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Avalanche Forecast

Archived

Jan 4th, 2021–Jan 5th, 2021

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

Regions

Northwest Inland.

Stick to sheltered terrain to avoid the intense wind transport and dangerous avalanche conditions on Tuesday.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack. Uncertainty is due to the complexity of the snowpack’s structure.

Weather Forecast

A windy storm will continue until Wednesday afternoon.

MONDAY NIGHT: Isolated flurries with up to 5 cm of snow, strong southwest wind, temperatures around -8 C

TUESDAY: Scattered flurries with 5-15 cm of snow throughout the day, strong to extreme south wind, temperatures around -4 C.

WEDNESDAY: Flurries continue with another 5-10 cm of snow by the morning then clearing in the afternoon, moderate south wind, temperatures around -6 C.

THURSDAY: Partly cloudy, moderate south wind, temperatures around -8 C.

Avalanche Summary

Over the weekend there were several reports of natural size 1-2 slab avalanches on northeast alpine slopes (see some photos here and here). Most of the activity was on wind loaded slopes. There was also a size 1.5 skier controlled storm slab avalanche at Ashman (see here). No persistent or deep persistent slab avalanches have been reported since Dec 22.

With more wind and snow in the coming days, natural and human triggered slabs remain a concern.

Snowpack Summary

Strong to extreme wind is moving around 30-50 cm of snow that has fallen since New Year's Day. Thick wind slabs can be found in exposed open terrain while sheltered terrain should have mostly low density snow with some isolated storm slabs potentially forming. Be cautious on steep rolls in sheltered areas where the recent snow may be sitting above weak surface hoar.

The lower snowpack has two crusts that potentially have weak snow around them. One is 60-90 cm below the surface and the other is near the ground. These weak layers produce avalanches on Dec 21 and 22, but since then have been trending towards dormancy. We are uncertain about how likely it is to trigger avalanches on these deeper layers, but would be suspect of shallow rocky alpine slopes.

Terrain and Travel

  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • Choose slopes that are well supported and have limited consequence.
  • Be especially cautious near rock outcroppings, on steep convexities and anywhere the snowpack feels thinner than average.

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.