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Avalanche Forecast

Archived

Jan 3rd, 2012–Jan 4th, 2012

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells.

Confidence

Fair - Timing, track, or intensity of incoming weather is uncertain

Weather Forecast

A small "Pineapple" (or maybe we should refer to it as a Papaya due to lack of moisture in the Interior) is pushing through the Southern ranges. Tuesday night snow amounts 5-10cms. Strong SW winds. Wednesday: Light snow amounts up to 5cms. Wind 60-90km/hr from the SW. Freezing levels rising to 2000m through the day. Wednesday night snow amounts 10-15cms. Freezing levels fall to 1000m. Thursday: Snow amounts 5-10cms. Freezing levels falling from 1000m to valley bottom by Thursday afternoon. Friday: Mix of sun and cloud. Light winds from the SE. Freezing levels valley bottom.

Avalanche Summary

One natural size 3 avalanche reported. This was at 2900m on a North aspect, and failed down to glacier ice. Explosive avalanche control missions on Monday reported one size 3 that was remotely triggered from 500m away. This was at 2800m on a NW aspect. Natural activity has tapered off, but I'd still be suspicious of the mid-December layer susceptible to rider triggers (especially sledders) please don't rule this out yet; especially with rising freezing levels and a bit of snow on it's way.

Snowpack Summary

The Purcell region has seen up to 140cms since the Christmas holiday. The new snow has settled into a cohesive slab, and the average total snowpack depth is near 200cms. The ridgetop winds have consistently blown from W-SW and have redistributed the storm snow into wind slabs. Wind slabs are widespread and found in exposed alpine and treeline locations. Even open areas below treeline are suspect for wind affected snow. In sheltered areas the snow surface is starting to form surface hoar crystals (feathery), and surface facets (sugary crystals). These crystals may create a weak layer in the snowpack once buried. Below the surface down 80-140cms lurks the mid-December surface hoar/ crust/ facet layer. This is a layer of concern. Test results on this are still variable in the moderate to hard ranges with sudden planar characteristics. It continues to be sensitive to large triggers like cornice fall and explosives. Between this layer and the bottom of the snowpack sits a well settled mid pack. At the bottom of the snowpack basal facets and depth hoar crystals live. These become a concern in thinner snowpack areas, and have become reactive under the new load of the post Christmas storms.

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.