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Avalanche Forecast

Archived

Dec 21st, 2020–Dec 23rd, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

North Rockies.

Exercise restraint as skies clear and visibility improves this week. Avalanche activity during the stormy period has been steady, with a tricky set of failure planes in play. Ease cautiously into low consequence terrain and watch for signs of instability.

Confidence

Moderate -

Weather Forecast

MONDAY NIGHT: Cloudy with isolated flurries. Light north winds.

TUESDAY: Becoming sunny. Light northwest winds increasing over the day and overnight. Alpine high temperatures around -12.

WEDNESDAY: Sunny. Moderate to strong southwest winds. Alpine high temperatures around -5, possibly cooler at lower elevations under a mild temperature inversion.

THURSDAY: A mix of sun and cloud. Light to moderate southwest winds. Alpine high temperatures around -5, possibly cooler at lower elevations under a mild temperature inversion.

Avalanche Summary

A report from the Tumbler Ridge rail corridor on Saturday detailed evidence of a 24-48 hour-old widespread natural avalanche cycle producing avalanches up to size 3 (very large) This cycle appeared to be linked directly to recent southwest winds. Subsequent explosives control yielded smaller, thinner wind slab releases.

Pine Pass reports on Friday (courtesy of your local AvCan field team) included natural, accidental, and remotely triggered avalanches up to size 2.5. That's a big slide to get tangled up in -- one party of three were caught and partially buried (one up to the neck) or fully buried (with a deployed airbag).

Slides were running at the bottom or the recent storm snow (down 40 cm) or last week's crust (65 cm deep). There was also a size 3 reported from Renshaw. Observations have been limited by lack of visibility.

Snowpack Summary

The upper snowpack is made up of new or recent snow from a parade of storms over the past week. A recent bout of strong southwest wind redistributed much of this snow in exposed areas at higher elevations, forming wind slabs in the process.

In some areas in the north (Pine Pass, possibly Wolverine) there is a weak layer of surface hoar near the bottom of this recent snow - now as deep as 60 or 80 cm. In some areas this layer may present as a combo of crust and faceted snow. This variable layer continues to react in snowpack tests and slope cuts and should figure into your terrain decisions wherever wind slabs aren't the dominant concern.

The mid and basal snowpack feature a variety of crusts. Recent reports have not highlighted them as a problem but they may be a concern in areas of the region with a shallow snowpack.

Snow depths are approaching 200 cm around Pine Pass, the MacGregors, and McBride and around 100 cm in the northeast around Tumbler Ridge.  

Terrain and Travel

  • Use small low consequence slopes to test the bond of the new snow.
  • Choose low-angled, sheltered terrain where new snow hasn't been wind-affected.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.