Avalanche Forecast

Issued: Feb 28th, 2020 5:00PM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs and Persistent Slabs.

Avalanche Canada astclair, Avalanche Canada

Winds are strong and shifting direction as snow accumulates, creating a tricky pattern of wind slabs at upper elevations. Use caution around drifted slopes and convex roll-overs, especially where these slabs may overly a buried weak layer.

Summary

Confidence

Moderate - Uncertainty is due to the track & intensity of the incoming weather system. Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather.

Weather Forecast

Friday night: Cloudy, 10-20 cm of snow, strong southwest winds, freezing level dropping to 500 m.

Saturday: Mostly cloudy, a trace of snow in the morning, northwest winds decreasing from moderate to light, freezing level 1000 m.

Sunday: Mostly clear, light variable winds, freezing level 1000 m. 

Monday: Partly cloudy, isolated flurries with trace accumulations, light southwest winds, freezing level 1100 m. 

Avalanche Summary

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 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.

Snowpack Summary

By Saturday afternoon, 10-20 cm of snow is forecast to accumulate as the freezing level drops below 500 m. Strong winds from the south are expected to switch to the northwest and decrease. This will likely create a complex pattern of reactive wind slabs on lee features at upper elevations and may bring cornices to their breaking point. Buried 40-70 cm deep, a weak layer of near surface facets or surface hoar from February 22 may persist and warrants careful evaluation.

Faceted snow and crusts exist near the base of the snowpack in much of the region. This layer is has been dormant recently.

Terrain and Travel

  • Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.

Problems

Wind Slabs

An icon showing Wind Slabs

10-20 cm of new snow with strong winds changing in direction (from south to west to northwest) may form 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. 

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 2

Persistent Slabs

An icon showing Persistent Slabs

A weak layer of near surface facets and surface hoar from February 22 may get buried down 40-70 cm. On solar aspects, these weak grains may be accompanied by a crust. As new snow and wind add to the slab above, there is concern for the size and consequences of avalanches on this persistent weak layer. 

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

1 - 2.5

Valid until: Feb 29th, 2020 4:00PM

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