Confidence
Fair - Timing, track, or intensity of incoming weather is uncertain for the entire period
Weather Forecast
The rather largo weather pattern that has been in place for the last week is being swept out of the province Monday setting the stage for an active weather week influenced by SW flow for the southern interior. Monday Evening: freezing level tops out around 1700m Monday evening as precip begins to fall. 10mm of water are expected, most of which will fall as rain as high as 1500m. Alpine elevations should receive 15 or so cm of snow. Winds should be moderate out of the NE, switching SW overnight. Tuesday: Freezing levels remain near 1500m, trace of precip expected. Treeline winds light out of the S, SW, Alpine winds Moderate, SW. Wednesday: Freezing level 1500m, 3mm water expected during the day. Winds light SE at treeline, moderate S in the Alpine. Look for clearing skies Thursday AM with precipitation expected to start back up in the afternoon.
Avalanche Summary
The strong spring sun initiated an avalanche cycle Sunday that included everything from small sluffing to size 2.5 avalanches in the afternoon. A natural cornice fall size 2.5 was reported from the high alpine on a NW aspect on Saturday. On Friday, a very large avalanche was triggered by a cornice in Gorman Lake (Dogtooth Range). It was wide, deep and covered sled tracks. Photos here: https://bit.ly/GMOYCv . There were four other close calls in the Purcell Mountains this week, all involving very large slabs releasing either from a distance or with a light load. On Thursday, a snowmobiler remotely triggered a size 3.5 slab on an east aspect at 2400m at Oldman Creek, east of Quartz Creek (video here https://bit.ly/GLHIbg). It failed 1-2 m deep, was 200-300 m wide and ran full path, taking out mature timber. A second slope also failed, either remotely or sympathetically.
Snowpack Summary
Warm alpine temperatures combined with strong solar radiation on Sunday caused moist snow up to about 2400 metres. The major concerns are the mid-February persistent weak layer, buried 1-2 m deep, and facets at the base of the snowpack. The February layer demands respect right now. It is very touchy and can be triggered remotely or by a light load on a variety of aspects, creating frighteningly large avalanches.
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.
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.