Bollettino valanghe

North Columbia

Feb 23 – 24, 2022Avalanche Canada
Vedi sulla mappa
Archiviato · scaduto 5 years ago

Pericolo valanghe

Fri
Sat
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
Apri la mappa completa

In sintesi

Recent storm snow is bonding poorly to the underlying surface. Reactivity on this weak layer may persist, particularly in wind-loaded terrain.

Reverse loading from strong northeast winds has built wind slabs on unusual aspects, which could take riders by surprise.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2.5

    Wind slabs will exist on various aspects in exposed terrain and around ridgelines. Last weekend's storm initially came in with southwest winds, but with the intrusion of arctic air, winds switched to the northeast, building slabs in less common locations.

    These slabs may have formed on top of old crusts and hard wind-pressed snow and there is uncertainty about how quickly they will bond to old surfaces.

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely - Possible
    Dimensione
    2–3.5

    There are two prominent weak layers in the upper meter of the snowpack.

    An upper layer exists down ~50-100 cm and is comprised of surface hoar in sheltered areas, a crust at lower elevations and on solar aspects, and hard wind-affected snow in the alpine and exposed treeline. Reports suggest that in many areas the storm snow is bonding poorly to this hard interface, and reactivity on this layer may persist. Be particularly cautious in sheltered areas where surface hoar may be preserved, or south facing aspects where recent wind-loading has occurred and a sun crust may be buried.

    The lower layer may be found roughly 150 cm deep in the snowpack. This layer consists of faceted snow, a sun crust on steep solar aspect, and most notably large surface hoar crystals in sheltered terrain. Over the past few weeks, reactivity on this layer had tapered, but the storm over the weekend caused this layer to 'wake up' on northerly aspects around treeline. The most reactivity on this layer has been seen between 1600-2200 m in open areas in the trees, in cut blocks, and on steep convexities. Terrain features to be suspect of include the lee side of ridges, openings in trees, cut blocks, and burns, especially on northerly aspects where the layer may not be capped by a firm crust.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

A weak disturbance will come through on Wednesday with a light amount of snowfall expected in the region. The ridge of high pressure will reestablish on Thursday. 

Tuesday Overnight: Mainly clear. Light northerly winds. Alpine temperatures around -30 C.

Wednesday: Increasing cloud cover, light snowfall. Light to moderate westerly winds. Alpine temperatures rising to -15 C.

Thursday: Partially cloudy. Light to moderate northwesterly winds. Alpine temperatures around -15 C.

Friday: Partially cloudy. Moderate to strong northwesterly winds. Alpine temperatures around -10 C.

Avalanche Summary

During the peak of the storm on Saturday, numerous natural and human-triggered storm slabs were reported on all aspects due to rapid loading. Larger natural and human-triggered wind slab avalanches were reported in wind-loaded terrain in the alpine and treeline. Several natural dry loose avalanches were reported out of steep terrain on all aspects.

As artic air invaded the province on Monday, strong northerly winds redistributed the storm snow into wind slabs on southerly aspects. A natural avalanche cycle occurred on southerly aspects as a result of this wind-loading. Several human-trigged wind and storm slab avalanches where also reported throughout the day.  

As this storm added load to the snowpack, the mid-January weak layer began to "wake up". Operators reported a large cornice-triggered natural persistent slab avalanche on an east aspect at treeline. In the neighboring South Colombia, two human triggered avalanche occurred on northerly aspects at or just above treeline.

Snowpack Summary

Last weekend's storm brought 30-100 cm of new snow accompanied by southwesterly winds. Below ~1200 m the majority of the precipitation fell as rain, with 10-20 cm of snow now overlying a thick melt-freeze crust. 

Up to 120 cm now overlies a weak layer formed in mid-February. This layer is comprised of surface hoar in sheltered areas, a melt-freeze crust at lower elevations, a sun-crust on steep solar aspects, and hard wind-affected snow in the alpine and exposed treeline. Reports suggest that in many areas, the storm snow is bonding poorly to this firm layer.h

The mid-January surface hoar/crust layer is now down 50-150 cm in the snowpack. There has been reactivity on this layer during last weekend's snowfall. In the neighboring South Colombia, there has been a few human-triggered avalanches on this layer in the past week as well as a few large avalanches occurring naturally as a result of large cornice failures and shallower slabs avalanche stepping down to this layer. See the avalanche summary for more details.

The facet/crust layer that formed in early December is now buried around 150 to 250 cm. Human triggering of this layer is unlikely, except perhaps on a re-loaded bed surface or in a shallow snowpack area. This layer may become a concern during periods of warming or heavy loading. 

Terrain and Travel

  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Keep your guard up at lower elevations. Wind slab formation has been extensive.
  • Closely monitor how the new snow is bonding to the old surface.
  • Shooting cracks, whumphs and recent avalanches are strong indicators of an unstable snowpack.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.

Regioni coperte