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Avalanche Forecast

Archived

Feb 19th, 2021–Feb 20th, 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 and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

North Rockies.

Snow will continue to accumulate Friday night and through the day Saturday. The load on a buried persistent weak layer is gradually increasing and may well reach a tipping point. Check out our Forecaster Blog for a more detailed explanation of the brewing situation.

Confidence

Low - Forecast snowfall amounts are uncertain. Uncertainty is due to whether buried persistent weak layers become active, triggering avalanches, with the arrival of the forecast weather.

Weather Forecast

FRIDAY Night: Cloudy with flurries, accumulation 3 to 8 cm, 60 km/h southwest wind, alpine temperature -8 C.

SATURDAY: Cloudy with snowfall, accumulation 5 to 10 cm, 40-60 km/h southwest wind, alpine temperature -7 C.

SUNDAY: Snow, accumulation 15-25 cm, 50-90 km/h west and southwest wind, alpine temperature -2, freezing level 1700 m.

MONDAY: Cloudy with flurries, accumulation 5 cm, 40 to 60 km/h southwest wind, alpine temperature -4 C.

Avalanche Summary

A few wind slab avalanches were triggered by riders in the north of the region earlier in the week.

The likelihood of triggering new wind and storm slab avalanches will increase quickly as the snow falls and the wind blows into the weekend. This is a good time to dial back your terrain choices, with some uncertainty on if/when the buried weak layer will become reactive.

Snowpack Summary

New snow and strong wind will form both wind and storm slabs on Friday night and Saturday. The wind is forecast to blow from the southwest, so watch for the deepest and touchiest deposits in lee slopes near ridges at treeline and alpine elevations. The air temperature is forecast to be quite a bit warmer than the past week, so slabs are anticipated to develop quickly.

The snow will load a widespread persistent surface hoar layer. Prior to the storm, the layer was 60 cm deep near Renshaw, 40 cm deep around Torpy and McBride, and 15 to 25 cm deep further north. Reports suggest this layer is most prevalent around treeline elevations, but likely reaches into the alpine and down into the trees too. It may sit above a crust below 1600 m. The likelihood of triggering this layer will increase as the snow turns into a cohesive slab.

There are presently no deeper concerns in the snowpack.

Terrain and Travel

  • Watch for avalanche hazard to increase throughout the day.
  • Wind slabs are most reactive during their formation.
  • Dial back your terrain choices if you are seeing more than 20 cm of new snow.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Approach steep open slopes at and below treeline cautiously, buried surface hoar may exist.

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.