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Avalanche Forecast

Archived

Jan 14th, 2021–Jan 15th, 2021

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

Regions

Sea To Sky.

New snow and wind will keep storm slabs and cornices fresh. As you tiptoe around these surface instabilities remember their potential, if triggered, to step down to persistent slab avalanches on deeply buried weak layers. Such avalanches have been large and destructive lately.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Thursday night: Rain/wet snow, 10-20 mm/cm, strong south winds, freezing level fluctuating between 2000 and 1300 m.

Friday: Mix of sun and cloud, moderate west wind, freezing level 1300 m.

Saturday: Mix of sun and cloud, light southwest wind, freezing level 1300 m.

Sunday: Snow, 5-15 cm, moderate southwest wind, freezing level 1200 m.

Avalanche Summary

On Wednesday, there were reports of a natural storm slab avalanches cycle up to size 3. There were also several that stepped down to deeper weak layers resulting in very large (size 3-4) persistent slab avalanches. Check out this MIN describing a large cornice-triggered avalanche in the Blackcomb sidecountry.

Last Saturday, a large (size 3) avalanche was remotely triggered by skiers on Cowboy Ridge near Whistler. The avalanche occurred on a north to northwest aspect at around 1900 m. The crown ranged reached up to 200 cm and was approximately 200 m wide. Check out this MIN for photos and more information. A similar avalanche was remotely triggered by skiers on a nearby westerly slope a few days before; check out this MIN for more info and photos.

Snowpack Summary

40-60 cm of recent storm snow accompanied by strong to extreme westerly wind has built reactive storm and wind slabs at upper elevations. These slabs may be extra sensitive where they overlie surface hoar in sheltered terrain or a crust on solar aspects. Warmer temperatures and a period of rain overnight Tuesday has formed a thin surface crust up to 1900 m.

The snowpack is currently quite complex. Layers of surface hoar as well as sugary faceted grains above a hard melt-freeze crust from early December may be found down 100 to 200 cm. Last weekend, several large natural and human-triggered avalanches on this layer occurred, with the hot spots being around the Whistler backcountry.

Near the base of the snowpack, there may be faceted grains above a crust from early-November, which was identified as the failure layer in recent sporadic large avalanche releases.

Terrain and Travel

  • In times of uncertainty conservative terrain choices are our best defense.
  • Cornice failure may trigger large avalanches.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.

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.