Don't let clear skies create complacency with a complicated snowpack. A tricky pattern of wind-drifted snow and an atypical buried weak layer require careful evaluation and terrain selection on Sunday.
20-30 cm of recent snow with strong winds changing in direction (from south to west to northwest) has formed a complex pattern of reactive wind slabs on leeward terrain features at upper elevations. These areas of concern overlap with where cornices may also be reaching their breaking point.
40-70 cm cm of snow is settling over a layer of surface hoar on sheltered slopes near and above treeline. On solar aspects, these weak grains may be accompanied by a crust. Recent snow, wind, and solar radiation have promoted cohesion in the slab above this layer, priming it for human triggering. Travelling near steep, open slopes at upper elevations warrants an investigation of these deeper layers.
Saturday night: Clear, light northwest winds, freezing level dropping to valley bottom.
Sunday: Mostly clear, increasing cloud in the afternoon, light variable winds, freezing level 1000 m.
Monday: Increasing cloud, isolated flurries with trace accumulations, light southwest winds, gusting strong at ridge-top, freezing level 1100 m.
Tuesday: Mostly cloudy, 10-15 cm of snow, moderate southwest winds, gusting strong at ridge-top, freezing level rising to 1300 m.
By Friday afternoon, small (size 1-1.5) wind slabs were easily human-triggered on lee features in the alpine and upper treeline. These avalanches were breaking 20-30 cm deep. On Saturday, several large (size 2) explosive-triggered avalanches were reported from operators, breaking 20 cm deep in the storm snow and including two cornices.
On Wednesday, a large (size 2) human-triggered avalanche releasing on the February 22 surface hoar was reported.
Reports from earlier in the week include a natural storm and wind slab avalanche cycle up to size 2, primarily on northeast aspects in the alpine and upper treeline, releasing on the February 22 interface. This MIN post includes a photo of a skier triggered wind slab.
20-30 cm of snow has accumulated since Friday. Strong winds from the south switched to the northwest and decreased. The winds redistributed the storm snow into a complex pattern of reactive wind slabs on lee features at upper elevations and rapidly loaded cornices. Periods of sun on Sunday are expected to initiate pinwheeling and rollerballs from strong solar radiation on steep, sunny slopes. Ice climbers should monitor for the effect of solar radiation on overhead snow and for the potential for small loose wet avalanches to have consequences.
Buried 40-70 cm deep, a weak layer of sugary faceted snow and feathery surface hoar from February 22 may persist and warrants careful evaluation. On south-facing slopes, this weak layer may be combined with a melt freeze crust. Check out the latest forecaster blog that offers a deeper dive into these conditions.
Faceted snow and crusts exist near the base of the snowpack in much of the region. This layer is has been dormant recently.
