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Avalanche Forecast

Archived

Dec 20th, 2019–Dec 21st, 2019

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

Regions

Lizard-Flathead.

Continued loading from snow and wind Friday night through Saturday will keep the avalanche danger at HIGH.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

Friday Night: Snow, accumulation 15-25 cm. Alpine temperature -3 C. Moderate to strong southwest wind. Freezing level 1600 m.

Saturday: Snow, accumulation 15-30 cm. Alpine temperature -2 C. moderate to strong southwest wind. Freezing level 1700 m.

Sunday: Flurries, 5-10 cm. Alpine temperature -3 C. Moderate southwest wind. Freezing level 1000 m.

Monday: Mix of sun and cloud. Alpine temperature -4 C. light southwest wind. Freezing level 1000 m.

Avalanche Summary

Expect a natural avalanche cycle to be ongoing through Saturday as slabs build due to heavy loading from snowfall, wind and warm temperatures. On Friday there were reports of widespread avalanche activity in the recent storm snow.

On Wednesday, two large natural avalanches in the Morrissey area were reported to have failed on a crust from mid-November. A cornice failure triggered a size 2.5 avalanche, and one size 2 slab avalanche failed naturally, both avalanches had crown depths 70-100 cm, were on east-northeast aspects, and suspected to have been triggered by strong winds.

Snowpack Summary

Snowfall at upper elevations has covered a previously variable and wind-affected surface and is expected to build and develop reactive storm slabs. Expect the surface snow at lower elevations to be moist due to rain.

Crust layers from November and October can be found 40-100 cm below the surface. These layers produced large avalanches with explosive triggers around Dec 13-14, but since then have appeared to gain strength, but may become overloaded with incoming snow.

Snowpack depths range between 60-130 cm at higher elevations and taper rapidly below treeline.

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow and wind.
  • Avoid exposure to overhead avalanche terrain, avalanches may run surprisingly far.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.

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.

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.