Bulletin d’avalanche

Banff Yoho Kootenay

Dec 26 – 27, 2022Avalanche Canada
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Archivé · expiré 4 years ago

Danger d’avalanche

Wed
Thu
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Points saillants

Increased winds, warmer temps, and new snow have added load to a very weak faceted snowpack.

The Christmas Eve Storm brought a spike in natural avalanche activity however this had already backed off with the fair weather Christmas Day. Incoming weather overnight promises to pack another punch for Boxing Day so further natural and human triggered avalanches should be expected.

A conservative approach to terrain is recommended through the holidays until things improve.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2.5

    Strong S-W winds and 10-25 cm of snow in the last 72 hrs has contributed to the development of slabs particularly in alpine and treeline terrain. These slabs overlie the weak and faceted snow that sat on the surface during the prolonged deep freeze. If triggered the slab may step down to the deep persistent slab or simply gouge into the facets at lower elevations. In either scenario a larger avalanche than expected will result.

  • Loose Dry

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible - Likely
    Taille
    1–1.5

    While we are not too concerned about the new snow 'surface sluffing', we are becoming concerned with the weak, unconsolidated facets found in sheltered and low elevation areas running as a sluff that entrains more facets and potentially gouges into the rotten basal layers. These may take a larger 'push' to initiate: a small slab or a hard ski cut, and are likely to only be an issue in very steep terrain, but once moving, they are likely to run further and with more force than a regular surface sluff.

  • Deep Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–3

    The whole snowpack is thin, weak and facetted but the biggest facets and depth hoar are found near the base. While the recent cold temperatures discouraged (and may have reversed) some of the slab development over this weak base, the rising temperatures, new snow and strong winds we are now encountering will only serve to promote slab development. We are expecting more avalanche activity on the basal facets over the next week.

Confidence

Moderate

Avalanche Summary

Fresh wind slabs have been reactive to skier and explosive control in the past couple of days and many fresh natural slabs to sz 2.5 were observed Christmas day following the storm overnight.

Most of these slabs were size 1.5 with failures occasionally stepping down to trigger deeper slabs on the basal facets.

In steep to very steep terrain particularly treeline and below, we are getting reports of ski cuts or small slabs initiating long-running sluffs that entrain the unconsolidated facets found beneath the new snow.

Bourgeau left-hand went sz 2.5

Snowpack Summary

15 to 30 cm now sits over the December 17th layer of facets and surface hoar and has been redistributed to form windslabs in open areas by moderate to extreme west to south winds. The Nov 16 facet, crust and surface hoar layer is down 40-70 cm, just above the weak basal facets.

The snowpack is generally quite shallow and has been weakened by prolonged cold temperatures driving the faceting process. This is particularly true BTL where there is very little cohesion to be found in the snow which makes for poor support of travelers and for poor coverage of hazards.

Total snowpack depths at treeline range from 60-120 cms.

Weather Summary

As the next frontal system approaches overnight Sunday, flurries will produce 2-5cm of snow, alpine winds will increase to 40-60km/h, and alpine temperatures will fall to -10 to -15C. Monday, another trough will develop over Alberta which will draw winds alpine into the 60-80km/h range as the freezing levels reach 1300m and 2-5 cm of snow / rain? falls. Alpine temps will drop to -5/-10C overnight into Tuesday as another 2-5cm of snow arrives and the strong to extreme alpine winds remain.

Terrain and Travel Advice

  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Uncertainty is best managed through conservative terrain choices at this time.

Régions couvertes