Bollettino valanghe

Northwest Inland

Mar 12 – 14, 2020Avalanche Canada
Vedi sulla mappa
Archiviato · scaduto 7 years ago

Pericolo valanghe

Sat
Sun
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
Apri la mappa completa

In sintesi

Fresh wind slabs will likely form in new places as winds change direction and increase. Stick to sheltered areas, be mindful of overhead hazards, and investigate buried weak layers on Friday. 

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible - Likely
    Dimensione
    1–2

    Winds are forecast to shift to the northeast and increase with the arctic outflow. These winds are expected to drift recent snow into fresh wind slabs that may be reactive to human triggering. Given recent variability in wind direction, few aspects will be spared from wind slab concerns. This problem is expected to be more reactive where surface hoar sits at the interface.

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible - Likely
    Dimensione
    1.5–2.5

    40 to 90 cm of settled snow rests above several layers of buried surface hoar that have produced avalanches over the past week. These slabs have been most sensitive to triggering in sheltered areas at treeline. 

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely - Possible
    Dimensione
    2.5–3.5

    Recent loading from new snow and wind has aggravated the region's deep persistent slab problem. Some very large and destructive avalanches have been sporadically failing on deeply buried weak layers, predominantly on north through east aspects in the alpine. Cornice falls are a likely trigger for these slabs.

Terrain and Travel

  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Thursday night: Decreasing cloud, moderate northeast winds, alpine temperature -16 C

Friday: Partly cloudy, moderate northeast winds with strong gusts at ridge-tops, alpine high temperature -12 C

Saturday: Clear, strong northeast winds, alpine high temperature near -7 C

Sunday: Clear, light northeast winds, alpine high temperature near -3 C

Avalanche Summary

Strong winds are expected to form fresh wind slabs that may be reactive to human triggers. Humans may be able to trigger these slabs in drifted areas and in areas where surface hoar may be found at the interface. Cornices have also grown large with the recent weather, and a cornice failure could trigger a wind slab avalanche on the slope below. 

Over the past several days, large (size 2) human-triggered avalanches have been reported releasing in this drifted snow/weak interface combination and breaking 20-80 cm deep. These avalanches primarily occurred on leeward features above 1400 m. 

Last week, there were reports of large (size 2-2.5) human-triggered avalanches failing on the March 1st surface hoar layer. Over the past two weeks, avalanches have been reported on an earlier surface hoar layer from February 19th as well as a deep persistent slab failing on basal facets. This pattern highlights how shallow avalanches in the surface snow have the potential to strain multiple weak layers in the snowpack and release larger avalanches.

Snowpack Summary

The storm on Monday/Tuesday delivered 10-20 cm of new snow to the region with strong winds. This new snow is likely sitting on a weak layer of surface hoar, and/or a crust on solar aspects. On Friday, winds are forecast to shift to the northeast and increase. These winds are expected to form fresh wind slabs that may be reactive to human triggering, especially where the snow is drifted or sitting over a layer of surface hoar. 

Loading from new snow and wind has made several deeper weak layers problematic over the past week. A surface hoar layer from March 1st may be found 40-80 cm deep, and another combination of surface hoar and sun crust from February 19 may sit 60-90 cm deep. These layers seem to be most sensitive to human triggering at treeline elevations. 

A couple of weak layers that formed in January are buried in close proximity to one another 80-140 cm below the surface. Below that, an early season crust/facet layer lurks at the base of the snowpack.

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