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Avalanche Forecast

Archived

Feb 17th, 2020–Feb 18th, 2020

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

Regions

South Rockies.

Be ready to step back from steep sun exposed slopes in the afternoon. Continue to avoid steep, rocky, and wind affected areas where triggering slabs is more likely.

Confidence

Moderate - Uncertainty is due to whether buried persistent weak layers become active, triggering avalanches, with the arrival of the forecast weather.

Weather Forecast

Monday night: Clear with cloudy periods and isolated flurries. Light to moderate northwest wins. Freezing level valley bottom.

Tuesday: Mix of sun and cloud. Light northwest wind. Freezing level 400 m.

Wednesday: Sunny. Light north wind. Freezing level 700 m.

Thursday: Sunny. Moderate west wind. Freezing level 1200 m.

Avalanche Summary

A natural size 2 persistent slab avalanche on the February rain crust was observed Monday. Explosive and skier control work on Sunday produced size 1.5-2 storm slabs, and size 1 loose dry. Solar triggered loose dry avalanches to size 1.5 were also observed in steep south facing terrain.

Reports have been trickling in of deep persistent avalanches on deep weaknesses near the bottom of the snowpack. The last was from one week ago, described in this MIN report from February 11. There was also a bigger avalanche reported in this MIN on February 9th (check out the photos, they're humbling). The very large avalanche ran on a north facing feature at 2400 m and was triggered by a snowmobile. 10 or so climbs had been made without incident in the same area in the days leading up to this avalanche. Activity on these layers often accompany abrupt changes in weather. We have our eye on the sun this week, forecast to come out in full force Wednesday.

Snowpack Summary

Up to 40 cm of snow (with highest amounts concentrated in the Castle area) from the past week has been redistributed into wind slabs in the alpine and exposed areas at treeline. 

A thick rain crust from early February sits 20-70 cm below the surface up to 2100 m. Recent reports indicate that faceting may be occurring at the interface of this crust and overlying snow in some areas, creating a weak layer between a slab and a slick bed surface. At least one avalanche was reported to have slid on this layer on Monday.

A well consolidated mid-pack overlies generally weak basal facets that may be possible to trigger in isolated shallow rocky start zones or with large loads. There is some uncertainty about whether the forecast sunny period will provide a strong enough input to resurrect this avalanche problem. 

Terrain and Travel

  • Be careful as you transition into wind affected terrain.
  • Caution around slopes that are exposed to cornices overhead.
  • Avoid exposure to steep, sun exposed slopes, especially when the solar radiation is strong.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.

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.