This is the first significant warm spell of the season and the snowpack is revealing its secrets, especially in alpine locations. Make conservative terrain choices as destructive avalanches are a real possibility.
Confidence
Fair - Freezing levels are uncertain
Weather Forecast
Expect clear skies and light southerly winds for the entire forecast period. An inversion is also meant to dominate the region with alpine temperatures sitting at about 1.0'.
Avalanche Summary
No new natural avalanches reported today. In the neighboring Lizard range numerous deep releases were reported on facets that formed in October. The same snowpack structure exists in many parts of the South Rockies region. This combined with the current warming at upper elevations means that you can expect an increase in avalanche activity this weekend.
Snowpack Summary
Strong SW winds have created stiff, yet reactive wind slabs on lee slopes. Cross-loading, and wind slabs are also found lower on the slopes and in unsuspecting terrain due to the strong push of the winds. Below 1700m a 4cm thick crust exists, I suspect it's from the rising freezing levels, and precipitation last Sunday. This crust is said to be supportive. Below this reports indicate the mid-pack to be fairly well settled. Deeper in the snowpack sits the mid-December surface hoar layer. This layer remains a concern for deeper releases. Basal facets and depth hoar also play a major role in this region. The big story for the weekend is warming at higher elevations due to an inversion. Cornices, and sun-exposed slopes will be a concern as will the possibility of stimulating deeper releases. In Elk Valley North and the Crowsnest, the average snowpack depth at treeline is 150 cms. Elk Valley S and the Flathead the snowpack depth is closer to 2 m.
Problems
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
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.
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.
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.