Avalanche Forecast

Archived

Dec 21st, 2013–Dec 22nd, 2013

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Olympics.

Bottom Line: Expect highly variable local snow conditions with unstable wind slab, storm slabs and persistent slabs.

Detailed Forecast

Light rain or snow showers should end Saturday morning giving way to mostly cloudy conditions. Moderate ridge top winds should persist Saturday. This should allow for slow settlement of newly formed unstable layers, however some new wind transport along ridges may maintain or continue to build wind slabs below ridges.

Expect to find a wide variety of local snow conditions and plan accordingly with flexibility in mind.

Snowpack Discussion

Warm weather last weak created another strong crust layer in most areas. Light amounts of low density some as well as some surface hoar formation occurred over the past few days. This has set the stage for the strong storm that moved through the region Friday. The latest strong storm began with light snowfall at very cold temperatures early Friday, adding to the weak snow overlying a strong crust. This weak near surface snow has been loaded by moderate to heavy amounts of snow at warming temperatures creating unstable conditions.

Friday morning control routes at Mt Baker Ski area were releasing sensitive ski triggered wind slabs on exposed northerly facing terrain in the Mt Baker area ranging up to 30 cm (1 foot). By mid-morning Friday additional loading had built further very sensitive wind slab layers.

The warming and heavy precipitation rates have tipped the balance in many areas given the weak underlying snow all atop of firm crust layers.

There has been a variety of snow crystal types reported falling through the day Friday, ranging from sleet, mixed rain and snow, graupel other unknown grains. The Crystal Mountain area reported a new crust layer forming by midday to the top of the ridges as a result of temperatures in the mid 30's and mixed precipitation forms.

As a result of high variability in temperatures as well as precipitation types experienced during Friday's storm, expect an equally wide variety of local snow stability and plan your routes with a high degree of flexibility based on local conditions. Expect to encounter new wind slab layers as well as storm slabs and persistent slabs, especially in areas receiving the greatest amounts of storm precipitation, such as the volcanic peaks and central WA Cascade Passes.

Shallow snow cover is limiting the avalanche danger at lower elevations in many areas.

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.