The storm may be over, but the mountains are currently primed for skier/sledder triggered avalanches.
Weather Forecast
Synopsis: The region should see one last burst of precip as the moisture laden system moves off to the east Saturday night. Sunday/Monday bring a drying trend that should persist through Monday evening. Saturday Night: 10/15 cm/mm snow/rain. Freezing level around 1500 at Sunset, steadily lowering through the night. Moderate to Strong SW winds.Sunday: Freezing level starts at 500m rising to 900m during the day. No precipitation expected. Cloudy skies. Winds switch to the NE, light at treeline, strong at ridge top. Monday: Freezing level at valley bottom. No precip. Clear skies. Winds light and variable.Tuesday: Freezing level at valley bottom, rising to 1000m during the day. Precip begins around lunch time as another deep upper low approaches the region.
Avalanche Summary
A widespread natural avalanche cycle occurred on Friday with many reports of avalanches running up to size 4 on all aspects and elevations. Remember that we define a size 4 as being large enough to destroy a railway car or semi truck. The observations are humbling to say the least.
Snowpack Summary
A heavy dump of moist new snow has added to recent storm totals (now 120-150 cm over a two-week period). The snow surface became wet as snow gradually turned to rain to approximately at 1900 m. I expect a crust to develop at lower elevations as temperatures drop Saturday night. Below the storm snow lies a highly reactive weak layer of surface hoar (and/or a crust) which was buried on February 12th. Strong southwest winds have redistributed the new snow into deep wind slabs on exposed lee slopes and cross-loaded gully features. The February 12 layer is primed for triggering and has been reacting readily to both natural and human triggers on all aspects and a wide range of elevations. Deeper persistent weak layers from early February and late January continue to show planar failures when moderate to hard forces are applied in snow pack tests. In particular, in the Monashees, a rain crust buried down 150 cm has been responsible for a couple of recent large, surprising avalanches. Immediately following the current storm, the presence of this deeper weak layer could further increase the potential size of avalanche events.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.