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Avalanche Forecast

Archived

Feb 19th, 2013–Feb 20th, 2013

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 possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Northwest Inland.

Confidence

Poor - Timing, track, or intensity of incoming weather is uncertain on Friday

Weather Forecast

Wednesday: Freezing Level: 500m Wind: SW, initially moderate increasing to strong by sundown.  5 – 10 expected during the day. Thursday: Freezing Level: 500m Wind: Strong SW backing off of to moderate SW in the evening.  5 - 15 cm expected during the day. Friday: Freezing Level: 500m Wind: Strong S decreasing to light W after dark.  10 -25 cm expected during the day.

Avalanche Summary

Soft slabs up to 25 cm in depth on N aspects around 1400m were reported Monday.  Yet another size 3 avalanche that went to ground was reported Monday on a rocky convexity with a shallow snowpack.  This was in the same area where a wind loaded NE facing slope around 1400m released naturally to ground resulting in a size 3 avalanche on Sunday.

Snowpack Summary

The storm on Friday the 19th left between 20 - 60 cm in it's wake with the Northern portion of the region receiving the most snow along with a significant amount of wind that created large deep hard slabs.  Most of the precip from the last storm fell as rain below 1200m or so which resulted in a crust below treeline.  The region received another 20 cm or so Monday along with some moderate winds from both the west and possibly even east which created soft slabs that were sensitive to skier triggering on Monday.There are a variety of old interfaces now down 40 - 100cm which include facets, crusts and isolated pockets of surface hoar.  There is very limited information about the nature of this interface. 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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.