Regions
Northwest Inland.
Recent shifting winds have been driving a wind slab problem at higher elevations. Slabs have proven especially touchy where they overlie a recent rain crust.
Weather Forecast
Monday: A mix of sun and cloud. Light to moderate southeast winds. Freezing level to 900 metres with alpine temperatures of -7. Tuesday: Cloudy with sunny periods and flurries beginning in the afternoon. Light southeast winds. Freezing level to 1000 metres with alpine temperatures of -3. Wednesday: Mainly cloudy with scattered flurries bringing a trace to 5 cm of new snow. Moderate to strong southeast winds. Freezing level to 1100 metres with alpine temperatures of -2.
Avalanche Summary
Reports from Saturday include observations of a natural avalanche cycle in the Howson Range, where several Size 2.5-3.5 storm slabs released naturally from north to east aspects between 1200-1700 metres. A MIN report from the Telkwa area also details a snowmobile triggered Size 2.5 wind slab. The March 14 crust mentioned in our snowpack discussion was identified as the failure plane in this occurrence as well as in a skier triggered Size 2 wind slab north of Kispiox. These close calls should draw attention to the extent of recent wind transport as well as the March 14 crust's excellent performance as a sliding layer.
Snowpack Summary
At treeline and above, ongoing winds have been redistributing recent storm snow and light new snow amounts into wind slabs in lee terrain. New snow amounts taper with elevation and lie over a supportive crust (March 14) at about 1300 metres and below. This crust has been observed at higher elevations in the Telkwa area, and wind slabs have proven especially reactive where they sit above it. At higher elevations, wind redistribution is incrementally loading a weak interface that was buried in late February. The interface is composed of facets, crusts, and surface hoar and may be 40-60 cm deep. There's still some uncertainty around the distribution and reactivity of this interface, but it has proven reactive under skier traffic in the north of the region. The mid-pack is well consolidated, but sits above weak, sugary snow near the ground. This deep basal weakness remains an ongoing concern in thin rocky start zones and in shallow snowpack locations.
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.
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.