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Avalanche Forecast

Archived

Dec 11th, 2011–Dec 12th, 2011

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

Regions

Purcells.

Confidence

Fair - Intensity of incoming weather is uncertain

Weather Forecast

Monday: Mostly clear, with a chance of clouds late in the day. Winds should remain light and westerly with temperatures reaching -5. Tuesday & Wednesday: A weak trough will enter the region giving unsettled conditions with a chance of flurries. Winds remain light and turn southwest with daytime temperatures reaching -5.

Avalanche Summary

Isolated explosive triggered avalanches to size 1.5.

Snowpack Summary

Around 3cm of new snow has fallen in the past couple of days. This overlies surface hoar (as large as 10mm in protected areas), surface facets, the odd old windslab and sun crusts (on steep south through west aspects). The new snow has fallen with limited winds effect. All of these newly buried surfaces are good sliding interfaces; all that's lacking is a large enough load. As more snow arrives and/or the wind picks up (as per the forecast) the avalanche danger will increase. Due to the slow, incremental increase in load (dribs and drabs of new snow and sporadic winds), this increase in danger is difficult to pinpoint and could be very site specific. Snow depths are quite variable through the region with 130-150cm in the alpine. There is still concern for triggering the weak basal facets/depth hoar, as these layers have shown sudden collapse results in stability test in shallow areas.

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.