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Avalanche Forecast

Archived

Dec 21st, 2018–Dec 22nd, 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 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.

Avalanche activity has slowed down, but the snowpack remains generally weak. Choose conservative terrain and pay close attention to signs of instability such as whumphing and cracking in the snow.

Confidence

Moderate - Due to the number of field observations.

Weather Forecast

FRIDAY NIGHT - cloudy with clear periods / west winds, 30-40 km/h / alpine low temperature near -12°c. SATURDAY - Cloudy with sunny periods / southwest winds, 15-35 km/h / alpine high temperature near -9°c. SUNDAY - Cloudy with isolated flurries / southwest winds, 40-60 km/h / alpine high temperature near -7°c,  low temperature near -9°c. MONDAY - Cloudy with isolated  flurries / light southwest winds / alpine high temperature near -5°c, low temperature near -7°c.

Avalanche Summary

Several natural and explosives triggered avalanches to size 2 were reported in the region on Friday. One size 2 explosives triggered avalanche was reported on a north aspect at 1800 m on Thursday. 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

60-90 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). Avalanche activity on this layer has slowed down, but in certain terrain features, it is likely that it would still be possible for humans to trigger avalanches on this layer. Features where the underlying ground surface is smooth, and areas where the snowpack varies from thin to thick would be the most likely places for humans to trigger an avalanche on this layer. 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.