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Avalanche Forecast

Archived

Dec 11th, 2019–Dec 12th, 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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Lizard-Flathead.

Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

WEDNESDAY NIGHT: Light flurries bring 5-10 cm of new snow, 50 km/h wind from the southwest, alpine temperatures around -6 C.

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

FRIDAY: 5-10 cm of new snow possible by the morning and then cloudy in the afternoon, 30 km/h wind from the west, alpine high temperatures around -4 C.

SATURDAY: Mostly cloudy, 30 km/h wind from the west, alpine high temperatures around -6 C.

Avalanche Summary

Recent avalanche activity is limited to a few small wind slab avalanches (size 1) triggered with explosives on Wednesday. Avalanche activity has seemed to quiet down since the weekend, when numerous large persistent slab avalanches (size 2-3) were reported at treeline and alpine elevations. Many of these avalanches ran on the November and October crust layers 40-100 cm deep. Triggering an avalanche on one of these deeper layers is still a possibility in steep rocky terrain.

Snowpack Summary

New snow and wind will likely form fresh wind slabs at upper elevations. In sheltered areas 30-40 cm of snow from last weekend is gradually settling. Crust layers from November and October can be found 40-100 cm below the surface and have recently produced large avalanches with explosive triggers, but the likelihood of human triggering is gradually reducing. Snowpack depths range between 50-100 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.