Bulletin d’avalanche

Purcells

Dec 9 – 10, 2020Avalanche Canada
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Archivé · expiré 6 years ago

Danger d’avalanche

Fri
Sat
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
3 · Marqué
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Points saillants

Hazard will increase to HIGH in areas where over 30 cm of total new snow fell. Limit exposure to avalanche terrain: New snow will not bond well to widespread surface hoar at tree line and below & fresh wind slabs lurk in the alpine. Be Careful & Watch for signs of instability!

Problèmes avalancheux

  • Storm Slabs

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

    At tree line, in lower elevations and in wind sheltered areas, the new snow rests on widespread layer of large surface hoar crystals. Even as little as 10-15cm of denser storm slabs atop these domino-like surface hoar and can form ⚡ electric⚡ propagation. Shooting cracks, remote triggering and settlements (whumphing) are classic signs of instability that buried surface hoar is becoming reactive. 

    Slab development depends on several factors and can take time so we don't always see slab avalanches right after a big snowfall. This doesn't mean the Surface hoar is no longer a problem & we will need to keep monitoring this surface hoar layer for a while. 

    At upper elevations where the wind is transporting the new snow, fat storm slabs formed in lee features (Northerly and Easterly slopes) will be especially touchy where they sit over a crust, facets or isolated surface hoar.

    Where snow remained cool, loose dry sluffing will likely be observed out of steep terrain and under your skis. However in low elevations, where snow is soggy or moist, loose wet stuffing may also be an issue.

  • Deep Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely - Possible
    Taille
    2–3

    A facet/crust layer from November rains (Nov 5 Crust) sits near the base of the snowpack. On the one hand, there hasn't been any associated avalanche activity in weeks. On the other hand, it's surrounded by weak crystals and still producing hard results in snowpack tests.

    We may see this layer "wake-up" with the new load or be triggered by a smaller avalanche in a step down.  

Confidence

Low - Uncertainty is due to the track & intensity of the incoming weather system. Forecast snowfall amounts are uncertain.

Weather Forecast

Mountainous regions of the Purcells will see higher overall snowfall amounts. Additionally, the "atmospheric river-like" storm will shift south overnight.

Tuesday Night: 10-20cm of snow overnight (more in the south), winds light to moderate Southwesterly, freezing level 1400m.

Wednesday: Cloudy with sunny periods, Nil to 15cm over the day (more in the south), winds light to moderate west wind (with strong NW gusts), freezing level 1400 m.

Thursday: Cloudy with sunny periods, isolated flurries, winds light to moderate Southwesterly, freezing level 700 m.

Friday: A mix of sun and clouds, isolated flurries, winds light to moderate Southwesterly, freezing level 700 m.

Avalanche Summary

An avalanche cycle is ongoing in adjacent regions where 30+cm of snow has already fallen. Large avalanches (Size 2) were reported from skier controlled and explosive control work on Tuesday in the storm snow.  

We expect heightened natural and skier triggered small and large avalanches in the Purcells with forecast snowfall amounts. Furthermore, smaller avalanches may trigger larger avalanches where they step down to the deeper persistent weak layer of the November crust.  

Our eyes and ears in the mountains are limited this winter due to fewer professional observations. If you go out in the mountains, please share your observations on the Mountain Information Network (MIN). Photos are especially helpful! Thank you so much for all the great MINs submitted so far! 

Snowpack Summary

Around 5-15 cm of new snow fell Monday night in the northern regions with another 5-20cm forecast for Tuesday night. This 10-30cm of new snow will sit on top of a melt-freeze crust on south aspects and widespread surface hoar tree line and below. Generally speaking, new snow does not bond well to these types of surfaces.

The lower snowpack is characterized by a couple of crusts, the most notable is from rains in early November (Nov 5 Crust). This crust is sitting near the base of the snowpack, likely surrounded by a weak layer of sugary facets. It is most likely to be reactive to human triggers on south facing slopes and in shallow, rocky areas in the alpine.

Terrain and Travel

  • Choose conservative terrain and watch for clues of instability.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Don't let the desire for deep powder pull you into high consequence terrain.

Régions couvertes