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Avalanche Forecast

Archived

Dec 18th, 2021–Dec 19th, 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 unlikely, human triggered possible.
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.

Regions

South Rockies.

Strong winds will likely have created hard wind slabs in the alpine and treeline. Seek out sheltered terrain where you can avoid wind slabs, find softer snow and enjoy better riding.

Confidence

Moderate - Uncertainty is due to the extreme variability of wind effect on the snowpack.

Weather Forecast

Saturday Overnight: Extreme southwest winds will ease to moderate overnight and shift to the west. 5-15 cm of new snow can be expected with alpine temperatures dropping to around -12 C.

Sunday: Partially cloudy with light flurries, trace to 3 cm accumulation. Light to moderate ridgetop winds from the west. Alpine temperatures around -10 C, lowering into the evening.

Monday: Mainly cloudy with flurries. Strengthening westerly winds at ridgetop, strong by the afternoon. Alpine temperatures around -12 C.

Tuesday: Partially cloudy with strong to extreme southwest winds. Light flurries, trace accumulation. Alpine temperatures rising throughout the day to around -6 C.

Avalanche Summary

On Saturday, operators observed cracking, whoomphing, and skier triggered wind slabs up to size 1.

On Friday, the South Rockies field team was out in the Window mountain area. They observed several natural wind slab avalanches in the alpine up to size 1.5. Read their full report here.

Earlier in the week, operators have reported several large (size 2) explosive-triggered avalanche releasing on the early December crust layer. 

Snowpack Summary

Yesterday and overnight the region received 10-20 cm of new snow. Strong to extreme southwest will have redistributed this new snow into wind slabs in the alpine and treeline. 

Below this new snow, more consolidated snow from the previous weekend storm sits over a substantial crust that formed in early December. This crust is likely 20 cm thick (or more) and is present across aspects below 2400m. A thin layer of weak facets (sugary snow) can found above this crust that has demonstrated reactivity in snowpack tests and avalanche activity. 

Snowpack depths vary due to strong to extreme southwest winds from early December that stripped snow off of exposed areas and deposited it onto lee slopes. Below 2300m, several early season crusts are breaking down and forming a cohesive lower snowpack. Snowpack depths range from 60-110 cm at treeline elevations and taper quickly below 1900m.

Terrain and Travel

  • Avoid freshly wind loaded terrain features.
  • Avoid shallow snowpack areas, rock outcroppings and steep convex terrain where triggering is most likely.

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.