Variable amounts of new snow throughout the region has been redistributed by recent moderate to strong southeast and east wind. When the sun comes out this time of year it usually packs a punch - steep southerly slopes are suspect right now.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Saturday
Weather Forecast
SATURDAY: Flurries, accumulation 5-15cm Friday night into Saturday / Light southeast wind / Alpine temperature -8 / Freezing level 900m SUNDAY: Cloudy with isolated flurries / Light west wind / Alpine temperature -12 MONDAY: Mix of sun, cloud and isolated flurries / Light west wind / Alpine temperature -12 NOTE: Convective flurries which are common during this time of year can result in widely varying snowfall amounts throughout a region. These spring squalls routinely drop 20+ cm of snow in one valley while the adjacent drainage remains dry. For this reason, the distribution of avalanche problems associated with new snow, such as wind slabs, can vary greatly within a region.
Avalanche Summary
Thursday's reports focus on wind and storm slab avalanches up to size 2 at treeline and in the alpine primarily on north and east aspects. Wednesday's reports either spoke to "no new avalanches" or small (size 1 to 2) slabs triggered by people. There was a single size 2.5 avalanche on a south aspect releasing on a layer approximately 70 cm deep.Prior to Wednesday, avalanche activity consisted of mainly loose dry sluffing, storm slabs, or wind slabs (skier triggered and skier remote) in the size 1 to 2 range. However, we've received reports of persistent slab avalanches where recent accumulations have been the highest. These avalanches, were remotely triggered (from a distance) or naturally occurring. We suspect the sun crust buried mid-February is the culprit for those on southerly aspects. Deep persistent avalanche activity has become less frequent over the past week.
Snowpack Summary
An additional 10-25cm of new snow fell Thursday into Friday, bringing the storm snow total this week to 40 to 70cm. Earlier in the last storm it was 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 50 to 80cm 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. March 01 today -- 'tis the season when solar aspects start to get complicated!There are several deeper layers that have shown signs of improving but still remain suspect as low probability - high consequence avalanche problems. I'm 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.