Bulletin d’avalanche

Kootenay Boundary

Dec 17 – 18, 2020Avalanche Canada
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Archivé · expiré 6 years ago

Danger d’avalanche

Sat
Sun
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Points saillants

It's a good day to make conservative terrain choices and assess how touchy weak layers react to new snow. Watch for signs of instability. Be careful about the possibility for remote avalanches. Be cautious in steep terrain and limit your exposure to overhead hazard.  

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    A rain crust from last week is down 40-75cm. This rain crust is widespread and can be found to mountain top throughout the regions. It appears to be most reactive where a thin layer of sugary facet crystals overlays it. 

    In some sheltered treeline locations there is also a buried surface hoar layer that is buried down 35-60cm.

    These layers have shown signs of instability in recent days, including remote avalanches, accidental skier triggered avalanches, cracking and easy sudden snow test results.

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    15-35cm of new snow is forecast to fall over the region by midday Thursday. New snow is falling with moderate to strong southwest winds and the resulting slabs will likely be the most reactive in wind loaded areas in the lee of ridgecrests. New snow will also be settling into a soft slab with warm temperatures even in wind sheltered terrain.  

Confidence

Moderate - We are confident the likelihood of avalanche will increase with the arrival of the forecast weather. Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather.

Weather Forecast

Wave after wave of frontal systems brings precipitation to the Kootney Boundary through the weekend. Wednesday night into Thursday will see snow with the first wave. Then a brief ridge of high pressure provides cloudy but dry weather Friday ahead of a stronger wave on precipitation that moves in late in the day Friday. A much stronger warmer wave hits the area for Saturday.  

WEDNESDAY NIGHT - Snow flurries with accumulations of 5-15cm, southwest wind 15km/h gusting 60 km/h, alpine low temperature -4C, & freezing levels 1100m.

THURSDAY - Snow flurries with accumulations of 5-10 cm, west wind 20km/h gusting 60 km/h, alpine high temperature -4C & freezing levels near 1000m.

FRIDAY - Snow Flurries with accumulations of 10-15cm, southwest wind, 25-40 km/h, alpine high temperature near -5C & low temperatures near -7C & freezing levels near 1000m.

Saturday - Snow with accumulations of 20-40cm, southwest wind 20km/h gusting to 65 km/h, alpine high temperature near 0C & low temperatures near -5C & freezing levels rising to near 1600m.

Avalanche Summary

On Tuesday, there were numerous reports of widespread natural and skier triggered small avalanches to size 1.5. These avalanches ran on a variety of surfaces including crusts and surface hoar. A skier accidental avalanche was reported to have failed on surface hoar that was buried up to 20cm deep. Other reports were of skier remote avalanches occurring from 40m away on the same buried surface hoar layers.  

Elsewhere, folks were finding a poor bond to the crust as reported in this MIN from Dec 15th S of Nelson. These signs of instability are a great warning sign of deteriorating avalanche conditions. 

Earlier in the week, avalanche activity has mainly been limited to size 1-1.5 skier triggered storm/wind slabs running on the recently buried crust. This great MIN from Friday near Nelson includes a photo of an accidental wind slab.  

As snow fall amounts increase we expect to see the size of avalanches increase.

Please submit your observations to the Mountain Information Network (MIN). Photos are especially helpful! Thank you so much for all the great MINs submitted so far!

Snowpack Summary

Forecast 5-15 cm of new snow overnight Wednesday and another 5-10cm on Thursday is incrementally loading weak interfaces. A total of 30-50cm will overlie a buried crust that extends to near mountain top and surface hoar by mid-day Thursday. The surface hoar is likely to be found in sheltered locations at treeline.

At upper elevations, moderate to strong southwest wind has likely blowing the new snow into fresh slabs that may be reactive to human triggers.

The early November crust is sitting about 10-30 cm up from the ground. There is a late November crust in the midpack in some places as well. Uncertainty with these crusts exists due to limited observations, but there haven't been any recent avalanches reported on these layers in this region.

Snowpack depths vary with elevation, treeline averages between 50-100 cm. 

Terrain and Travel

  • Don't be too cavalier with decision making, storm slabs may remain sensitive to human triggering.
  • Be aware of the potential for larger than expected storm slabs due to the presence of buried surface hoar.
  • Keep in mind the crust offers an excellent bed surface for avalanches.
  • Approach steep open slopes at and below treeline cautiously, buried surface hoar may exist.

Régions couvertes