New snow, strong winds and rising temperatures make a great recipe for avalanches. Add in a touchy weak layer buried in the upper snowpack and things may get a bit spicy.
Confidence
Fair - Intensity of incoming weather systems is uncertain
Weather Forecast
Moderate to heavy precipitation is expected to continue until late Saturday. Anticipated amounts: by Friday morning – 20-35 cm; Friday – 15-25 cm; Saturday – 10-15 cm. The freezing level is the big question. It looks like it will hover around 1800 m, dropping towards 1200 m on Saturday. Winds are moderate to strong from the S-SW.
Avalanche Summary
A size 2.5 avalanche was observed from a distance on a north aspect at 2300 m. It is suspected to have been remotely triggered by a snowmobiler. A skier accidentally triggered a size 3 hard wind slab on Monday, which failed on the March crust/facet layer. Skiers also remotely triggered size 1.5-2.5 slabs, most on northerly aspects, some of which failed on the March crust/facet layer. A natural and human-triggered cycle of wind slabs was reported on Sunday. A round of storm slab activity is expected over the next few days in response to forecast snow and wind. Because of the buried persistent weak layer, avalanches might be bigger than expected for the amount of new snow.
Snowpack Summary
New snow will be landing on old hard wind slabs and wind-scoured surfaces. New storm slabs are expected to develop. Cornices may also be fragile. A crust/facet persistent weak layer, buried anywhere from 10 to 60 cm down, has started to play up (see avalanche summary). Avalanche problems associated with this layer may linger for a while with the potential for large and destructive avalanches. A deeper crust (formed by heavy rain in February) is down around 70 cm. Below the deeper crust, the snowpack is reported to be generally well-settled and strong. Below treeline, the diminishing snowpack is trending isothermal (same temperature throughout).
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.