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Avalanche Forecast

Archived

Dec 19th, 2019–Dec 20th, 2019

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

Regions

Lizard-Flathead.

New snow (and maybe rain at lower elevations), gusty winds, and rising temperatures are priming the snowpack for a natural avalanche cycle. Hazard will rise through Friday, peaking later in the day.

Confidence

Moderate - We are confident the likelihood of avalanche will increase with the arrival of the forecast weather. Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

Thursday Night: Snow, 10-20 cm. Alpine temperature -7 C. Southwest wind 35 gusting to 65 km/hr.

Friday: Snow, 15-40 cm. Alpine temperature -3 C. Southwest wind 20-40 gusting to 65 km/hr. Freezing level rising to 1600 m, potentially producing wet snow and rain at lower elevations.

Saturday: Continued snow and flurries, 10-30 cm. Alpine temperature -1 C. Southwest wind 30-40 gusting to 75 km/hr. Freezing level 1500 m.

Sunday: Flurries, 5-15 cm. Alpine temperature -6 C. Southwest wind 20 gusting to 60 km/hr. Freezing level dropping to valley bottom.

Avalanche Summary

Natural wind slab avalanches (size 1.5) were reported Wednesday morning, with crowns 50-60 cm deep, these avalanches were suspected to have failed overnight Tuesday as strong winds reached 90 km/hr.

On Wednesday, two large natural avalanches in the Morrissey area were reported to have failed on a deeper instability, 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. This layer has been trending towards unreactive recently, but this report suggest that triggering is still possible with large loads or in areas with a thin and variable snowpack. 

Explosives testing was able to trigger a handful of small (size 1) slab avalanches on Tuesday from steep alpine terrain.

Snowpack Summary

Prior to this weather system, wind had impacted and redistributed loose snow, and formed wind slabs in exposed alpine terrain and around ridge features. In sheltered areas, 20-40 cm of recent snow was gradually settling. New snow will fall on a variable and wind-affected surface.

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.

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.