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Avalanche Forecast

Archived

Nov 29th, 2021–Nov 30th, 2021

Alpine
Widespread avalanches certain.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Widespread avalanches certain.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Sea To Sky.

Another atmospheric river storm will bring lots of precipitation and rising freezing levels. As a result, expect to see a widespread avalanche cycle with the potential for large, full-path avalanches. Avoid all avalanche terrain for the next days.

Confidence

High - We are confident the likelihood of avalanche will increase with the arrival of the forecast weather.

Weather Forecast

The third of a series of atmospheric river winds its way down the BC coast and the Sea to Sky ranges will see rain ramp up through this afternoon and evening.  

Monday night: Periods of snow; 20-35 cm. Strong southwest winds. Alpine low temperatures -4 C with freezing levels at 1300 metres.

Tuesday: Heavy snow changing to rain in the afternoon; 30-50 cm with even more overnight. Strong to extreme southwest winds. Alpine high temperatures +1 C with freezing levels at 2000 metres.

Wednesday: Wet snow mixed with rain; 30-40 cm. Strong to extreme southwest winds. Alpine high temperatures +2 C with freezing levels at 2200 metres.

Thursday: Mainly cloudy with scattered flurries; 5 cm. Moderate westerly winds. Alpine high temperatures -7 C with freezing level at 800 metres.

Avalanche Summary

Numerous natural and triggered avalanches, including large storm slabs avalanches (size 3 and 4) sliding on buried surface hoar and deeper buried crusts, have been reported Saturday during last storm. With another atmospheric river storm affecting the area Tuesday and Wednesday, expect to see a widespread avalanche cycle with the potential for large, full-path avalanches.

Snowpack Summary

Recent snowfalls and strong southerly winds are developing reactive storm slabs with thicker accumulations in leeward alpine and treeline features. This new snow continues to bury a surface hoar layer and a combo of thick crust / facets observed 50-100 cm down the surface in the alpine and at treeline. Surprising large avalanches sliding on these weak layers have been reported over the last few days. Heavy loading coming with this intense storm will likely produce more avalanche activity on these layers.

Average snow depths at treeline are now likely closer to 150-200 cm; 250+ cm in the alpine. Snowpack depths decrease dramatically below treeline and may still be below threshold for avalanches in some areas. Early season hazards such as rocks, stumps, and creeks are still a concern at these lower elevations.

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.
  • If triggered, storm slabs in-motion may step down to deeper layers and result in very large avalanches.
  • Be aware of the potential for large, deep avalanches to run full path or even longer.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.