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Avalanche Forecast

Archived

Jan 9th, 2021–Jan 10th, 2021

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

Regions

South Columbia.

The slope testers have been out in full force and we've seen a sharp drop in natural and human triggered avalanche activity since the end of the week. Even so, watch out for lingering wind slabs and keep in mind the potential for large and destructive persistent slab avalanches.

Confidence

High - The number, quality, or consistency of field observations is good, and supports our confidence.

Weather Forecast

Saturday night: Increasing cloud, isolated flurries, light to moderate southeast wind, freezing level valley bottom.

Sunday: Mostly cloudy, moderate southwest wind, alpine high -7, freezing level 600 m.

Monday: Flurries, up to 5 cm, moderate southwest wind, alpine high -6, freezing level 1000 m.

Tuesday: Snow 5-10 cm, strong southwest wind, alpine high -5, freezing level 1200 m.

Avalanche Summary

Avalanche activity during the week was generally large (averaging size 2), a combination of wind and persistent slabs, triggered both naturally and by humans.

  • Wind slab - Numerous natural storm/wind slab avalanches were observed on all aspects at alpine and treeline elevations as recently as Friday, averaging size 2.

  • Cornice - A few reports of cornice falls size 1-2 have triggered wind slabs on the slopes below.

  • Human triggered - Thursday and Friday were eventful days for human triggered avalanches in the busiest areas of the region. RMR slackcountry saw four size 1.5-2 skier triggered avalanches, including a partial burial. Check out some of these MIN reports here, here and here. Crowns ranged from 30 to 70 cm, likely running on the most recent storm interface at the shallow end, and on the Christmas persistent layer at the deeper end.

  • Persistent slab - On Wednesday and Thursday, there were four natural persistent slab avalanches size 2.5-3 reported in the region on northerly aspects in the alpine. Three of these were reported to have run on the early December persistent weak layer.

Snowpack Summary

Around 40-80 cm of recent snow sits on a weak layer of surface hoar that was buried around Christmas. This layer is most prevalent at and below treeline. Previous moderate to strong winds have redistributed surface snow, creating wind slabs at wind exposed elevations.

A couple of persistent weak layers buried in early/mid-December are down about 90-150 cm. Generally, they can be found as a layer of surface hoar above in close proximity to a crust/surface hoar layer. The form and distribution of these layers are highly variable and conditions may differ significantly from one valley to the next. 

Terrain and Travel

  • Be careful with wind slabs, especially in steep, unsupported and/or convex terrain features.
  • Avalanche hazard may have improved, but be mindful that deep instabilities are still present.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.