It's complicated: 1) fresh snow may disguise reactive wind slabs; 2) a widespread, touchy weak layer may just be waiting for a trigger; 3) clearing skies in the afternoon could ramp up reactivity.
Avalanches have been large and human triggering remains likely at all elevations.
Incremental snowfall and recent strong winds formed reactive wind slabs in the alpine and open areas at treeline that remain likely to human-trigger. If triggered, wind slabs have the potential to step-down to deeper weak layers. This problem overlaps with where cornices may be reaching their breaking point.
A weak layer of surface hoar is buried 40-80 cm deep, and human-triggering is likely. Over the past several days, large avalanches have been observed extensively across the region. The distribution of this problem extends to all aspects and elevations, but north to east aspects near treeline have been a hot-spot for reactivity.
Thursday night: Cloudy, 5-10 cm of snow, moderate southwest winds, alpine temperature -7 C,freezing level dropping to valley bottom.
Friday: Decreasing cloud, isolated flurries with trace accumulations, light southwest winds, alpine high temperature -2 C, freezing level 1100 m.
Saturday: Increasing cloud, up to 5 of snow, light variable winds, alpine high temperature -4 C, freezing level at valley bottom.
Sunday: Partly cloudy, 5-10 cm of snow overnight, light southwest winds, alpine high temperature -7 C, freezing level at valley bottom.
Over the past several days, a natural wind slab avalanche cycle was observed. These avalanches were small to large (size 1-2.5) breaking 25-50 cm deep on leeward features at upper elevations. In several cases, cornice falls triggered these wind slabs.
There has been a flood of reports of avalanches releasing on the February 22 surface hoar since Saturday. Both human-triggered and remotely-triggered avalanches have been reported by nearly every operation region-wide, even as professionals tiptoe around and avoid suspect terrain features. The problem is touchy but tricky. In many instances, slopes were ski cut with no results, only to have the third or fourth skier in the group trigger the slab.
Avalanche size has increased since the weekend, with more of the activity releasing size 2+. Observations extend to all aspects and elevations, but north to east aspects near treeline have been a hot-spot for reactivity. Avalanches are also occurring at unusually high elevations for surface hoar, with several cases of wind slabs stepping down in the alpine. This combination has produced very large (size 3-4) avalanches.
Incremental snowfall and recent strong winds formed reactive wind slabs in the alpine and open areas at treeline that remain likely to human trigger. Convective showers in the wake of the storm may obscure the evidence of these wind slabs.
A weak layer of widespread surface hoar is buried 40-80 cm deep. It may sit over a crust on solar aspects. Incremental loading from successive storms, strong winds, and mild temperatures have strengthened the slab character and increased the depth of the snow above, creating the potential for larger avalanches with serious consequences. Read more about surface hoar on our forecaster blog!
