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Avalanche Forecast

Archived

Dec 8th, 2019–Dec 9th, 2019

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

There is uncertainty about whether the new snow has increased the potential for triggering large avalanches on buried weak layers. This uncertainty is best managed with conservative terrain choices.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

SUNDAY NIGHT: Clear with cloudy periods, 20-30 km/h wind from the northwest, alpine temperatures drop to -10 C.

MONDAY: Mix of sun and cloud, 20-30 km/h wind from the northwest, alpine high temperatures around -6 C.

TUESDAY: Mix of sun and cloud, 20-30 km/h wind from the northwest, alpine high temperatures around -6 C.

WEDNESDAY: Mix of sun and cloud, 30-50 km/h wind from the northwest, alpine high temperatures around -4 C.

Avalanche Summary

Numerous storm slab and persistent slab avalanches were reported during the stormy weather on Saturday. Explosive control in the Lizard Range produced numerous large slab avalanches (size 2-2.5) at treeline and alpine elevations. The avalanches ran on the mid-November crust layer 40-80 cm deep. Smaller (size 1) skier triggered slabs were also reported, including this MIN report from Harvey Pass. Preliminary reports from Sunday suggest more large persistent slab avalanches (size 3) were triggered with explosives.

While avalanche activity likely peaked during the storm, triggering storm slab and persistent slab avalanches remains a concern as the snowpack adjusts to the weight of the new snow.

Snowpack Summary

30-40 cm of fresh snow is settling into a storm slab at upper elevations. Crust layers from November and October can be found 40-80 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

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.