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Avalanche Forecast

Archived

Jan 9th, 2022–Jan 10th, 2022

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 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 possible, human triggered probable.

Regions

Lizard-Flathead.

Wind slabs will form in lee terrain features and below ridges with moderate to strong southwest wind. Be alert to conditions that change with elevation and wind exposure. 

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Sunday night: Clear, moderate southwest wind with strong gusts, treeline low around -7 °C, possible inversion.

Monday: Sunny with increasing clouds in the afternoon, moderate to strong southwest wind, treeline high around -3 °C, freezing level at valley bottom.

Tuesday: Cloudy, up to 10 cm new snow, moderate to strong southwest wind gusting to extreme, treeline high around -1 °C, freezing level rising to 1500 m.

Wednesday: Cloudy, 10-20 cm new snow and rain at lower elevations, strong southwest wind gusting to extreme, treeline high around +2 °C, freezing level rising to 1800 m.

Avalanche Summary

On Saturday, storm slab avalanches up to size 2.5 were triggered by explosives. Overnight, a large (size 3.5) natural storm slab avalanche released and a large (size 2.5) cornice failure occurred. 

On Friday, storm slab avalanches released naturally up to size 2.5. Explosives triggered storm slabs to size 2.5, and skiers triggered avalanches to size 2. A large (size 2) natural cornice failure was reported likely having failed in the overnight storm.

Evidence of a natural storm slab avalanche cycle size 2-2.5 was observed on NE aspects Wednesday and Thursday, likely having run during the storm earlier in the week. 

Last week on Sunday, a size 2.5 persistent slab avalanche was triggered on an E aspect at treeline. This avalanche is detailed in a MIN post and features in our latest blog post, Photos of recent persistent slab avalanches in the southern interior.

Snowpack Summary

The recent storm snow totals 40-50 cm. In open terrain and upper elevations, wind developed more reactive deposits around lee and convex features. This recent snow covered a hard, faceted snow surface which formed during cold temperatures and may take a little time to bond. 

The most notable layer of concern in the snowpack is a crust that was formed in early December and is now down 100-270 cm. Activity on this layer has been sporadic, the most recent persistent slab avalanche in the region was on January 2. This layer has created a low likelihood, high consequence scenario which is best managed through conservative terrain choices and disciplined backcountry travel techniques.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • 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.