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Avalanche Forecast

Archived

Dec 1st, 2021–Dec 2nd, 2021

Alpine
Natural avalanches possible, human triggered probable.
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

Sea To Sky.

Bring your guard up as you approach the alpine. Your search for dry snow will place you in the same terrain where avalanche problems still exist.

Confidence

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

Weather Forecast

Wednesday night: Diminishing flurries bringing 5-10 cm of new snow. Moderate southwest winds, easing.

Thursday: Mainly sunny. Light west winds. Treeline temperatures cooling from -3 to -7 over the day.

Friday: Initially clear, clouding over by afternoon. Light northeast winds. Treeline high temperatures around -7.

Saturday: Cloudy with lingering flurries from the overnight period bringing 5-10 cm of new snow. Light to moderate southwest winds, easing over the day. Treeline high temperatures around -7.

Avalanche Summary

A natural storm slab avalanche cycle occurred over Tuesday night. Details are still limited but it's safe to assume numerous large or even very large avalanches occurred during this period.

Looking forward, alpine areas where dry snow accumulated during the storm will remain a real concern for Thursday. Cooling temperatures should aid in stabilizing the snowpack but the alpine may need more time to adjust to the significant load of new snow.

Snowpack Summary

The warm, wet storm from early in the week brought 50+mm of precipitation to the region. Above about 2100 metres, this fell as snow (up to 50 cm) under the influence of extreme southwest winds. At these upper alpine elevations, the new snow added to 20-50 cm of previous storm snow. A breakable crust (from Nov 27) exists about halfway through its depth. 

The new and older storm snow collectively overlies a mix of surface hoar, decomposing grains and faceted snow resting on a melt freeze crust from mid-November. Surprising large avalanches sliding on these weak layers were reported during the last storm. 

Below the storm's snow line, a rain-wetted 50 cm of older storm snow overlies the same crust. Areas where wet snow exists at or very near the surface will quickly stabilize as temperatures cool Wednesday night.

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.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Watch for areas of hard wind slab on alpine features.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

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.