Expect 5-10 cm new snow on Saturday. If more than this arrives, elevate the alpine danger to Considerable.
Confidence
Fair - Intensity of incoming weather is uncertain on Saturday
Weather Forecast
Saturday: Snowfall accumulations of 5-10 cm are expected. Treeline temperatures around -6C. Strong southwest winds in the morning, becoming light easterly in the afternoon. Sunday and Monday: Flurries or light snowfall. Temperatures -10C, dropping to -18C on Monday. Wind light on Sunday, gaining strength again on Monday.
Avalanche Summary
One size 2.5 natural slab avalanche occurred in the Elk Valley on a NE aspect at around 2200m on Tuesday. The fracture depth was 40-50 cm and it is believed to have run on the mid-December weak layer. Other slab avalanches were triggered nearby using explosives on Wednesday--these were also on NE aspect slopes and ran on the mid-December weak layer, pulling out to a depth of around 50 cm. A sled-triggered size 3 slab avalanche was reported from the Flathead/ Lodgepole area on Saturday. The slide was 75cm deep and 100m wide and occurred on a northeast aspect at the 2100m elevation. While not numerous, this smattering of avalanches involving deeper weak layers indicates persistent slab problems are still an issue in this region.
Snowpack Summary
The average HS (height of snow) in the Crowsnest North is 130cms and the Crowsnest South has 160cms. As we move further West into the Elk Valley North the HS is an average of 140cms. The Elk Valley South sits at 180cms.In the Flathead the avg. HS is 180cms. All snow depths indicated are at the 2000 m elevation.The new snow from earlier this week has been substantially redistributed in open areas onto lee aspects. This, coupled with variable snow depths across the region, means variability is the key word here--both in terms of snowpack structure and reactivity of wind slabs and buried weak layers. In more sheltered locations, the upper slab is still relatively unconsolidated due to cool temperatures. In the mid pack, weak buried surface hoar layers can be found on N-NE slopes at treeline and below down approx. 30-55cms. The surface hoar may co-exist with facets that are result of the early December dry spell. Activity has tapered off on this layer, but a couple of notable releases have occurred in the last week, indicating the possibility for triggering a dangerous avalanche on this layer is still there. At the bottom of the snowpack a variable facet/crust/facet sandwich is located with depth hoar up to size 5mm below the crust, and facets size 2mm above. There has been little in the way of recent activity on basal layers, but the nature of a release would be highly destructive.
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.