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Avalanche Forecast

Archived

Dec 3rd, 2021–Dec 4th, 2021

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

Regions

South Coast Inland.

Shifting winds may form fresh wind slabs on new aspects. Be alert to conditions that change with elevation as you get above the trees.

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

Friday night: Increasing cloud. Light variable winds. Freezing level valley bottom.

Saturday: Flurries, up to 5 cm. Moderate southwest winds shifting northwest. Treeline high temperatures around -9.

Sunday: Mainly sunny. Light northwest winds. Treeline high temperatures around -12.

Monday: Mainly sunny. Light northwest winds. Treeline high temperatures around -12.

Avalanche Summary

Wind slab was reactive to explosives in the Hurley on Thursday. Reports from explosive control work in the Duffey Lake area Wednesday showed debris from releases up to size 2 (large) scouring to ground in tracks and reaching quite low in runouts. This followed the widespread natural avalanche cycle up to size 3.5 (very large) that occurred over Tuesday night. These sizes suggest that some avalanches may have involved persistent layers discussed in the snowpack summary.

Snowpack Summary

A windy conclusion to last week's storm has likely left wind slabs perched in leeward terrain features in the alpine and upper treeline. Wind slabs may see some redistribution Saturday as winds shift northwest. Below 2100 meters, this wind affected snow overlies a moist upper snowpack which is likely solidifying and bonding in the cold temperatures. Rain-wetted surfaces below 1800 m have refrozen into a thick, supportive crust that should effectively lock the snowpack in place at lower elevations.

A couple of buried weak layers produced large avalanches during a previous storm. Snowpack models indicate that these layers have been reset by recent rain at treeline, but uncertainty remains around their status in the alpine. These include a layer of surface hoar down an estimated 90-120 cm and deeper crust/facet combo layer.

Average snowpack depths in the alpine are now likely closer to 150-200 cm. Below treeline, depths of 30-50 cm have been reported around 500 m, decreasing dramatically with elevation, and still below threshold for avalanches in many areas. Early season hazards such as rocks, stumps, and creeks are still a concern below the alpine.

Terrain and Travel

  • Be alert to conditions that change with elevation and wind exposure.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • When a thick, melt-freeze surface crust is present, avalanche activity is unlikely.

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.