Avalanche Forecast

Archived

Dec 16th, 2018–Dec 17th, 2018

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

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.

The potential to trigger large persistent slab avalanches warrants conservative terrain choices.

Confidence

Moderate - Intensity of incoming weather systems is uncertain

Weather Forecast

SUNDAY NIGHT: A weak front passes overnight bringing 3-10 cm of snow, strong wind from the southwest, alpine temperatures around -2 C.MONDAY: Flurries easing off by the morning before the next storm arrives in the evening, moderate wind from the southwest with strong gusts, freezing level around 1400 m, alpine high temperatures around -1 C.TUESDAY: 15-30 cm of snow, strong wind from the southwest, freezing level around 1500 m, alpine temperatures possible reaching 0 C.WEDNESDAY: Another 5-10 cm of snow, strong wind from the southwest, freezing level dropping to 1300 m, alpine temperatures around -3 C.

Avalanche Summary

On Saturday, a group of skiers remotely triggered a large (size 2) avalanche on a northeast facing slope at treeline in the Corbin area. The skiers were in dense trees and the avalanche released roughly 60 m above them. One skier was fully buried and the group successfully extricated them without significant injuries. For full MIN incident report follow this link.Some recent large (size 2-3) natural avalanches were also reported at treeline elevations in the Harvey area. Most of them were 30-40 cm deep storm slabs, but a few appeared to step down to deeper crust layers. See photos in this MIN report. Smaller wind slabs (size 1) were occurring naturally and reactive to skiers throughout the region on Saturday.

Snowpack Summary

Strong wind has affected most open terrain and formed fresh wind slabs at higher elevations.Roughly 30-50 cm of recent storm snow sits above a weak layer composed of large surface hoar, facets, and/or sun crust. There have been numerous signs of the new snow bonding poorly to this layer including remote triggering from low angle terrain, shooting cracks, and wide propagations in avalanches.Several other weak layers have been observed in the lower snowpack such as early season crusts with weak facets. The most concerning crust is prevalent at higher elevations and is likely most problematic on north-facing features, especially those that are large and planar in nature.

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.