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Avalanche Forecast

Archived

Nov 30th, 2021–Dec 1st, 2021

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

Regions

Lizard-Flathead.

Another wet storm will increase avalanche danger through the day Wednesday. Avoid all avalanche terrain including low elevation runout zones during this period of heavy loading from new snow, wind, and rain.

Confidence

High - We are confident the likelihood of avalanche will increase with the arrival of the forecast weather.

Weather Forecast

Another wet one.

Tuesday night: Flurries starting 5-10 cm. Strong southwest wind. Freezing level rising 1800 to 2500 m by morning.

Wednesday: Wet snow and rain, 20-40 mm. Strong to extreme southwest wind. Freezing level 2500 m.

Thursday: 10-20 mm rain turning to snow overnight then clearing. Moderate southwest wind. Freezing level dropping 2000 to 1700 m.

Friday: Mix of sun and cloud. Light winds. Freezing level valley bottom.

Avalanche Summary

No new avalanches were reported on Tuesday. A natural size 2 was observed in the alpine on Monday. Several natural storm slab avalanches were observed in the alpine on Sunday, a few notably stepping down to deeper layers (suspect the mid November crust).

A natural avalanche cycle occurred overnight Thursday-Friday, large storm slab avalanches (size 2-2.5) in alpine and upper treeline features were reported Friday morning. Through the day, explosives triggered large (size 2) storm slab avalanches.

Snowpack Summary

Overnight snow turning to rain soaks the dense upper snowpack at most elevations. Wet snow accumulations may form heavy storm slabs in the high alpine.

Below 40 cm of recent storm snow lies a thick mid-November rain crust which has been the suspected bed surface of a few recent step-down avalanches. Moist snow is still found below the crust and to the ground.

Snowpack depths range from 40-70 cm at treeline elevations. Expect to find a deeper snowpack at higher elevations and in wind-loaded areas. Snowpack depths decrease rapidly below 1500 m.

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.
  • Avoid areas with overhead hazard.
  • Avoid lingering or regrouping in runout zones.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.