Regions
Northwest Coastal.
The incoming storm will build touchy wind slabs and possibly wake up weak layers buried by last week's storm.
Confidence
Moderate - Due to the number and quality of field observations
Weather Forecast
SUNDAY: Cloudy with afternoon flurries, southwest winds picking up throughout the day reaching 60 km/h by the afternoon and 100 km/h overnight, temperatures around -10C.MONDAY: Stormy with 20-40 cm of new snow, 40-60 km/h southwest winds, temperatures around -5C.TUESDAY: Stormy with 10-20 cm of new snow, 20-40 km/h southwest winds, temperatures around -5C.
Avalanche Summary
Wind slabs continued to be reactive on Friday, as reports included a rider-triggered size 2 soft slab on a north aspect at treeline and numerous explosive-triggered size 1.5-2.5 avalanches on east and northeast aspects. More reports confirm a large natural avalanche cycle producing up to size 3 avalanches occurred during last week's storm. In the northern part of the region where the snowpack is thinner, skiers have remotely triggered several size 1.5-2 deep persistent slab avalanches in steep shallow start zones (failing on weak sugary facets near the ground). With winds picking up on Sunday, expect wind slabs to remain touchy.
Snowpack Summary
Clear cold weather has limited settlement of the recent storm snow. New surface hoar (up to 15 mm) and sun crusts have been reported forming on the snow surface, and intense snow transport has been observed in the alpine. The region received up to 125 cm of new snow between 17 and 21 December. The new snow arrived with extreme winds, which has left widespread hard wind slabs in exposed terrain. All this new snow sits over a variable interface composed of hard wind slabs in wind exposed terrain, weak faceted (sugary) snow, and 5-10 mm feathery surface hoar in sheltered areas. The bond between the newer storm snow and this interface is a concern and could develop into a persistent slab problem. In areas with less snow, there are two additional layers that should be on your radar. The first is a layer of surface hoar that was buried in mid-November and may now be down around a metre in thinner spots. The second is an old rain crust from November that may have developed weak facets, particularly in the north of the region.
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.