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Avalanche Forecast

Archived

Feb 17th, 2017–Feb 18th, 2017

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

Regions

South Rockies.

Danger will remain elevated throughout the weekend. The lingering possibility of deep persistent slab avalanches warrants extra caution around big open slopes.

Confidence

Moderate - Timing, track, or intensity of incoming weather system is uncertain on Sunday

Weather Forecast

SATURDAY: Cloudy with light flurries starting in the afternoon and 5-10 cm of new snow overnight, light to moderate south winds, freezing level around 1300 m.SUNDAY: Cloudy with isolated flurries, moderate southwest winds, freezing level around 1300 m.MONDAY: Storm starting midday with up to 5 cm of new snow by the afternoon, strong southwest winds, freezing level around 1700 m.

Avalanche Summary

Recent avalanche reports in the region include size 1-2 loose wet avalanches in rain soaked snow at lower elevations. On Thursday, a small slab in Waterton stepped down in a shallow snowpack area below a cliff to release a size 2 deep persistent slab avalanche. Earlier in the week, a few size 2 solar-triggered persistent slab avalanches were observed in the Elkford area.For the weekend, watch for reactive wind slabs at higher elevations. Also keep in mind that the deep persistent slab problem is a low probability/high consequence scenario that warrants extra caution around and below large open slopes.

Snowpack Summary

5-20 cm of new wind affected snow can be found above 1800 m, while lower elevations will have a combination of moist snow and rain crusts as freezing levels fluctuate. Moderate southwest winds have redistributed the new snow into reactive wind slabs in exposed terrain. Beneath the new snow, you'll find a mix of hard old wind slabs, melt-freeze crusts, and moist snow from the recent warm spell. The midpack consists of 50-100 cm of settled snow from last weeks storm. In deep snowpack areas, the lower snowpack appear to be well settled with only isolated concerns about the mid-December facet layer which comprises the bottom third of the snowpack. In shallow snowpack areas, this layer is weak and faceted. Watch this video from the South Rockies field team for some recent test results on this layer.

Problems

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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.