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Avalanche Forecast

Archived

Mar 20th, 2012–Mar 21st, 2012

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

Purcells.

Confidence

Fair - Forecast snowfall amounts are uncertain on Wednesday

Weather Forecast

Snow is expected to continue during the day on Tuesday and taper off during the night, bringing an additional 5-10 cm for the interior regions by Wednesday morning. A weak upper ridge is forecast to build over the southern interior on Wednesday that should cause light southwesterly winds and scattered flurries. There may be periods of sun in the eastern Monashees and Selkirks, and in the northern Purcells. A weak upper trough is forecast to push up from the U.S. border on Thursday bringing moderate precipitation to the southern and Eastern regions. Freezing levels are expected to climb up to about 1800 metres as the trough advances. The upper flow is expected to shift to the southeast with light alpine winds, broken skies with sunny periods, and freezing levels up to 1800 metres in the south and east regions of the interior.

Avalanche Summary

There were two reports of accidentally triggered size 3.0 avalanches that released on the deeply buried persistent weak layer from mid-February. Both were reported by groups of snowmobilers who were in the southern Purcell mountains near Kimberley in the Kootenay-Boundary region. Reports of avalanches on this layer are getting less frequent. Rising freezing levels forecast for Thursday and Friday may continue to promote releases that result in wide propagations on this layer.

Snowpack Summary

Strong southerly winds combined with new snow to develop thin new windslabs in the alpine and at treeline. Operators report that periods of broken skies on Sunday did not create widespread moist surface snow. It was reported to be warm enough to help settle the recent new snow and storm slab. The combined slab above the mid-february persistent weak layer is now between 150-200 cm. Some operators continue to get easy-moderate sudden planar character shears on the mid-february surface hoar in shallow areas. Snowpack conditions continue to be tricky to evaluate, and are variable throughout the region. Forecast strong wind and new snow may develop new windslabs in the alpine and at treeline.

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.

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.

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.