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Avalanche Forecast

Archived

Jan 11th, 2021–Jan 12th, 2021

Alpine
Natural avalanches unlikely, human triggered possible.
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, 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.

Avalanche hazard will gradually rise over the day and into Tuesday night as new snow and wind form fresh wind slabs at upper elevations.

If you see more than 20 cm by the end of the day, treat avalanche danger as CONSIDERABLE at elevations with wind blowing the snow around.

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system.

Weather Forecast

Monday night: Flurries, up to 5 cm, high wind gradient with elevation, light at 2000 m, strong at 3000 m southwest, freezing level 500 m.

Tuesday: Flurries, up to 10 cm, high wind gradient with elevation, light at 2000 m, strong at 3000 m southwest, alpine high -5, freezing level 1100 m.

Wednesday: Snow overnight then clearing, 10-20 cm, strong southwest wind, alpine high -4, freezing level 1500 m dropping through the day.

Thursday: Sunny, strong southwest wind, alpine high -7, freezing level 700 m with a weak inversion setting up.

Avalanche Summary

Over the weekend we received reports of a couple of natural wind slabs size 1.5, and solar triggered loose snow avalanches size 1-2.

Two human triggered avalanche incidents occurred last Tuesday; a size 1.5 on surface hoar in a northeast facing burn near Glacier National Park (view MIN report here), and a size 2.5 avalanche on an east aspect at treeline in the Lower Holt area, near Golden (view report here).

We have not had any reports of deep persistent slab avalanches on the basal weak layers for over a week.

Snowpack Summary

Small amounts of new snow fall on wind affected surfaces at upper elevations, surface hoar in wind sheltered areas and a thin crust on solar aspects. Winds are likely redistributing new snow into wind slabs and growing cornices. 

A few persistent weak layers exist in the upper to mid snowpack:

  • The upper layer, found 40-60 cm down, is a spotty layer of surface hoar at treeline and below. 
  • The lower layer, found 90-140 cm down, is widespread and can present as surface hoar and/or a crust.

Avalanche activity on these layers has dwindled since last week and snowpack tests results have been moderate to hard planar, but still showing propagation. Incoming snowfall in the forecast for this week will add additional load to these layers and may shed some light on whether they will remain a concern.

The lower snowpack is characterized by more crusts, the most notable is a rain crust from early November that is near the base of the snowpack, surrounded by a weak layer of sugary facets. It is most likely to be reactive to human triggers in steep, shallow, rocky areas with a thin to thick snowpack.

Terrain and Travel

  • Fresh wind slabs will likely form throughout the day, diligently watch for changing conditions.
  • Dial back your terrain choices if you are seeing more than 20 cm of new snow.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

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.