Watch for new wind slab formation on Sunday as new storm snow accumulates. Older wind slabs may also still be reactive to human-triggers.
In the east of the region, buried persistent weak layers may still be reactive to human-triggering in north-facing terrain around treeline.
Watch for new wind slab formation on Sunday as new snow accumulates with moderate to strong southwest wind.
Lingering old wind slabs from the recent northeast wind may still be reactive to human-triggering in exposed high elevation terrain. These would be most likely where they overlie a weak layer from mid-February or in steep, unsupported terrain features.
Two buried persistent weak layers are expected to create problems as the new storm snow accumulates.
A weak layer from mid-February was down 20-30 cm prior to the storm and is expected to become reactive as the overlying snow forms a thicker and more cohesive slab.
Two weak layers of surface hoar from January were down 40-70 cm prior to the storm and were primarily a concern in the east of the region on northerly aspects around treeline. Smaller storm avalanches have the potential to step down to these deeper layers.
The first in a series of storm systems is forecast to reach the region on Sunday morning but it is expected to be relatively weak. A more substantial storm is expected on Monday.
Saturday Night: Increasing cloud cover, moderate SW wind, treeline low around -10 °C.
Sunday: Light snowfall 5-10 cm, moderate to strong SW wind, treeline high around -5 °C.
Sunday night: Snowfall 10-20 cm, moderate to strong SW wind, treeline low around -7 °C.
Monday: Snowfall 20-40 cm, strong SW wind, treeline high around -1 °C.
Monday night and Tuesday: Periods of heavy snowfall continuing overnight and then easing during the day, moderate to strong SW wind, freezing levels reaching around 2000 m.
On Friday, natural and human-triggered loose dry avalanches were reported. Just north of the region, two human-triggered persistent slab avalanches were reported.
On Thursday, four skier triggered avalanches up to size 1.5 were reported on south and southeast aspects at treeline in the northwest of the region.
Earlier in the week, several size 1-1.5 skier-triggered wind slabs were reported near ridge crests. Surface hoar layers buried in January have also been responsible for two recent size 2 avalanches in the east of the region.
The new snow will bury a highly variable snow surface which includes wind-affected surface in exposed terrain, a thin sun crust on solar aspects which may be faceting, surface hoar in sheltered areas, and/or old storm snow on shaded slopes which may be faceting.
A weak interface from mid-February was down around 20-30 cm prior to the storm. It consists of a sun crust on solar aspects, facets, and/or surface hoar. This layer was previously only reactive in wind-loaded terrain where a slab had formed but it is expected to become a more widespread problem as the storm progresses and creates a more widespread slab.
Two layers of buried surface hoar from January were down around 40-70 cm prior to the storm. Most recently, this surface hoar was most reactive in sheltered areas around treeline in the east of the region, and appeared to be dormant through the rest of the region. As the storm progresses and more load is added to the snowpack, this surface hoar will get tested and avalanches have the potential to step down to these deeper layers.
