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Avalanche Forecast

Archived

Dec 20th, 2018–Dec 21st, 2018

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 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.

The storm and associated natural avalanches have tapered off to some degree, but give the snowpack some time to adjust to recent loading as the new sits on a weak layer that will likely need more time to gain strength.

Confidence

Moderate - Due to the number of field observations.

Weather Forecast

THURSDAY NIGHT: Cloudy with flurries, up to 10 cm, southwest winds, 60-90 km/h, alpine low temperature near -5°c, freezing level 2000m. FRIDAY: A mix of sun and cloud, west winds, 60-70 km/h, alpine high temperature near -7°c, freezing level 1200m and dropping to valley bottom by the evening. SATURDAY: Sunny with cloudy periods, southwest winds, 20-40 km/h, alpine high temperature near -5°c, low temperature near -8°c. SUNDAY: Cloudy with sunny periods and isolated flurries, southwest winds, 50-70 km/h, alpine high temperature near -5°c, low temperature near -8°c.

Avalanche Summary

One size 2 explosives triggered avalanche was reported on a north aspect at 1800 m on Thursday. No new natural avalanches were reported in the region. Numerous natural and explosives triggered avalanches to size 2.5 were reported in the region on Wednesday. Many of these were up to 100 cm deep. A notable avalanche on Wednesday was a size 3.5 explosives triggered avalanche in which the explosives were placed on a cornice. When the cornice failed, it triggered a 200 cm deep slab that failed on the crust at the base of the snowpack. This avalanche occurred on a northeast aspect at approximately 2000 m.

Snowpack Summary

50-80 cm of recent snow sits on a 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 rain crusts and facets that formed in late October/early November.

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.