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Avalanche Forecast

Archived

Dec 20th, 2021–Dec 21st, 2021

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

Regions

Lizard-Flathead.

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 find great riding.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

Monday Overnight: Partially cloudy with the chance of light flurries, trace accumulation. Light to moderate west winds at ridgetop. Alpine temperatures near -15 C.

Tuesday: A mix of sun and cloud, increasing cloud cover in the afternoon. Moderate to strong west winds at ridgetop, increasing in the afternoon. Snowfall beginning overnight with 5-15 cm of accumulation. Temperatures rising to -7 C in the alpine by morning.

Wednesday: A stormy day. Moderate to strong southwest winds will accompany steady snowfall. There is the potential for locally heavy accumulations of upwards of 30 cm throughout the day. Freezing level rising to 1000m in the afternoon. Continued snowfall overnight with another 10-40 cm of accumulation.

Thursday: Continued snowfall, another 10-30cm of accumulation heaviest in the morning. Moderate to strong southwest winds easing into the afternoon to light to moderate. Freezing levels dropping in the evening to valley bottom.

Avalanche Summary

On Monday, operators ski cut storm slab and loose dry avalanches up to size 1. 

On Sunday, operators reported a natural avalanche cycle from overnight, with avalanches up to size 3 running almost full path. Explosive control work produced size 1-2.5 storm slabs in the alpine and treeline. 

On Sunday, one notable ski cutting result produced a size 1.5 persistent slab avalanche at treeline on the early December crust.

Snowpack Summary

An intense storm came through the region on Saturday, bringing 40-70 cm of new snow. Overnight, northwest winds will have redistributed this new snow into wind slabs in the alpine and treeline. The northerly component to winds will potentially creating loading in more atypical areas at higher elevations. Throughout the day, winds will shift southwest and continue to transport available snow into fresh wind slabs.

Below this new snow, 50-100 cm of more consolidated snow sits over a substantial crust that formed in early December. This crust is up to 20 cm thick and is present across all aspects below 2400m. A layer of weak facets (sugary snow) can found 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. 

The lower snowpack consists of a variety of early season crusts and mainly moist snow. Snowpack depths range from 100-200 cm at treeline elevations and taper quickly below 1800 m.

Terrain and Travel

  • Be careful as you transition into wind affected terrain.
  • Be aware of highly variable recent wind loading patterns.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

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.