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Avalanche Forecast

Archived

Feb 4th, 2013–Feb 5th, 2013

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

Regions

Purcells.

Confidence

Fair - Forecast snowfall amounts are uncertain on Tuesday

Weather Forecast

Tuesday: The trailing cold front has brought temperatures back down to seasonal norms. The next well developed system brings snow amounts near 10 cm. Ridgetop winds will blow strong from the SW and alpine temperatures will be near -7. Freezing levels will hover around 1400 m. Wednesday: Upper disturbances embedded in the SW flow will bring continued stormy conditions. Snow amounts 10-15 cm throughout the day, tapering off overnight. Ridgetop winds will be strong from the West. Alpine temperatures near -9 and freezing levels around 1100 m. Thursday: Weak unsettled conditions will prevail with no significant precipitation. Ridgetop winds will blow light from the SW. Alpine temperatures will be near -9 and freezing levels will hover at 1000 m in the afternoon, then falling overnight.

Avalanche Summary

No new natural avalanche activity has been reported. An explosives control mission triggered a slab avalanche size 2.5 on a SE aspect around 2500 m.

Snowpack Summary

Forecast strong SW winds and new snow will continue to build wind slabs in the alpine and at treeline. 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 touchy. Continued freezing temperatures overnight have helped to strengthen the moist surface layers that developed during the weekend warm period. Rising freezing levels forecast during the next pulse of moisture may weaken the old storm slab at lower elevations. The mid-pack is well settled and there are no reported deep layers of concern.

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.