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Avalanche Forecast

Archived

Dec 19th, 2020–Dec 20th, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
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

Purcells.

Wind slabs continue to produce avalanches that are most problematic along ridge lines and in lee terrain features. New snow and wind Saturday night into Sunday will likely exacerbate this problem. Best to put together a travel plan that avoids wind exposed terrain.

Confidence

Moderate - Recent weather patterns have resulted in a high degree of snowpack variability within the region.

Weather Forecast

The storm parade continues for the foreseeable future.

SATURDAY NIGHT: Freezing level around 1000 m, strong southwest wind, 5 to 10 cm of snow expected.

SUNDAY: Broken cloud cover, freezing level around 1500 m, strong west/southwest wind, 1 to 5 cm expected through the day.

MONDAY: Overcast, freezing level around 700 m, light south wind with moderate gusts at ridgetop, 10 to 20 cm of snow possible.

TUESDAY: Broken cloud cover, freezing level near valley bottom, light to moderate northerly wind at ridgetop, potential for a trace of snow.

Avalanche Summary

On Friday and Saturday the buried surface hoar continued to show it's stuff making for touchy avalanche conditions which produced slabs to size 1.5, check out this great MIN report from the north of the region Saturday.

Natural wind slabs and cornice failures were reported on alpine features in the central portion of the region to size 1.5. 

In the south of the region the snowpack is thin and there have reports of whumphing at ridgetop. Check out all of the MIN reports for the region here

On Thursday in the Kicking Horse backcountry, small (size 1-1.5) wind slabs 15-25 cm deep were reactive to skiers, and were found in predictable wind loaded areas.

This MIN report from the KHMR backcountry documents poor bonding between the new snow and old surface resulting in sluffing and loose, dry avalanches, and small windslabs in steep rocky terrain. 

A MIN report from the Clamshell near Golden reported a natural size 2.5 avalanche last Wednesday (Dec 9). This avalanche appears to have run on a weak layer roughly 40 cm below the surface, which seems to be an isolated problem in this area.

Snowpack Summary

Incremental loading has stacked 15-30 cm loose snow around the region (with the higher amount to the north around Golden). Southwest winds have built slabs in lee features at upper elevations, reactive cornices have grown too. 

This recent snow overlies a crust up to 2400 m in the south of the region and 1800 m in the north. Around treeline, where there crust isn't found, the recent snow may sit over a weak layer of surface hoar or facets. Reports on this weak layer come from the Golden area (MIN report), finding weak surface hoar in isolated locations, 30-40 cm below the surface. Keep this layer in mind a forecasted snow (on Monday) accumulates, a touchy weak layer could emerge.

The lower snowpack is characterized by a couple of crusts, the most notable is from a rain event in early November that is 50 to 100 cm deep. This crust is sitting near the base of the snowpack, surrounded by a weak layer of sugary facets. It is most likely to be reactive to human triggers or in a step-down from a smaller avalanche in shallow, rocky areas.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Be careful with wind slabs, especially in steep, unsupported and/or convex terrain features.
  • In areas where deep persistent slabs may exist, avoid shallow or variable depth snowpacks and unsupported terrain features.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.