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Avalanche Forecast

Archived

Feb 5th, 2013–Feb 6th, 2013

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

Regions

Purcells.

Confidence

Fair - Forecast snowfall amounts are uncertain

Weather Forecast

Wednesday: Today the region sits under a weakening zonal flow maintaining moderate-strong ridgetop winds from the West. An upper disturbance embedded in the flow will bring snow amounts 5-10 cm. Treeline temperatures steady -4 and freezing levels steady around 1100 m. Thursday: Unsettled conditions continue, bringing light snow accumulations. Ridgetop winds will switch and blow from the SW. Treeline temperatures near -3 and freezing levels hovering around 1300 m, falling to valley bottom overnight.Friday: Outlook shows surface and upper ridge building, bringing dryer conditions and valley cloud. Treeline temperatures near -4 with freezing levels hovering around 1100 m. Ridgetop winds blowing moderate from the NW.

Avalanche Summary

Skier triggered size 1 slab avalanches were reported. These were initiated in the immediate lee of slopes primarily on NE aspects around 2200 m. No new natural activity has been reported.

Snowpack Summary

Forecast strong westerly winds and new snow will continue to build wind slabs in the alpine and at treeline on lee slopes and behind ridgelines. The recent storm slab continues to be reactive down 30-50 cms at treeline and below. This slab sits on a variety of old interfaces, however areas that have buried hoar surface seem to be most touchy. Continued freezing temperatures overnight have helped to strengthen the moist surface layers that developed during the weekend warm period. The mid-pack is generally well settled. The average snowpack depth at treeline elevations is near 130 cm.

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.