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Avalanche Forecast

Archived

Feb 9th, 2019–Feb 10th, 2019

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

Regions

Purcells.

Watch for wind slabs in unusual locations Sunday. Large avalanches may remain sensitive to human triggering due to the presence of buried surface hoar. Click here to see our Forecasters' Blog for a more detailed description of current conditions.

Confidence

Moderate - Wind effect is extremely variable

Weather Forecast

Cold, dry, arctic air continues to dominate the weather pattern for the foreseeable future.SATURDAY NIGHT: Freezing level at valley bottom, around -20 C in the alpine, light variable winds at all elevations, no significant snowfall expected.SUNDAY: Clear skies at dawn with cloud cover increasing throughout the day, freezing level at valley bottom, around -20 C in the alpine, light variable wind at valley bottom, light southwest wind in the alpine, no significant snowfall expectedMONDAY: Scattered cloud cover, freezing level at valley bottom, around -20 C in the alpine, light variable wind at all elevations, no significant snowfall expected. A few centimeters of snow possible Monday night.TUESDAY: Scattered cloud cover, freezing level at valley bottom, around -20 C in the alpine, light variable wind at most elevations with moderate southwest wind at ridgetop, a few cm of snow possible.

Avalanche Summary

On Saturday a skier remote triggered a size 1 wind slab on a south/southeast facing slope at 2500 m.On Friday a skier in the "Molars South" zone near Golden accidentally triggered a size 2 avalanche on a southeast facing slope at 2300 m with a crown 40 to 50 cm in depth, the mid January persistent weak layer was likely in play. Also on Friday, a natural size 1 persistent slab avalanche was reported from an east facing slope at 1900 m. Finally, a natural size 1 wind slab avalanche was reported from an east facing slope at 2300 m on Friday. The mid-January persistent weak layer continues to be sensitive to human triggering and is sensitive enough to trigger remotely (from a distance). Human triggered avalanches size 1.5 to 2 were reported almost everyday last week. This MIN report from February 3rd illustrates the potential for humans to remotely trigger the mid January layer.

Snowpack Summary

Approximately 30-60 cm of old storm snow sits on, surface hoar (feathery crystals), facets (sugary snow) and a crust on sun-exposed slopes. A very notable feature in the snowpack is a persistent weak layer that was buried in mid January, which is now 50-90 cm below the surface. This layer consists primarily of surface hoar, however there is also a crust associated with it on sun-exposed slopes. This layer has been most reactive at treeline and below.The base of the snowpack has a deep persistent weak layer near the ground. This layer consists of facets over a crust. This weak interface continues to produce large and destructive avalanches that are sporadic in nature, and very difficult to predict. This layer is most likely to be triggered from areas where the snowpack is shallow and weak. Rocky alpine bowls, ridge crests and rocky outcroppings are some examples of the kind of terrain to be wary of.

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.

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.