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Avalanche Forecast

Archived

Dec 23rd, 2021–Dec 24th, 2021

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 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.

Regions

South Rockies.

Touchy wind slabs will likely exist on Friday. Be conservative with your terrain choices and watch for signs of instability like natural avalanches, cracking and hollow sounding snow in the upper snowpack.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

Light snow and flurries will continue in most areas as a cold northeasterly flow begins to assert itself at the surface. This will bring persistent cloudiness and continued light snowfall amounts through the Christmas weekend.

Thursday Night: New snow 5-10 cm accompanied by moderate southwest wind. Treeline temperatures -8 and freezing level at the valley bottom.

Friday: New snow 5-10 cm with strong southerly winds. Treeline temperatures near -10 and freezing levels near 800 m. Temperatures continue to drop overnight.

Saturday: Cloudy with 5-15 cm accompanied by moderate southwest wind. Treeline temperatures near -20 and freezing levels at the valley bottom.

Sunday: Cloudy with some flurries up to 5 cm. Light winds from the southwest and colder temperatures at -25. 

Avalanche Summary

No new avalanches were reported on Thursday afternoon. 

On Wednesday, the South Rockies field team reported several wind slabs that appeared to have slid on the early December crust and up to size 2. 

Natural avalanche activity may taper on Friday but wind slabs could be primed for human triggering. 

Snowpack Summary

15-25 cm of recent storm snow accompanied by strong to extreme winds have likely formed wind slabs at most elevations in open terrain. 

Wind exposed areas have likely been stripped back down to the early December crust and old wind slabs. A lot of variability exists.

Below the surface exists a well-consolidated upper snowpack that overlies a substantial crust that formed in early December. This crust is approximately 50-90 cm below the surface, 20 cm thick on average, and is present across all aspects to at least 2400 m. A layer of weak facets (sugary snow) has been reported above this crust. Snowpack tests are generally showing hard results on this interface, however, areas, where avalanches have recently failed on this crust, continue to be problematic once reloaded with new snow. 

Below 2300 m, several early-season crusts make up the lower snowpack. Snowpack depths range from 80-150 cm at treeline elevations and taper quickly below 1900m.

Terrain and Travel

  • Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
  • Fresh wind slabs will likely form throughout the day, diligently watch for changing conditions.
  • Start with conservative lines and watch for clues of instability.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.

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.