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
Tuesday: 5-12cm of new snow / Extreme westerly winds / Alpine temperature of -13Wednesday: Mix of sun and cloud with light flurries / Moderate to strong southwest winds / Alpine temperature of -8Thursday: Mix of sun and cloud with light flurries / Light southerly winds / Alpine temperature of -5
Avalanche Summary
In recent days, several natural storm slab avalanches to size 2 were observed failing in response to new snow and wind. These avalanches ran on all aspects above 1800m. On Sunday in the southwest of the region a skier accidentally triggered a size 2 persistent slab avalanche which failed on the mid-February layer. The avalanche failed on a southwest facing slope below treeline and completely buried the skier who was successfully rescued by their party. Another noteworthy occurrence was a naturally-triggered size 3 persistent slab avalanche which failed on the mid-January surface hoar. 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. Although this avalanche occurred north of Hwy 1 just outside the South Columbia region, it 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
About 30-50 cm of new snow now covers 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. 50-80 cm below the surface you'll likely find another interface with similar character and distribution that was buried mid-February. This layer has been reactive in recent snowpack tests and has produced large avalanches, particularly in the south of the region where the overlying slab has become deep and cohesive.There are several persistent weak layers that are showing signs of improvement 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.