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Avalanche Forecast

Archived

Dec 6th, 2022–Dec 7th, 2022

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

Regions

Lizard-Flathead, Akamina, Flathead, Lizard.

Fresh, reactive wind slabs likely exist at upper elevations. Riders may seek more sheltered areas around treeline to avoid these wind slabs but should consider that this is where the persistent slab problem may be lurking. It's a good time to keep your terrain choices conservative.

Confidence

Moderate

Avalanche Summary

On Monday, explosive testing targeted the persistent slab problem with no results, however, skier-controlled wind slabs were initiated up to size 1 as well as explosive-controlled wind slabs size 1.5.

On Sunday, a MIN report from Tunnel Creek reported reactivity in a snowpack test on the Mid-November layer on a south aspect. In this area, the layer is presented as facets on a crust. They also observed a large natural avalanche on a west-facing slope at treeline.

I suspect wind slabs will be present and reactive on Wednesday as well as loose dry avalanches (sluffing) within the recently fallen low-density surface snow, especially from steep slopes and terrain features.

Snowpack Summary

Up to 15 cm of low density, new snow fell overnight Tuesday and Wednesday. This new snow will add to the ~25 cm of storm snow that fell early in the weekend. In wind-affected terrain, westerly winds are expected to redistribute this new snow into fresh wind slabs. In sheltered terrain, cold temperatures will likely maintain low-density, powdery snow.

The middle of the snowpack consists of weak sugary layers of facets and surface hoar. A widespread rain crust remains near the ground at treeline and below treeline.

At treeline, snowpack depths vary from 120-200 cm.

Weather Summary

Tuesday Night

Cloudy with light snowfall amounts 2 to 5 cm. Westerly winds 40-70 km/h at ridgetop. Treeline temperatures drop to -13 C.

Wednesday

Cloudy with possible sunny breaks, generally 5 cm accumulation. Moderate to strong westerly winds at ridgetop. Temperatures reach a high of -10 C.

Thursday

Cloudy with snow forecast later in the day and overnight, with possible accumulations 10-15 cm. Strong southwest winds at ridgetop. Average temperature near -8 C.

Friday

Cloudy with snowfall, up to 5 cm of accumulation. Gusty wind from the west/ southwest at ridgetop. Treeline temperatures reach a high of -6 C and a low of -10 C.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Expect slab conditions to change drastically as you move into wind exposed terrain.
  • Back off if you encounter whumpfing, hollow sounds, or shooting cracks.
  • Fresh snow rests on a problematic persistent slab, don't let good riding lure you into complacency.

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.