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Avalanche Forecast

Archived

Mar 19th, 2013–Mar 20th, 2013

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

Regions

Northwest Coastal.

Confidence

Fair - Forecast snowfall amounts are uncertain on Wednesday

Weather Forecast

Synopsis: A frontal system is spreading moderate snowfall to the region today and through Wednesday. Conditions remain unsettled in the wake of the system on Thursday before high pressure develops on Friday resulting in sunshine.  Tonight and Wednesday: 10-15 cm (maybe more near Bear Pass). The freezing level is around 400-500 m. Winds ease to moderate from the southwest, shifting to northwest.  Thursday: Mainly cloudy with a chance of flurries. The freezing level drops to valley bottom and winds ease to moderate from the west. Friday: Mainly sunny. Temperatures remain cool with the freezing level at valley bottom. Overnight lows near treeline could be in the -15 range. Watch for outflow winds to develop.

Avalanche Summary

Natural, human triggered, and remotely triggered avalanches up to size 2 continued on Monday with most avalanches releasing on the March 9th layer. A very large (size 3.5) avalanche was triggered in Cherry Bowl in the Shames backcountry on Sunday. Click here for a full report. This slide illustrates clearly the potential for deep and destructive releases on the March 9th surface hoar layer. On Saturday, natural slab avalanches were reported up to size 3 on all aspects in the alpine.

Snowpack Summary

30-60 cm of recent storm snow rests on a variety of old snow surfaces, including crusts, previous wind slabs and surface hoar. Warmer temperatures and recent strong southwest winds have now set this new snow into a reactive slab, with wind slabs building in exposed lee areas. The surface hoar (buried March 9th) is reported to be well-developed and fairly widespread, at treeline and alpine elevations. Recent snowpack tests have produced sudden failures with moderate loading force at this interface. The mid snowpack is generally well settled and strong. Cornices are large and untrustworthy, especially when the sun is out.

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.