Thursday's snowfall will continue to add to the existing storm slabs which have been reactive recently and may have a poor bond to the underlying surface. The new snow is expected to be most reactive in wind loaded terrain features and ongoing cornice growth is expected.
Fragile new cornices have been reported recently. Ongoing cornice development is expected on Thursday with new snow and moderate wind in the forecast, and natural cornice failures are possible.
A weak layer of faceted grains above a hard melt-freeze crust may be found around 50-80 cm deep. Last week this layer produced large avalanches. The problem was most prevalent on north to northeast aspects between 1600 and 2100 m.
The older storm snow from the weekend may still be bonded poorly to the underlying surface and is still capable of producing large avalanches. The new snow on Thursday will continue to add to this existing slab or may form thin new slabs over the old storm snow. The new snow will be most reactive in wind loaded terrain features.
A relatively weak storm system is expected to impact the region Wednesday night and Thursday. A break is then expected before the next system arrives Friday.
Wednesday night: Snowfall 3-5 cm, moderate to strong SW wind, freezing level around 1000 m.
Thursday: Snowfall 10-15 cm, moderate SW wind, freezing level reaching around 1200 m.
Friday: Snowfall 3-5 cm, moderate SW wind, freezing level reaching around 1300 m.
Friday night: Snowfall 10-20 cm, strong SW wind, freezing level around 1000 m.
Saturday: Lingering flurries in the morning, a mix of sun and cloud in the afternoon, light variable wind, freezing level reaching around 1200 m.
On Tuesday, skiers triggered a size 2 storm slab on a northeast aspect at around 2100 m which was typically 20-30 cm thick but up to 80 cm where it was wind loaded. This avalanche propagated over 150 m wide. Explosives and ski cutting produced a variety of size 1-2 storm slabs. This MIN report and this MIN report both describe human-triggered avalanches on Tuesday.
On Monday, a variety of human-triggered and explosive-triggered avalanches were reported including this MIN report and this MIN report. Some small natural storm slab avalanches were also observed in recently wind-loaded terrain around treeline.
Persistent slab activity has dwindled since last week. The most recent activity was on March 8 when two avalanches were triggered by riders on northeast aspects at treeline and alpine elevations. While this layer has not produced any avalanches recently, it still remains a concern for professionals in the region. At this point, the most likely triggers would be a cornice fall or a storm slab avalanche stepping down.
Prior to Thursday's storm, 25-40 cm of old storm snow from the weekend typically overlies a variable surface which includes surface hoar in shady, wind-sheltered areas and a hard melt-freeze crust on all aspects below 1500 m and on sun-exposed slopes into the alpine. Recent wind has redistributed this storm snow into reactive slabs in wind loaded terrain and has formed new cornices.
Around 50-80 cm deep, a weak layer of facets may be found above a melt-freeze crust that formed in February. This layer had been most reactive on north and northeast aspects between 1600 and 2100 m. While this layer is now likely dormant in most areas, it still remains a concern for professionals in the region and could still be capable of producing isolated large avalanches if triggered. Check out this forecaster blog for more info.
