Confidence
Fair - Timing, track, or intensity of incoming weather is uncertain on Saturday
Weather Forecast
Friday should be another dry day, with some clouds. Freezing levels should stay at valley bottom with alpine temperatures around -10C. Strong ridge top winds are expected from the west. On Saturday, a frontal system crosses the region bring light amounts of snow (2-4 cm expected). Temperatures should stay cool and strong SW winds are expected. On Sunday, flurries will likely continue with no significant accumulations. Winds will ease slightly and temperatures will remain cold.
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 2000m elevation.A nice dump of around 20 cm of low density new snow arrived Monday night. Winds were initially light following this storm, although recent gusty winds have started to redistribute snow onto lee aspects in exposed areas such as behind ridge lines. The upper slab is still relatively unconsolidated - the cooler temperatures will maintain this situation for a little while to come. In the mid pack, weak buried surface hoar layers can be found on North-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. This interface is still reactive and has produced numerous natural avalanches over the past week.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.As a side note, the snow depths alone indicate that the region is highly variable in terms of snowpack structure. You should anticipate that layering and reactivity of weaknesses will also be variable throughout the region.
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.