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Avalanche Forecast

Archived

Jan 5th, 2018–Jan 6th, 2018

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

Regions

South Rockies.

Expect persistent slab avalanche activity to continue as the snow sitting above the persistent weak layer further settles and consolidates. Adopt a conservative approach in the coming days.

Confidence

Moderate - Due to the number of field observations

Weather Forecast

Occasional flurries with minor accumulations through the forecast period.SATURDAY: 2-5cm Friday overnight into Saturday, with another 2-5cm possible throughout the day / Strong to moderate southwest wind / Alpine temperature -4 / Freezing level 900mSUNDAY: Cloudy with flurries / Moderate west wind / Alpine temperature -4 MONDAY: Cloudy with isolated flurries, accumulation 3-10cm possible / Light southwest wind / Alpine temperature -3

Avalanche Summary

Continued avalanche activity was observed in steep low elevation terrain (road cut banks) in the Elk Valley on Wednesday to size 1.5. These were triggering very easily. See here for details in the MIN. On Monday, a machine triggered Size 1.5 persistent slab avalanche was reported running on the mid December surface hoar layer, down 35 cm at that location (1700m). See the details here in a Mountain Information Network (MIN) post. Expect to see persistent avalanche activity continue as temperatures revert to a normal regime (warmer in the valleys and colder with elevation) and snow sitting above persistent weak layers settles more rapidly.

Snowpack Summary

Up to 45 cm of snow fell to end 2017. Since then, temperatures have warmed up from the deep freeze and moderate to strong winds (primarily from the south west) have redistributed that snow into wind slabs at tree line and above. Some windward slopes have been scoured to ground. The main concern in the snowpack is a weak layer buried mid-December, which consists of a crust on solar aspects and feathery surface hoar in sheltered terrain at and below tree line. This layer has consistently shown to be reactive to human triggers. The lower snowpack is composed of mostly soft sugary faceted snow and early season crusts. Avalanche hazard for tree line elevations and below may be one step lower in areas east of the divide.See here for a recent forecaster blog for a summary of the tricky conditions in the snowpack and strategies on how to work around a persistent slab problem.

Problems

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.

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.