Extreme winds will be driving the Avalanche Danger on Thursday. Watch for widespread wind slab development.
Confidence
Moderate - Wind effect is extremely variable on Thursday
Weather Forecast
Expect overcast skies and occasional light flurries over the entire forecast period. Extreme southwest winds are expected on Thursday and Friday with light winds forecast for Saturday. Freezing levels may reach 2000m on Thursday and Friday, and then drop to valley bottom for Saturday.
Avalanche Summary
In the Elk Valley North area on Tuesday, several natural wind slab avalanches to size 1.5 were observed in wind-loaded, high elevation terrain. A size 1 persistent slab avalanche was also noted on a steep cut bank below treeline. Extreme winds forecast for Thursday will likely spark a new round of wind slab activity. In areas where buried persistent weak layers exist, wind-deposited snow will also add to the likelihood and consequences of triggering the overlying slab.
Snowpack Summary
Up to 17cm of new snow fell on Tuesday night. Extreme southwest winds forecast for Thursday will redistribute much of this loose surface snow into fresh and reactive wind slabs at all elevations. East of Crowsnest Pass any new snow will overlie mainly scoured surfaces or hard stubborn wind slabs from last week. In general, the mid and lower snowpack are strong and well-settled in these areas.In areas to the west of the divide you're more likely to find a mix of persistent weak crystals which may lie up to 30cm below the surface. These crystals, which formed at the beginning of January, seem most reactive in open areas below treeline and consist of surface hoar, facets and a hard crust on steep solar aspects. Additional snow load will add to the size and reactivity of this developing persistent slab. About 50cm below the surface you may also find a surface hoar layer which formed in December. Although no avalanches have been reported on this layer, it continues to produce sudden results in snowpack tests and may come to life with a large trigger or additional storm loading.
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.