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Avalanche Forecast

Archived

Dec 28th, 2018–Dec 29th, 2018

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

Regions

Lizard-Flathead.

Rising freezing levels and increased wind will accompany snowfall. If more than 20 cm snowfall accumulates, the avalanche hazard will rise to CONSIDERABLE.

Confidence

Low - Freezing levels are uncertain

Weather Forecast

FRIDAY NIGHT: Cloudy with isolated flurries, up to 10 cm accumulation. Light to moderate southwest wind. Alpine low of -12C.SATURDAY: Flurries, 5-15 cm accumulation. Moderate southwest wind gusting strong to extreme. Alpine high -4, freezing level rising above 1400 m.SUNDAY: Mix of sun and cloud with scattered flurries, trace to 5 cm accumulation. Light northwest wind gusting moderate to strong. Freezing level dropping to valley bottom, alpine high -3C.MONDAY: Mix of sun and cloud. Light northwest wind with occasional moderate gusts. Alpine low -15C.

Avalanche Summary

Small (size 1) wind slabs have been reactive to skier traffic in the region.A great MIN report from Corbin on Thursday identifies a few slabs initiating in thin, rocky areas on a south aspect. Check out the report here.Over the last weekend, the persistent weak layer was active, explosives triggered avalanches to size 2.5 and a cornice triggered a size 2 avalanche. Skier traffic was also able to trigger wind slab avalanches to size 2. Though avalanche activity on this layer has slowed, avalanches failing on this layer have the potential to be large and destructive.

Snowpack Summary

Around 20 cm fresh snowfall accumulated through the week has been redistributed around lee areas at treeline and alpine elevations. In total, 60-110 cm of snowfall through December has formed a slab that sits on a persistent weak layer that formed in early December. This layer mostly consists of facets (sugary snow) with some isolated areas also containing small surface hoar (feathery crystals). This overlies several other weak layers observed in the lower snowpack such as crusts and facets that formed in late October/early November. With this weak basal snowpack, it is likely that an avalanche triggered on the persistent slab would step down to these lower layers, resulting in a full depth avalanche.

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.