Fresh and reactive wind slabs will continue to form as the winds increase over the next couple of days.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
A change in the weather pattern is upon us as the jet shifts to a westerly flow allowing a series of Pacific frontal systems to track across the Interior regions.Tuesday: Mix of sun and cloud. Alpine temperatures near -14 and ridgetop winds moderate with strong gusts from the West. Wednesday: Cloudy with scattered flurries 5-10 cm. Alpine temperatures near -12 and ridgetop winds moderate with strong gusts from the West. Thursday: Mostly cloudy with a trace of new snow. Alpine temperatures near -10 and ridgetop winds moderate-strong from the West. Freezing levels rising to 1200m.
Avalanche Summary
On Sunday, numerous natural storm slabs were reported up to size 1.5 from steep NE terrain at 2200m. The deep persistent slab problem is a low probability/high consequence scenario that warrants extra caution around large open slopes, especially in shallow snowpack areas. There was a report last Tuesday of a size 3.5 avalanche at Mt Hosmer in the Lizard/Flathead region that released on or stepped down to the deep weak layer near the ground. On Wednesday we had a report from the Lizard range of another size 3.0 deep persistent avalanche on a northeast aspect in the alpine.
Snowpack Summary
Up to 20 cm of storm snow fell on Saturday night, with Castle Mountain being the clear winner. This storm snow sits on last weeks ageing storm slab. The mid-pack in this region is well settled, but the bottom third of the snowpack is composed of weak facets (sugary crystals). Recent snow profile tests have produced moderate failures that released suddenly down 75 cm on the facetted crystals. In the Crowsnest North late last week up to 35 cm of recent storm snow lies above various old surfaces. Near Elkford up to 20 cm of old storm snow sits above a melt/freeze crust. Approximately 100-120 cm of settled snow sits above the weak layer of sugary facets that developed during the cold spell in December. Snow profile tests indicate hard shears in this location where the facets are sitting on a hard wind crust.
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.