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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, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Lizard-Flathead.

Human triggered avalanches are still possible, especially in deep wind-loaded pockets that overly a crust. Be ready to step back from sun exposed slopes if the sun is strong in the afternoon.

Confidence

High -

Weather Forecast

Monday night: Scattered clouds with isolated flurries. Light to moderate northwest winds. Freezing level valley bottom.

Tuesday: Mix of sun and cloud. Light northwest winds. Freezing level 500 m.

Wednesday: Sunny. Light northwest winds. Freezing level 700 m.

Thursday: Sunny. Light southwest winds. Freezing level 1000 m.

Avalanche Summary

Natural activity over the last few days has been limited to small (mostly size 1) loose dry and thin slab avalanches out of steep headwalls, as well as a size 1.5 cornice triggered slab. A few skier triggered slabs have been reported, including this great MIN report of a skier triggered size 2 avalanche from an east/southeast facing feature at 2000 m on Sunday. Explosive control work conducted on Sunday and Monday produced slab and loose snow avalanches up to size 1.5. 

Snowpack Summary

20-30 cm of snow from the past week has seen some redistribution in exposed alpine areas, with soft wind slabs forming in immediate lees of ridge crests. In sheltered areas, the recent snow is settling and may contain various sun crusts and layers of small surface hoar crystals. 

A thick rain crust sits 30-60 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. A weak layer sitting between a dense slab and a slick bed surface is a recipe for a future persistent avalanche problem.

The bottom 20 cm of the snowpack consists of basal facets and decomposing crusts that have not been an active avalanche problem since December, but could reemerge as a problem after sustained warming.

Terrain and Travel

  • Don't be too cavalier with decision making, storm slabs may remain sensitive to human triggering.
  • Extra caution is needed around cornices under the current conditions.
  • Brief periods of sun could quickly initiate natural avalanche activity.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.