Avalanche Forecast

Archived

Feb 27th, 2016–Feb 28th, 2016

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

Purcells.

Forecast snowfall and wind on Sunday could build fresh and touchy wind slabs in exposed terrain. Choose your lines carefully and avoid exposure to terrain traps.

Confidence

Moderate

Weather Forecast

Sunday: Cloudy with light snow 5-10 cm. The freezing level is around 1400-1600 m. Ridge winds are moderate from the S-SW. Monday: Cloudy with sunny breaks and a chance of flurries. The freezing level rises to 1000-1200 m during the day. Ridge winds are light. Tuesday: A mix of sun and cloud. The freezing level jumps up to 1500 m and winds remain light.

Avalanche Summary

Avalanche activity on Friday consisted mainly on loose wet sluffs on steep sun-exposed slopes and natural cornice falls to size 2. On Thursday, numerous natural loose avalanches up to size 2.5 were reported from steep sun exposed slopes. Explosives triggered a bunch of cornices up to size 2.5 and a couple natural cornices releases was also reported. Most of these cornices were on north aspects at treeline and in the alpine. Two cornice releases slabs on the slopes below. One was a 35cm thick wind slab on north aspect at 2600m and the other was 60cm thick slab on a north aspect at 2100m. Wind slabs were reported to be reactive to ski cutting in immediate leeward features in the far north of the region.

Snowpack Summary

A dusting of new snow overlies a melt-freeze crust on solar aspects and lower elevation terrain, and might be covering a layer of surface hoar on shady and sheltered slopes at treeline and in the alpine. New wind slabs are expected to form over the weekend with forecast moderate southwest winds in the alpine. The surface hoar and/or crust layer which was buried February 10 is now down 40-70cm and has been responsible for some large avalanches recently. This layer is expected to become much less reactive over the weekend with cooling temperatures. Large cornices have recently been a concern but should also gain strength will colder temperatures. The early January surface hoar/facet layer is typically down 70-120cm. Triggering an avalanche on this layer has become unlikely but it still has isolated potential to produce very large avalanches with a heavy trigger. In general, the lower snowpack is well settled and strong, apart from some thin snowpack areas where basal facets exist.

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.

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.