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Avalanche Forecast

Archived

Feb 18th, 2017–Feb 19th, 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.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

South Rockies.

Small amounts of new snow will form fresh wind slabs. The lingering possibility of deep persistent slab avalanches warrants extra caution around big open slopes.

Confidence

Moderate - Intensity of incoming weather systems is uncertain on Sunday

Weather Forecast

SATURDAY NIGHT: Flurries with about 5 cm of new snow in western parts of the region and 2 cm in eastern parts, moderate southwest winds, alpine temperatures around -5 C.SUNDAY: Cloudy with isolated flurries, moderate southwest winds, freezing level around 1500 m.MONDAY: Cloudy, strong southwest winds, freezing level around 1400 m.TUESDAY: Flurries with 5-10 cm of new snow, strong southwest winds, freezing level around 1700 m.

Avalanche Summary

Recent avalanche observations are limited. Earlier in the week, there were isolated reports of size 2 natural deep persistent slab avalanches around Waterton and Elkford. A few size 1-2 loose wet avalanches were reported at lower elevations during Thursday's storm, but cooling temperatures have improved the stability of the old storm snow. On Sunday, watch for fresh 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 large open slopes, especially in shallow snowpack areas.

Snowpack Summary

Expect light amounts of new snow on Sunday with moderate to strong southwest winds forming thin wind slabs in exposed terrain. The new snow sits above settled snow and hard rain crusts below 1900 m. The midpack consists of 50-100 cm of settled snow from last week's storm. In deep snowpack areas, the lower snowpack appears 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.