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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

Lizard-Flathead.

Storm slabs will likely be touchy to human triggering on Friday. Especially on wind-loaded leeward slopes. 

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 -15 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 -15. 

Avalanche Summary

On Thursday, numerous storm slabs were triggered by machines and explosives. The majority of them were size 2 with one size 3 reported. 

Natural avalanche activity may taper on Friday but slab avalanches likely remain primed for human triggering.

Snowpack Summary

30 to 50 cm of recent storm snow sits above a surface hoar interface and pockets of old wind slab. Low-density storm snow exists in wind-sheltered sheltered terrain.

Below the new snow exists a well-consolidated upper snowpack which overlies a substantial crust that formed in early December. This crust is approximately 90-150 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.

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

  • Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
  • Don't be too cavalier with decision making, storm slabs may remain sensitive to human triggering.
  • Start with conservative lines and watch for clues of instability.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.