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Avalanche Forecast

Archived

Dec 27th, 2018–Dec 28th, 2018

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

Persistent slabs are tricky to manage. Avalanche activity is receding, but if triggered, an avalanche falling on the persistent weak layer could have serious consequences. Read the latest Forecaster Blog on the persistent slab problem here.

Confidence

Low - Due to the number of field observations

Weather Forecast

THURSDAY NIGHT: Mix of sun and cloud. Light west wind. Alpine low of -13C.FRIDAY: Mainly cloudy with isolated flurries, trace accumulation. Light to moderate west-southwest wind. Alpine high of -11C.SATURDAY: Flurries, 5-15 cm accumulation. Moderate southwest wind with strong gusts. Freezing level rising to 1500 m.SUNDAY: Mainly cloudy with isolated flurries, trace accumulation. Light to moderate west-southwest wind. Alpine high of -4C.

Avalanche Summary

The persistent weak layer was active last weekend, explosives triggered avalanches to size 2.5 and a cornice triggered a size 2 avalanche. On Sunday, wind redistributing new snow resulted in numerous natural wind slabs avalanches (size 1) noted in the region.

Snowpack Summary

Up to 20 cm of fresh snow is being redistributed around lee areas at treeline and alpine elevations. A combined total of 60-110 cm of recent snow 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). Several other weak layers have been 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.