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Confidence
Fair - Timing, track, or intensity of incoming weather is uncertain
Weather Forecast
Overnight Monday: Heavy snowfall, especially for the southern part of the region. Up to 25-30cm is possible. Strong to extreme south-westerly winds. Freezing level around 1100 m. Tuesday: Heavy snowfall will continue in the morning but become lighter in the afternoon. Winds should also start to diminish. Freezing level should fall slightly. Wednesday: There may be a lull between storms, although snowfall is expected to start up again later in the day.Thursday: Another frontal system arrives in this region, bringing further heavy snowfall, strong south-westerly winds. Freezing level around 1200m.
Avalanche Summary
On Monday, several size 2-2.5 slabs released naturally (likely from wind-loading), which are suspected to have failed on the mid-February persistent weakness. A helicopter remotely triggered a size 1.5 slab, part of which stepped down to basal facets. A skier was also swept over a cliff in a size 1 slab which failed on the mid-Feb layer. On Sunday, a group of skiers triggered a size 3.5 slab 900m wide and 80-140cm deep on a north aspect in the southern Purcells. Late last week, large (size 2.5-3.5) human and explosive-triggered slabs were reported on all aspects, with crowns 40-120cm deep.
Snowpack Summary
Up to half a metre of recent storm snow, driven into wind slabs by moderate to strong southerly winds, overlies moist snow at low elevations or old hard wind slabs in the alpine. The early February surface hoar, about 1-1.6m deep, remains a key concern. Storm slabs, wind slabs or a light load can trigger surprisingly destructive avalanches on this layer. Basal facets have only been reactive on steep, shallow, and rocky slopes, but operators continue to monitor this layer.
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.
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.
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.