Confidence
Fair - Due to the number of field observations
Weather Forecast
A couple of relatively small disturbances are expected to bring some freezing drizzle Tuesday night and Wednesday.Tuesday: Dry during the day, flurries in the evening. Ridgetop winds westerly 20-30 km/h. Alpine temperature reaching around 0C.Wednesday: Freezing Drizzle, turning to snow with 2-5cm expected. NW'ly winds gusting 20-30 km/h at ridgetop. Alpine temperatures around 0C.Thursday: Dry with a mixture of sun and cloud. Ridgetop winds increading to 40-50 km/h from the NW. Alpine temperatures around -4C.
Avalanche Summary
Recent explosives avalanche control in the region produced 2 size 3 slab avalanches which failed at ground level. They occurred on a northeast aspect, likely on upper elevation slopes.On Saturday, a size 2.5 deep persistent slab avalanche was reported to have failed naturally on a southeast facing slope at about 2300m. The avalanche failed on basal facets. On the same day, a snowmobiler triggered what appears to be at least a size 2 slab avalanche form a rocky alpine slope in the Allen Creek area. Little is known about the avalanche.
Snowpack Summary
Snowpack depths vary, but in general there is now 200 to 250cm of snow at tree line and as much as 300 cm in select places in the alpine. In some areas we're still dealing with a relatively thin snowpack. Roughly 100cm of well settled storm snow exists as a stubborn hard wind slab in many areas. Heavy wind scouring is reported to have occurred on many exposed windward slopes. Steep, sun exposed features are most likely seeing a daily melt-freeze cycle while surface hoar has been growing in shaded terrain. Below the recently formed storm slab you may find surface hoar buried around January 8th. Its distribution and reactivity seems highly variable. Where it exists, reactivity seems most likely below treeline.A basal facet/crust combo (weak sugary snow above and below a crust) near the ground is still a concern in some areas, and was the culprit with some of the very large recent avalanches. Likely triggers at this interface would include cornice fall, rapid temperature changes or thin spot triggering on an unsupported alpine or treeline elevation slope.
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.
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.