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Avalanche Forecast

Archived

Dec 13th, 2021–Dec 14th, 2021

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

Regions

South Rockies.

Choose smaller features, and avoid slopes that steepen as they descend. Adding a rider's weight to the recent storm snow may start a large avalanche that slides on a hard crust.

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

Some significant snowfall potential overnight. Cloud and intermittent light flurries keep hanging around for the next few days.

MONDAY NIGHT: Overcast. 6-15 cm of snow expected. Light southeast winds at treeline and strong south or southwest in the alpine. Freezing level around 500m. 

TUESDAY: Mostly cloudy potential clearing in the afternoon. 0-5 cm of snow expected. Light to moderate southwest winds. Freezing level rising to around 750m.

WEDNESDAY: Partly cloudy. 0-2 cm. Light to moderate southwest winds, up to strong on higher peaks. Warming through the day, alpine high around -10.

THURSDAY: Brief clearing overnight, partly cloudy through the day. Possible trace of snow. Light to moderate southwest winds, with strong to extreme westerlies on higher peaks. Alpine high around -8, 

Avalanche Summary

This MIN Report paints a good picture of the avalanche problem right now. They spotted a natural avalanche in a wind loaded feature, and whumpfing in shallow areas.

Reports indicated that the snowpack was quite touchy this weekend. Avalanche control with explosives and ski cuts produced several size 2 avalanches, and one size 3. Most failed on top of the rain crust from the late November/early December rain events.

Snowpack Summary

A refresh of snow overnight keeps the snowpack in a state of change. Mostly light winds are forecast through the storm, so the new snow should be relatively unconsolidated. Where this new snow has been wind transported, (most likely at higher elevations) it could form reactive slabs. 

Moderate to strong southwest winds during the last storm formed wind slabs that are still taking time to bond to the rest of the snowpack. These slabs are now buried by the new snow. While that makes them harder to find, it does not mean they are less likely to avalanche.

Most of the recent avalanche observations have run on the early December rain crust. We suspect that weak, facetted snow has begun to form on top of the crust. 

Snowpack depths are extremely variable due to strong to extreme southwest winds from early December that stripped snow off of exposed areas, and made deep drifts in lee slopes.

Below 2300m, several early season crusts are breaking down and forming a cohesive lower snowpack.

Snowpack depths range from 50-110 cm at treeline elevations. The snowpack tapers quickly below 1900m.

Terrain and Travel

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Choose simple, low-angle, well supported terrain without convexities.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.

Problems

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.

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.