Recent new snow combined with wind has promoted wind slab development as well as cornice growth. As the clouds part in the coming days remember that the sun packs a punch this time of year.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
SUNDAY: Mix of sun, cloud and isolated flurries / Light to moderate southwest wind / Alpine temperature -11 MONDAY: Mix of sun, cloud and isolated flurries / Light to moderate west wind / Alpine temperature -10 TUESDAY: Mix of sun, cloud and isolated flurries / Light southwest wind / Alpine temperature -7
Avalanche Summary
Reports of avalanche activity on Thursday and Friday includes natural storm and wind slab avalanches to size 2 on north, south and west aspects from tree line into the alpine. On Friday there were two skier triggered size 2.5 persistent slab avalanche between 1800 and 1900m on south and southeast aspects. These ran on a crust buried mid-February 70cm deep. There were also a few size 1 skier triggered avalanches reported between 1400 and 2000m on Thursday. These were primarily on southerly aspects. On Wednesday a size 2 remotely triggered (from a distance of 10m) persistent slab avalanche was reported to run on surface hoar sitting on a crust that was buried mid February. This was on a southwest aspect at 1800m.
Snowpack Summary
40 to 60 cm of storm snow from the past week has been redistributed into wind slabs in wind-exposed terrain on most aspects. Below this is a layer buried mid-February that presents as a sun crust on solar aspects, and spotty surface hoar on sheltered slopes. This layer looks most concerning on on solar aspects where it's associated with with small facets or surface hoar sitting above. There are several deeper layers in the mid-pack that have shown signs of improving but remain on the radar as having a low probability of triggering yet a high consequence avalanche if they are triggered. We're talking about surface hoar layer buried back in December and January. Near the base of the snowpack is a November crust combined with loose sugary snow. These layers may "wake-up" with strong inputs such as sustained warming, sustained snowfall, large triggers (e.g. cornice fall, smaller avalanches coming down from above); human triggering is also possible in shallow snowpack areas with variable snow depth and convoluted terrain.
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.