Avalanche Forecast

Archived

Mar 24th, 2013–Mar 25th, 2013

Alpine

Natural avalanches possible, human triggered probable.

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.

Confidence

Good

Weather Forecast

Synopsis: Stable pattern under a ridge of high pressure expected to continue. In other words, little change.Overnight and Monday: Dry, light westerly winds, solid overnight freeze (-10 to -15 C) with daytime warming (around -5 in alpine). Monday freezing level rising to near 1600 m but above freezing temperatures likely to go higher on slopes facing the sun.Tuesday: essentially a nearly identical forecast to Monday.Wednesday: some high cloud drifting in. Tuesday night may stay warmer limiting the overnight freeze and crust recovery at treeline elevations and lower.

Avalanche Summary

Natural and explosive cornice releases are a major theme. Explosives control in several parts of the region resulted in some large cornice falls that released some slab avalanches in the tracks that were restricted to the recent storm snow.A size 3 avalanche taking out the whole basin was triggered by snowmobilers Saturday in Hell Roaring Creek, just south of the region resulting in one fatality. Details are limited at this time; it may be indicative of the persistent slab problem (surface hoar and/or crust buried about 70 - 120 cm) we continue to describe.

Snowpack Summary

Moist snow may be found on solar aspects up to about 2000 metres; surface faceting elsewhere. New melt/freeze crusts are developing due to cold clear nights and periods of strong solar radiation during the day. The new storm slab is about 30-50 cms deep, with wind transported pockets that are about 70 cms deep. Downflowing or katabatic winds have created small slabs in gulley and atypical low alpine and treeline elevations. Recent cornice growth is reported to be poorly bonded and breaks off easily with light triggers. A weak layer of surface hoar and/or a crust is buried down about 70-120 cms in the alpine and treeline elevations; this layer continues to produce "sudden" results. This surface hoar layer is producing variable results in snow profile tests, and continues to be a concern for professionals.

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.

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.

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.