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Avalanche Forecast

Archived

Dec 17th, 2018–Dec 18th, 2018

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

Regions

Lizard-Flathead.

HIGH danger ratings are based on the assumption the regions gets over 25 cm of snow by Tuesday afternoon.

Confidence

Moderate - Intensity of incoming weather systems is uncertain

Weather Forecast

MONDAY NIGHT: 10-15 cm of snow as a front approaches, strong wind from the southwest, freezing level around 1300 m.TUESDAY: Heavy snowfall with accumulations of 15-20 cm, strong wind from the southwest, freezing level up to 1600 m.WEDNESDAY: Flurries easing throughout the day with localized accumulations of 5 cm, moderate wind from the southwest with strong gusts, freezing level drops to 1200 m, alpine high temperatures around -2 C.THURSDAY: The next system arrives late in the day, moderate wind from the west, alpine high temperatures around -2 C.

Avalanche Summary

Small loose dry avalanches (size 1) were observed in alpine terrain on Monday.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.Large (size 2-3) natural avalanches were also reported at treeline elevations in the Harvey area on Saturday. 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.

Snowpack Summary

Tuesday's storm will build fresh storm slabs, especially at higher elevations where strong wind will form extra thick slabs.The new snow is falling on moist snow below 1600 m and hard wind packed snow in the alpine. Roughly 30-50 cm of snow is settling above a weak layer composed of large surface hoar, facets, and/or sun crust. There have been numerous signs over the past few days that this layer remains weak (i.e. remote triggering from low angle terrain, wide propagations in avalanches), and the layer could become extra reactive with the additional load of the new snow on Tuesday.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

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.