Confidence
Fair - Forecast snowfall amounts are uncertain
Weather Forecast
Wednesday: A Pacific frontal system starts to affect the interior regions. Cloudy with light to locally moderate snowfall, moderate to strong west winds. Freezing level in the valley bottom.Thursday: Moderate snowfall in the western portions of the region, light snowfall in the eastern portions, alpine temperatures -7, moderate west winds.Friday: Light snowfall, alpine temperatures -5, freezing level 1200m, winds moderate west and southwest
Avalanche Summary
Recent reports from the last 3 days indicate that the natural avalanche activity seems to be isolated to a windslab and cornice releases up to size 2.5. These are running within the storm snow in the alpine and at treeline elevations. There was also a report of a glide crack release running naturally to size 3.
Snowpack Summary
Up to 50cm of recent new snow lies on top of old wind slabs, a thin melt freeze crust below 2100m and numerous weak layers. Down 40 -100cm is the mid December layer comprised of surface hoar, stellars and/or a crust. Around 70 -140cm down is the early December surface hoar, faceted snow and/or crust.There are two deeper layers of note. The late-November persistent weak layer consists of a sun crust on steep south facing slopes and surface hoar in sheltered areas and is now buried 90-160cm below the surface. The October deep persistent weak layer consists of a layer of facets sitting on a crust at the base of the snowpack. This layer is predominantly found on northerly aspects at treeline and in the alpine. The depth of both these layers makes directly triggering an avalanche on them unlikely (maybe a heavy load on a thin spot in steep terrain). Their presence could, however, increase the size of a potential avalanche through step-down.
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.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.