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Avalanche Forecast

Archived

Feb 26th, 2013–Feb 27th, 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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
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 on Friday

Weather Forecast

Wednesday: Possible sunny breaks with isolated flurries, winds moderate from the west and alpine temperatures -10. Freezing level rising to 1100m.Thursday: Light snow, winds light from the west and alpine temperatures of -7. Freezing level rising to 1500m.Friday: Light snow throughout the day, light southwesterly winds and alpine temperatures -4. Freezing level rising to 1600.

Avalanche Summary

Slab avalanches to size 1.5 were ski cut in the region on Sunday. Recent explosives testing in the southern portion of the region producing slab avalanches to size 2 isolated to the storm snow.

Snowpack Summary

Light to locally moderate amounts of new snow have fallen adding to an ongoing storm snow instability, particularly on the west side of the region where accumulations have been highest. The new snow is most likely reactive as a wind slab  in exposed terrain or as loose snow in sheltered areas.  The variably reactive February 12th and 15th surface hoar layers now exist up to 80cm below the surface and seem most prevalent on the west side of the region. This interface may also be reactive on solar aspects where a sun crust exists. A surface hoar layer buried on February 23 is still on the radar of some operators. This layer has been less likely to trigger by skiers, but may still be sensitive to large loads like avalanches in motion or cornice fall. The mid and lower snowpack are generally well settled and strong.

Problems

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.

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.