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Avalanche Forecast

Archived

Dec 20th, 2021–Dec 21st, 2021

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

Wicked winds have returned! Northwest winds may form wind slabs in areas that are usually more windward, which could catch riders off guard. 

Seek out sheltered terrain where you can avoid these wind slabs and better riding.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast. Uncertainty is due to the extreme variability of wind effect on the snowpack.

Weather Forecast

Monday Overnight: Winds increasing overnight, strong from the northwest at ridgetop. Partially cloudy with the chance of very light flurries, trace accumulation. Alpine temperatures around -15 C.

Tuesday: A mix of sun and cloud. Strong to extreme west winds, increasing in the afternoon. Temperatures rising to -8 C in the alpine. Snowfall beginning over night with trace to 5cm of accumulation. 

Wednesday: A stormy day. Strong to extreme southwest winds rage on with 5-15 cm of new snow accumulation. Freezing levels rising to around 1000m. Continued snowfall overnight with another 5-15 cm of accumulation.

Thursday: Snowfall continuing into the morning, 5-15 cm of accumulation. Easing into the afternoon with winds backing off moderate to strong southwest at ridgetop. Freezing levels dropping to valley bottom with possible clearing in the afternoon. 

Avalanche Summary

On Monday, operators reported one size 2.5 persistent slab avalanche triggered with explosives on a south east aspect at treeline. They also ski cut isolated pockets of wind slab up to size 1. 

On Sunday, operators reported isolated size 1 wind slabs with explosive control. They reported whoomphing in previously unskied/uncontrolled terrain.

Snowpack Summary

Saturday's 10-30cm of storm snow will continue to be redistributed by west winds throughout the day. In exposed areas, strong to extreme winds will strip many areas back down to the early December curst and create deposits of hard wind slab in lees. This wind loading may be lower down on slopes than you may expect as a result of the high wind values.

Below this new snow, a layer of more consolidated snow sits over a substantial crust that formed in early December. This crust is up to 20cm thick and is present across all aspects below 2400m. Up to 10cm of faceting has been reported above this crust. This problem is particularly hard to predict and tricky to manage. For this reason wide, conservative terrain margins and disciplined backcountry travel techniques will be very important. Get more details and photos in our forecaster blog. 

Snowpack depths vary due to strong to extreme southwest winds that stripped snow off of exposed areas and deposited it onto lee slopes. Below 2300m, 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

  • Be careful as you transition into wind affected terrain.
  • Look for signs of instability: whumphing, hollow sounds, shooting cracks, and recent avalanches.
  • 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.