Last weeks Chinooks decimated the snowpack in much of our forecast area. The new snow is being accompanied by strong westerly winds, but should provide for some good skiing in sheltered areas near the divide.
Weather Forecast
A couple of weak fronts (warm followed by cold) will bring snowfall of up to 10cm Friday night into Saturday morning accompanied by cooling temperatures. Unsettled conditions will persist through the weekend in the wake of these fronts, with cloudy skies, cool temps, variable light to moderate winds and scattered flurries through Monday.
Snowpack Summary
Up to 12mm of precip fell Thursday, accompanied by freezing levels dropping from 2200m to valley bottom and strong westerly winds. This over firm surfaces previously affected by Strong to Extreme Chinook winds and warm temperatures. Two PWLs linger as a concern; a layer of crust or surface hoar down 40-70cm, and a crust facet layer down 70-120cm.
Avalanche Summary
Several loose wet avalanches up to size 1.5 were observed on Thursday below 2200m. An isolated size 2.0 natural slab avalanche (likely a fresh windslab) was observed Friday morning from a steep Easterly alpine slope. Neighboring ski operations have reported isolated large deep persistent slab avalanches failing on the Dec. 9th crust recently.
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.
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.