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Avalanche Forecast

Archived

Dec 18th, 2019–Dec 19th, 2019

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

Regions

Lizard-Flathead.

Triggering an avalanche is most likely in wind affected terrain and steep, rocky slopes.

Confidence

Moderate - Uncertainty is due to the timing of the incoming weather system.

Weather Forecast

Wednesday Night: Mostly cloudy. Alpine temperature -8 C. Southwest wind 25-35 km/hr.

Thursday: Cloudy with isolated flurries, trace to 10 cm. Alpine temperature -5 C. Southwest wind 25-45 km/hr.

Friday: Snow, 20-40 cm. Alpine temperature -1 C. Southwest wind 20-40 gusting to 65 km/hr. Freezing level rising to 1900 m.

Saturday: Continued flurries, 5-15 cm. Alpine temperature -2 C. Southwest wind 35 gusting to 75 km/hr. Freezing level 1600 m.

Avalanche Summary

Two natural wind slab avalanches were reported Wednesday morning, these avalanches were suspected to have failed overnight as strong winds reached 90 km/hr.

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

Snowpack Summary

Wind has redistributed loose snow, and formed wind slabs in alpine terrain and around ridge features. In sheltered areas 20-40 cm of loose snow is gradually settling.

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. Large avalanches on these layers may still be possible to trigger in steep rocky terrain.

Snowpack depths range between 60-120 cm at higher elevations and taper rapidly below treeline. 

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.

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.