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Avalanche Forecast

Archived

Feb 8th, 2021–Feb 9th, 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.

A reactive layer of surface hoar lurks in the snowpack and requires conservative terrain selection - stick to low angle slopes, be suspect of open glades and any slope with terrain traps, and bring an extra puffy - it's cold.

Confidence

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

Weather Forecast

MONDAY NIGHT - Cold and mostly clear / light north wind / alpine low temperature near -30

TUESDAY - Cold and sunny / light northwest wind / alpine high temperature near -18

WEDNESDAY - Cold with a mix of sun and cloud / light northeast wind / alpine high temperature near -24

THURSDAY - Ongoing cold with a mix of sun and cloud / moderate and increasing northeast wind / alpine high temperature near -26

Avalanche Summary

On Sunday a cornice failure triggered a small (size 1) avalanche 50 cm crown and skiers reported sudden results in test profiles on prominent surface hoar buried 50 cm deep.

On Saturday riders continued to trigger slab avalanches 20-60 cm deep, documented in a handful of MIN reports (Trespass, Mammoth, Spicy, Different Day). Explosives triggered several large (2-2.5) persistent slab avalanches on northerly aspects below 2000 m/

On Friday, there were numerous reports of natural and human triggered persistent slab avalanches up to size 2.5. Some of these were triggered remotely, as outlined in a MIN report that can be viewed here. There were also numerous natural and explosives triggered dry loose, and storm slab avalanches reported up to size 2. 

Last week was busy for avalanche activity, on Wednesday (Feb 3) persistent slab avalanche activity really picked up with reports of natural, human, and explosives triggered storm and/or persistent slab avalanches up to size 2 everyday through Saturday (Feb 6). 

Many thanks for all of the great MIN reports over the past week!

Snowpack Summary

Recent variable winds have formed slabs in open terrain on a variety of aspects. 20-80 cm recent snow has formed a reactive slab on top of a persistent weak layer that consists of surface hoar, facets, and a crust buried in late January. Below 1600 m a hard melt-freeze crust is underneath 20-40 cm recent snow.  

A solid mid-pack sits above a deeply buried crust and facet layers near the bottom of the snowpack (150-200 cm deep), which is currently unreactive. 

Terrain and Travel

  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
  • Be aware of highly variable recent wind loading patterns.
  • Look for signs of instability: whumphing, hollow sounds, shooting cracks, and recent avalanches.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.

Problems

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.

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.