Avalanche Forecast

Archived

Feb 21st, 2013–Feb 22nd, 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 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

Northwest Inland.

The avalanche danger may jump to HIGH in areas that receive more than 20 cm of new snow on Friday. Local observations are crucial.

Confidence

Fair - Due to limited field observations

Weather Forecast

Synopsis: A series of frontal systems is expected to affect the region over the next few days. The strongest system is on track for Friday. Friday: Moderate to locally heavy snowfall – 15-20 cm. The freezing level remains around 600 m and winds are moderate to strong from the west.Saturday: Cloudy with light snow. The freezing level is around 500 m and winds ease to moderate from the southwest. Sunday: Moderate snow. The freezing level is around 500 m.

Avalanche Summary

Natural avalanches activity has tapered off but some observers continue to report older (+72 hrs) natural slab avalanches up to size 3 from steep northeast facing terrain. These events may involve the basal weak layers.

Snowpack Summary

New wind slabs have formed in exposed lee (N-E aspects) terrain and cross-loaded gullies. The previous storm snow seems to have settled and strengthened over the past couple days. 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 before committing to larger and steeper slopes.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

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.

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.