Avalanche Forecast

Archived

Mar 20th, 2013–Mar 21st, 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, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

South Coast.

Confidence

Fair

Weather Forecast

Synopsis: Cooler and unsettled conditions are expected for Thursday and Friday. A ridge of high pressure should start to develop on Friday resulting in drier and sunnier weather. Thursday: Mainly cloudy with a chance of flurries. The freezing level drops to around 400 m overnight and winds are moderate from the northwest. Friday and Saturday: A mix of sun and cloud. The freezing level drops to near sea level overnight and rises to 800 m during the day. Winds are light to moderate from the north-northwest.

Avalanche Summary

There is one new report of a size 1.5 skier triggered slab avalanche from the South Chilcotins on Tuesday. This avalanche occurred on a steep north aspect in the alpine and was triggered mid-slope. On Monday, a cornice fall triggered a size 3 slab avalanche on a NE aspect with a crown depth up to 100 cm. There is limited information on an accidentally triggered slab avalanche on Sunday in the Duffey Lake area that resulted in 2 people being buried.

Snowpack Summary

Freezing levels climbed to 1700-1800 m with the system on Wednesday, resulting in moist or wet snow below treeline. Recent snowfall amounts vary significantly throughout the region. In the Coquihalla area, 10-15cm of dense new snow sits on a melt-freeze crust or up to 90 cm of previous storm snow. At the base of the old storm snow is another melt-freeze crust buried around a week ago. Further to the north (Duffey Lake) there is 10-15 cm of new snow sitting on a sun crust, wind slab, or settling storm snow. A buried surface hoar layer can be found down 50-70 cm and may be associated with a sun crust on southerly aspects at treeline. For all areas, expect extensive wind slabs and cornices in the alpine.

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.