Changing winds may transport recent storm snow into deep pockets on several aspects.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Saturday
Weather Forecast
Tonight: Overcast with light west winds, flurries, and alpine temperatures -15. Saturday: Overcast with light snow and periods of intense flurries. Expect 5-10 cm of new snow combined with moderate westerly winds. Sunday: Overcast with light easterly winds and 3-5 cm of new snow, and alpine temperatures -15. Monday: Overcast with flurries and southerly winds.
Avalanche Summary
Explosives control near Fernie resulted in a deep persistent avalanche on Friday; this avalanche was size 2.5 on a north aspect at 2000 metres, and is believed to have released on the mid-December facets. There was a report on Tuesday of a natural avalanche size 3.5 on Mt Hosmer on an East aspect at 2100 metres that appears to have released on or stepped down to the weak deep persistent layer of sugary facets near the ground. On Wednesday we had a report of a size 3.0 avalanche on "Big Steep Mother" bowl on a northeast aspect at 2100 metres in the alpine in the Lizard range. This avalanche appears to have been started by a cornice fall that released a storm slab that then "stepped down" to deeply buried weak facets.
Snowpack Summary
There is now 50-60 cm of storm snow that is expected to settle into a storm slab. This slab sits above a thick rain crust below 1900 m and a generally well settled snowpack. Isolated basal weaknesses may still exist in shallow snowpack areas. These weak areas appear to be on north-northeast aspects in the alpine. If the winds shift direction over the next few days, watch for new wind loading on south-west aspects.
Problems
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
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.