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Avalanche Forecast

Archived

Dec 10th, 2019–Dec 11th, 2019

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

Regions

Lizard-Flathead.

Resist the temptation to travel into steep or complex terrain, triggering large avalanches on buried weak layers is a concern right now.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

TUESDAY NIGHT: Cloudy with some isolated flurries, 40 km/h wind from the southwest, alpine temperatures drop to -8 C.

WEDNESDAY: Cloudy with isolated flurries and up to 5 cm of new snow, 40 km/h wind from the west, alpine high temperatures around -5 C.

THURSDAY: 5-10 cm of new snow, 50 km/h wind from the west, alpine high temperatures around -2 C.

FRIDAY: Mostly cloudy, 30 km/h wind from the west, alpine high temperatures around -2 C.

Avalanche Summary

Numerous recent storm slab and persistent slab avalanches have been reported. Explosive control in the Lizard Range over the weekend produced large slab avalanches (size 2-3) at treeline and alpine elevations. The avalanches ran on the both the November and October crust layers 40-100 cm deep. Evidence of natural avalanches with wide propagation have also been reported from the Flathead area.

Fewer avalanches have been reported since the weekend, but the potential for triggering large persistent slab avalanches remains a concern.

Snowpack Summary

30-40 cm of snow from the weekend has been redistributed by the wind to form wind slabs in the alpine and near ridgetops at treeline. Crust layers from November and October can be found 40-100 cm below the surface and have recently produced large avalanches with explosive triggers. While these layers are likely widespread across the terrain, we are uncertain about whether they could also be triggered by humans. Snowpack depths range between 50-100 cm at higher elevations and taper rapidly below treeline.

Problems

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.

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.