A new persistent weak layer has "woken-up" and warrants your attention. Read the whole bulletin for details.
Confidence
Moderate - Wind effect is extremely variable
Weather Forecast
Wednesday: Overcast skies with light flurries / Moderate to strong southwest winds / Alpine temperature of -12Thursday: Overcast skies with light flurries / Moderate southerly winds / Alpine temperature of -7Friday: Overcast skies with light flurries / Light and variable winds / Alpine temperature of -10
Avalanche Summary
In recent days, several natural and human-triggered storm slab avalanches to size 2 were observed failing in response to new snow and wind. These avalanches ran on all aspects above 1800m. In the Selkirks, 2 slab avalanches (up to size 2) were remotely triggered on the Mid-February layer. These avalanches failed in south-facing alpine terrain.Another noteworthy event was a naturally-triggered size 3 persistent slab avalanche which failed on the mid-January surface hoar on Sunday. The avalanche, which is thought to have been triggered by sloughing from above, occurred in the Monashees near Revelstoke on a northeast facing slope at 1900m. This avalanche points to the continued reactivity of deeper persistent slabs in isolated terrain. Continued light snowfall snow and strong winds will promote ongoing wind slab activity, especially in exposed higher elevation terrain.
Snowpack Summary
Since Saturday night, 20-50 cm of new snow fell. These accumulations cover old surfaces which include faceted powder, a sun crust on solar aspects and surface hoar on shaded, wind-sheltered slopes. I suspect strong winds will have redistributed much of the new snow into fresh wind slabs in exposed terrain. 40-70 cm below the surface lies another interface with similar character and distribution. This layer, which was buried in mid-February, has just started to produce large avalanches and is definitely worth keeping an eye on as the overlying slab gains thickness and cohesion.There are several persistent weak layers that are showing signs of improving but still remain suspect as low probability - high consequence avalanche problems. Two surface hoar layers buried in January are now 100-150 cm below the snow surface. Deeper in the snowpack (150 - 200 cm deep) is a facet/crust/surface hoar layer buried in December. Near the base of the snowpack is a crust/facet combo layer buried in late November. These layers may "wake-up" with strong inputs such as solar radiation, rapid loading/warming, or a cornice fall. 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.