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Avalanche Forecast

Archived

Mar 24th, 2017–Mar 25th, 2017

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

Regions

South Rockies.

Watch out for fresh wind slabs at upper elevations. Avoid lingering in the runout of avalanche paths as wind loading and cornice fall could trigger large avalanches.

Confidence

Low - Timing, track, or intensity of incoming weather system is uncertain

Weather Forecast

There is significant uncertainty regarding precipitation amounts for Friday into Saturday. Unsettled weather and isolated flurries Sunday onwards. SATURDAY: 5-15cm possible Friday overnight with strong southwest winds. Light flurries easing off throughout the day on Saturday. Freezing level 1600m, alpine temperature around -4 C. SUNDAY: Sunny with some cloudy breaks, moderate southwest wind, freezing level 1700m. MONDAY: Isolated flurries with 2-4 cm possible. Moderate southwest wind, freezing level around 1900 m with alpine temperature around -3 C.

Avalanche Summary

On Thursday, several small Size 1 wind slabs were ski cut on north through east aspects between 1700 and 2200m. Evidence of a large natural avalanche cycle during last week's storm has been reported around the Crowsnest Pass / Sparwood area as well as around Waterton (typically size 2.5 deep persistent slabs). A fresher size 3 avalanche in the Alexander Creek drainage was possibly triggered by a cornice fall on Sunday. Widespread natural activity in the neighboring Kananaskis region suggest conditions may be touchier north of Elkford.Looking ahead, the primary concern is the potential for large deep persistent slab avalanches triggered by natural triggers such as a cornice fall. See the forecaster blog for advice on managing a low probability - high consequence problem (here) and the accompanying photos (here).

Snowpack Summary

Over the past two days we've had 5-15 cm of heavy snow at higher elevations, which sits on a thick hard crust that is quite solid above 1700m. In some locations winds have been strong from the southwest, creating wind slabs on northerly aspects, and isolated features at treeline. Rain has soaked the snow below about 2000 m. Recent rain and sun have left a variety of crusts that are thicker and most supportive in open south facing terrain. Below the new snow and crusts, roughly 50-90 cm of snow sits above the February crust and facet interfaces. In some areas there may still be a poor bond to these interfaces. The bottom third of the snowpack is composed of weak facets and full-depth avalanches over this basal weakness remain a concern.

Problems

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.

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.