Regions
Northwest Coastal.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain on Saturday
Weather Forecast
Synopsis: A low pressure system over northwest BC will track south eastward through BC and into Alberta.Tonight: Cloudy with flurries, freezing level around 200 metres, winds from the south light occasionally moderate.Thursday: Cloudy with sunny periods and possible flurries, trace of precipitation, freezing level around 700 metres, winds light, from the west.Friday: Cloudy with flurries, Freezing level around 600 metres, light ridge top winds, occasionally gusting to moderate.Saturday: Cloudy with snow, 15 to 20cm of precipitation, freezing level around 800 metres, ridge top winds, light from the south east.
Avalanche Summary
Not many reports of avalanche activity from Tuesday, which is more likely because of poor weather and visibility, rather than lack of avalanche activity. Large natural avalanche cycle up to size 3's reported on Monday, initiating in the storm slab and running on the early March and Feb. 10th persistent weak layers. With continued loading and wind, rider triggering is definitely possible at this time.
Snowpack Summary
35 to 50 cm of 24 hr. precipitation combined with moderate to strong winds continues to build wind slabs at tree line and above, increasing the depth of the storm slab that now overlies the March persistent weak layer. This persistent weak layer is now buried well over a metre in most parts of the region. The recent storm snow has fallen on a melt freeze and/or rain crusts that exist in most parts of the region below 1200 metres. This will most likely become reactive on solar aspects when the sun finally comes out.The March weak layer is a combination of hard wind-scoured slab surfaces in exposed terrain, facets and/or surface hoar in sheltered and north aspects, and sun crust on steep solar aspects and is widespread throughout the forecast region.A late January/early February crust/facet/surface hoar combo is still showing up in snowpack tests with moderate to hard shear tests. This layer will be with us for a long time, and a slide triggered on this layer would be very large and destructive. Recent warming temperatures will be good for bonding on this layer, but a sudden increase in load, or a large rainfall event could "wake up" this layer and result in very large avalanches.
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.