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Avalanche Forecast

Archived

Feb 11th, 2019–Feb 12th, 2019

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
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.

The pieces are in place for a significant snowfall enhancement as the incoming storm collides with embedded arctic air. Avalanche danger will increase over the day as new snow accumulates and is affected by wind.

Confidence

Moderate - Forecast snowfall amounts are uncertain

Weather Forecast

Monday night: Increasing cloud with isolated flurries beginning with a trace of new snow. Light southwest winds, increasing to strong in the alpine.Tuesday: Cloudy with periods of snow bringing 20-30 cm of new snow, decreasing overnight. Light southwest winds, increasing to strong or extreme in the alpine. Alpine high temperatures around -11.Wednesday: A mix of sun and cloud with continuing isolated flurries and a trace of new snow. New snow totals up to 40 cm. Light west winds. Alpine high temperatures around -9.Thursday: Mainly cloudy with isolated flurries and a trace of new snow. Light southeast winds shifting to southwest in the evening. Alpine high temperatures around -7.

Avalanche Summary

A social media post from the Marten area of Elk Valley on Thursday describes touchy persistent slab conditions, with lots of smaller slabs releasing over the mid-January surface hoar layer. Check out the post here. Of note is the fact that this area was previously untracked, leaving the mid-January layer undisturbed. This is something to bear in mind as dry conditions persist and the motivation to step out of well-traveled terrain increases. Areas where this layer is undisturbed are far more likely to hold reactive persistent slabs.

Snowpack Summary

Forecast new snow will bury a variable surface of heavily wind affected old storm snow at alpine and wind-exposed treeline elevations, while adding to 25-35 cm of lower density storm snow from last week in sheltered areas above 1700 metres. In these sheltered areas, this older storm snow may cover a layer of weak, feathery surface hoar crystals. Below 1700 metres, the new snow will accumulate above a thinner cover (5-10 cm) of the same storm snow that instead overlies a melt-freeze crust.The mid-January layer of surface hoar or a crust is buried around 35 to 45 cm deep. The surface hoar is found on shaded and sheltered slopes and is most prominent between 1600 m and 1900 m but has been found up to 2200 m. The melt-freeze crust is found on south aspects at all elevations. This layer was the subject of a recent Special Public Avalanche Warning.The middle of the snowpack is generally consolidated. The bottom half of the snowpack is unconsolidated and composed of weak and sugary faceted grains. The basal snowpack is becoming even weaker under prolonged cold temperatures, especially in thin snowpack areas.

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.

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.