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Avalanche Forecast

Archived

Dec 26th, 2021–Dec 27th, 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 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

Lizard-Flathead.

Watch for more reactive deposits in areas influenced by the wind. Be ready for the cold.

Confidence

Moderate -

Weather Forecast

Sunday night: Cold with isolated flurries, 5 cm. Moderate decreasing east wind. Overnight temperature dropping below -27. 

Monday: Cold and cloudy, with sunny breaks. Light west wind increasing to moderate through the day. Cold temperatures with a high of -21. 

Tuesday: Cold, mostly cloudy, and isolated flurries. Light southwest wind and a high temperature of -20.

Wednesday: Cold, with a sun and cloud mix. Light east wind and a high temperature of -24.

Avalanche Summary

Evidence of a natural avalanche cycle was observed Saturday morning, including a large (size 2.5) natural storm slab avalanche which ran 900 m. Explosives also triggered storm slab avalanches size 1.5-2, noting that although low density slabs, fast moving snow was running far and entraining a lot of snow.

On Friday, 3 large (size 2-2.5) natural slab avalanches were reported, all failed on a persistent weak layer 100-200 cm deep, likely triggered by the new load of snow. Explosives also triggered large (size 2.5) storm slabs.

On Thursday, skiers and machines triggered storm slab avalanches to size 2, explosives triggered slab avalanches to size 3. Poor visibility prevented good alpine observations.

Snowpack Summary

Recent storm totals reached over 100 cm! Cold temperatures and calm wind will keep snow light and fluffy, but snow will be redistributed easily with any wind. Reports also indicate the recent snow buried a surface hoar interface and pockets of old wind slab. For now, we don't expect the new snow to bond well to the old interface. Sluffing is to be expected in steep terrain.

A crust from early December is now 90-150 cm below the surface, and is found up to 2400 m. A layer of weak facets (sugary snow) has been reported above and below this crust. This is the layer of concern in our persistent avalanche problem, recent snowpack tests are generally showing hard results on this interface.

The lower snowpack consists of a variety of early-season crusts and mainly moist snow. Snowpack depths range from 100-250 cm at treeline elevations and taper quickly below 1800 m.

Terrain and Travel

  • Start with conservative lines and watch for clues of instability.
  • Pay attention to the wind, once it starts to blow fresh sensitive wind slabs are likely to form.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.

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.