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Avalanche Forecast

Archived

Feb 28th, 2020–Feb 29th, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Sea To Sky.

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.

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

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.