Bulletin d’avalanche

Lizard-Flathead

Feb 6 – 7, 2021Avalanche Canada
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Archivé · expiré 6 years ago

Danger d’avalanche

Mon
Tue
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
3 · Marqué
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Points saillants

Persistent slabs and fresh storm slabs may be touchy and reactive on Saturday, especially to skier and rider triggering. Snowfall amounts are uncertain so if you see more than 25 cm in your local riding area the avalanche hazard is HIGH.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1.5–2.5

    A cohesive slab 50-80 cm thick now sits above a buried weak layer of surface hoar, surface facets and crust. These slabs have been very reactive to human triggers showing wide propagations in moderate to low angle terrain. 

    Persistent slabs have been catching people by surprise, so pay attention to signs of instability and stay clear of terrain traps like creeks, cliffs, and depressions below you.

  • Storm Slabs

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

    20-40 cm of new storm snow has slowly accumulated over the past few days. Changing wind directions may deposit deeper and stiffer slabs on multiple aspects. I suspect that anywhere the new snow feels stiff and more cohesive the slab may be more reactive, especially where it sits above a buried weak layer. Loose-dry avalanches may be seen from steeper slopes where there is little cohesion in the recent storm snow.

    Cornices are large and fragile and don't respond well to rapid change- like new load from snow, rapid warming or rapid cooling. They deserve respect and a wide berth from above and below. 

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

A series of upper disturbances are expected as the arctic front pushes West of the Divide bringing localized periods of snow to the region. It's hard to pin-point snowfall amounts and they may be heavier than what the models suggest.

Overnight Saturday: Light snow 5-10 cm. Ridgetop wind mostly light with strong gusts from the northwest. Alpine temperatures near -10.

Saturday: Snow amounts 10-20 cm. Ridgetop wind moderate from the East. Alpine temperatures -10.

Sunday: Mix of sun and cloud. Ridgetop wind light from the East and alpine temperatures dipping to -20. 

Monday: Mix of sun and cloud. Ridgetop wind light from the East and temperatures -25. 

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Avalanche Summary

On Friday, numerous explosive controlled loose-dry and storm avalanches were triggered up to size 1.5. On Thursday several MIN reports suggested that things are still spicy out there and that the persistent slab problem continues to be reactive. Check this out MIN Report.

On Wednesday, numerous natural storm slab avalanches were reported up to size 2, and several more slabs were triggered by explosives up to size 1.5. Loose-dry avalanches were seen up to size 1. 

The persistent slab proved to be reactive on Wednesday with skier and rider remote triggers and naturals up to size 2. Check out the awesome MIN posts here: MIN report, MIN report, MIN report. Sending a big shout out of appreciation to everyone posting these MIN's and sharing this pertinent information.

As natural avalanche activity tapers, wind and persistent slabs are still be primed for triggering. These will likely propgate far and can catch you by surprise even in low angle terrain. 

Snowpack Summary

Up to 80 cm of recent snow has formed a cohesive and reactive slab that sits above a weak interface of surface hoar and surface facets (weak sugar or feather-like snow crystals) and a crust that was buried in late January. This slab of recent snow sits on top of a plethora of old snow surfaces comprising of hard wind slab, scoured areas, sastrugi, and isolated pockets of soft snow. Below 1800 m a hard melt-freeze crust underneath the new snow.  

A solid mid-pack sits above a deeply buried crust and facet layers near the bottom of the snowpack (150-200 cm deep), which are currently unreactive. 

Terrain and Travel

  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
  • Look for signs of instability: whumphing, hollow sounds, shooting cracks, and recent avalanches.
  • Avoid terrain traps where the consequence of any avalanche could be serious.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.

Régions couvertes