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Avalanche Forecast

Archived

Feb 14th, 2013–Feb 15th, 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 possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Northwest Inland.

Confidence

Fair - Forecast snowfall amounts are uncertain on Friday

Weather Forecast

Friday: Light to locally moderate snowfall continuing overnight / Strong to extreme southwest winds / Freezing level at 900mSaturday: Light snowfall / Moderate to strong northwest winds / Freezing level at 500mSunday: Light snowfall / Light southwest winds / Freezing level at 300m

Avalanche Summary

Natural activity to size 2.5 was reported in the north of the region. The cycle ran in steep terrain on all aspects at treeline and above, and occurred in response to recent storm loading.

Snowpack Summary

Up to 20cm of recently fallen snow has been distributed into deeper pockets of wind slab at treeline and in the alpine. In general, between 20-40cm of storm snow sits over a variety of old surfaces which include facets, crusts and isolated pockets of surface hoar (sheltered treeline and below treeline). There is very limited information about the nature of this interface, with the only reports suggesting reactivity in sheltered, shady treeline and below treeline slopes (preserved surface hoar). I would stress the importance of digging down to find and test weak layers.A strong mid-pack currently overlies a weak base layer of facets/depth hoar. It is worth noting that the snowpack in general is quite shallow compared to averages; triggering the basal weakness may still be possible from thin spots, rocky outcrops or under the weight of larger triggers such as cornice fall.

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.