Avalanche forecast

North Rockies

Feb 21 – 23, 2021Avalanche Canada
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Avalanche danger

Tue
Wed
Alpine
4 · High
Treeline
4 · High
Below
3 · Considerable
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Highlights

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.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–3

    Snow and an overall warming trend will likely make a weak layer of surface hoar that developed late-January reactive to riders. Prior to this storm, the layer was found 15 to 60 cm deep across the region, being deepest in the south and shallowest in the north. This layer has been the most sensitive right at treeline, but it may extend up into the alpine, and be found in openings in the trees below treeline. The likelihood of triggering this layer is expected to increase once a cohesive slab develops above it.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely - Almost Certain
    Size
    1–2

    New wind and storm slabs will form as the snow falls and the wind blows strong to extreme on Saturday night and Sunday. The wind is forecast to be strong to extreme from the southwest, so use particular caution near ridges in lee terrain features. 

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–1.5

    The freezing level is expected to rise to 1500 m through the day on Sunday. As a result incoming precipitation is expected to fall as rain at lower elevations causing the surface snow to sluff.

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.

Regions covered