Avalanche Forecast

Moderate to strong Southwesterly winds are expected to accompany the cold front as it passes overnight into the morning of the 12th. These will create thick, dense slabs on leeward slopes and features. Some of these slabs are sitting on old weak snow, and may surprise you by breaking well upslope. As a result, it is best to avoid steep, wind drifted terrain today.

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.

Storm totals from the 11th and 12th are expected to be substantial. The deep new snow is not bonding well to the old snow surface, which is weak and faceted. During the day on the 11th, professional observers reported natural, and remotely triggered avalanches on the old snow layer on a variety of aspects and as low as 4700ft. You are likely to trigger an avalanche that will break on the interface of the new and old snow on all aspects. Expect shooting cracks through the snow, collapses, and avalanches that break widely over terrain features. It will be prudent to avoid avalanche terrain today. 

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.