afternoon Thursday. Snow totals, along with the avalanche danger will noticeably increase as you gain elevation.
Don’t let the excitement of this first major winter storm distract you from making informed and conservative decisions. Be cautious if you decide to venture into the backcountry on Thursday. Ease into terrain slowly and gather information along the way. Getting caught in even a small avalanche would be a rough ride with high consequences in these early season conditions. Expect to encounter shallowly buried objects and challenging travel conditions, especially below 5000ft where the new snow will fall on bare ground.
Early season conditions on the west face of Glacier Peak as seen from Mt. Dickerman above the Mountain Loop Highway. (Observation) 12/8/19. Photo: Gary Brill
Snowpack Discussion
Regional Synopsis Coming Thursday, December 12, 2019
Avalanche Problems
Storm Slabs
Heavy snowfall will build fresh storm slabs near and above treeline over the next 24 hours. Avalanches are most likely where new snow falls on upper elevation slopes with pre-existing snow cover. Wind-loaded terrain will develop thicker and more reactive slabs. The new snow may not bond well to old snow surfaces, and mid-storm layers may be problematic as well. Watch for shooting cracks, collapses, and recent avalanches. It may be a good day to avoid going into avalanche terrain altogether, steering clear of big terrain features like large bowls and chutes.
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