Confidence
Fair - Forecast snowfall amounts are uncertain
Weather Forecast
Overnight and Saturday: Heavy snow – around 15-20 cm by morning and another 20 cm on Saturday. Winds are strong from the S-SW. The freezing level may jump to around 1500 m briefly before dropping slightly on Saturday.Sunday: Moderate to locally heavy snowfall continues – 15-20 cm. The freezing level should be around 500-800 m and winds are strong from the W-NW. Monday: Periods of snow. The freezing level should jump to around 1500 m and winds remain strong and gusty from the W-NW.
Avalanche Summary
On Friday morning fresh storm slabs, 20-30 cm thick, were easily triggered on moderately steep terrain and were propagating quite far. It's likely that some areas experienced a natural avalanche cycle on Friday. The size and likelihood of avalanches is certainly on the rise through the weekend. Buried persistent weaknesses may also become reactive with heavy loading from snow and wind. Be aware of what is above you. Very large avalanches running full path are possible.
Snowpack Summary
Another 20-30 cm of snow fell on Thursday night bringing a total of around 50 cm of storm snow sitting on a variety of snow surfaces ranging from older stiff wind slab, a soft layer of facetted snow and/or surface hoar. 70-90cm below the surface from you may find at various depths a thin melt freeze crust below 2100m and a couple of buried weak layers (surface hoar, facetted snow, and/or a crust) that continue to be reactive in snowpack tests.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
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.
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.