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Avalanche Forecast

Archived

Feb 20th, 2021–Feb 22nd, 2021

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

Regions

North Rockies.

A warm front is expected to impact the region bringing moderate to heavy snowfall, strong to extreme winds and a rising freezing level. This is a recipe for HIGH avalanche danger. Minimize your exposure to avalanche terrain on Sunday.

Confidence

Moderate - 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

SATURDAY: Cloudy with snowfall, accumulation 10 to 25 cm, 60-90 km/h southwest wind, alpine temperature -6 C, freezing level at valley bottom.

SUNDAY: Snow, accumulation 15-25 cm, 80-110 km/h west and southwest wind, alpine temperature -3, freezing level 1500 m.

MONDAY: Cloudy with flurries, accumulation 2-5 cm, 30 to 60 km/h west wind, alpine temperature -5 C, freezing level 1000 m.

TUESDAY: Cloudy with flurries, accumulation 2-5 cm, 30 km/h northwest wind, alpine temperature -6 C.

Avalanche Summary

There have been no reports of avalanches in past few days, however there were several reports of cracking and whumpfing on Friday as outlined in these MIN posts here and here.

Expect wind, storm and persistent slab avalanche activity to increase as the snow falls and the wind howls Saturday night and through the day Sunday. This is a good time to dial back your terrain choices.

Snowpack Summary

New snow and strong to extreme wind are expected to form both wind and storm slabs on Saturday night and Sunday. The wind is forecast to blow from the west 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 60-70 cm deep near Renshaw, 40-50 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

  • Minimize exposure during periods of heavy loading from new snow and wind.
  • If triggered loose wet avalanches may step down to deeper layers resulting in larger avalanches.
  • Watch for avalanche hazard to increase throughout the day.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.