Avalanche Forecast

Archived

Mar 20th, 2012–Mar 21st, 2012

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 unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

South Columbia.

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

We are still getting reports of large avalanches that are a couple of days old that failed on the mid-February persistent weak layer. There were a couple of new reports of small avalanches that failed in the top 50-70 cm of recent storm snow. There were two size 3.0 avalanches that were triggered accidentally by snowmobilers in the far south of, or just outside the region near Kimberley. The PWL is becoming a low probability, high consequence avalanche problem.

Snowpack Summary

Strong southerly winds combined with 10-15 cm of new snow created new windslabs in the alpine and at treeline. Sunny periods on Sunday developed a sun-crust on southerly aspects up well into the alpine. The recent storm snow is reported to be bonding well to the old surface. There is a persistent weak layer (PWL) of buried surface hoar from mid-february that is buried between 100-200 cm that is the main concern. This PWL shears with a fracture character that promotes wide propagations that result in very large avalanches. Reports of avalanches on this layer are becoming less frequent.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.