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
TUESDAY: Mix of sun, cloud and isolated flurries / Light southwest wind / Alpine temperature -10 WEDNESDAY: Mix of sun and cloud / Light southwest wind / Alpine temperature -9 THURSDAY: Mainly cloudy with scattered flurries / Light to moderate southwest wind / Alpine temperature -6
Avalanche Summary
Reports through the weekend highlighted numerous natural wind and storm slab avalanches size 1.5-2.5 at treeline and in the alpine on all aspects. Additionally there were also reports of skier triggered wind and storm slab avalanches to size 1.5 as well as a remotely triggered size 1.5 persistent slab avalanche on a northwest aspect at 1300m. On Sunday there was a skier triggered size 1.5 persistent slab avalanche on a southwest aspect at 2100m in the southern portion of the region. On Saturday there was a report of skier triggered size 1.5 persistent slab avalanche on a northeast aspect at 1500m with a 100cm fracture in the southern portion of the region as well.
Snowpack Summary
Storm snow total last week is 50 to 90cm and has now mostly settled. This new snow has been redistributed into localized wind slabs. It covers old surfaces including faceted powder, a sun crust on solar aspects, and surface hoar on shady slopes. Around 70 to 110cm below the surface there's a layer buried mid-February that's a sun crust on solar aspects and spotty surface hoar on sheltered slopes. This layer looks most concerning on solar aspects where it's associated with with small facets or surface hoar. There are several deeper layers that have shown signs of improving but still remain suspect as low probability to trigger yet will result as a high consequence avalanche if triggered. We are 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, rocky 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.