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Avalanche Forecast

Archived

Feb 1st, 2021–Feb 2nd, 2021

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
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.

Avoid avalanche terrain and overhead hazards on Tuesday as the new snow accompanied by a strong southwest wind will build touchy and reactive slab avalanches. They will especially be reactive in sheltered terrain where they sit above a surface hoar/ facet/ crust interface. 

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

Enhanced snowfall amounts may occur with the strong southwest flow followed by the cold front.

Monday Night: Snow amounts 5-10 cm with 20-60 km/hr ridgetop wind from the South. Alpine temperatures near -1 and freezing levels 1300 m.

Tuesday: Snow 10-25 cm. Strong and gusty southwest wind. Alpine temperatures near -1 and freezing levels 1400 m.

Wednesday: A mix of sun and cloud with isolated flurries. Alpine temperatures -6 and freezing levels at the valley bottom. Ridgetop wind generally light from the southwest.

Avalanche Summary

On Monday, several storm slab and loose dry avalanches up to size 2 were triggered with the use of explosives. Reports of natural cornice failures were seen up to size 2. On Sunday, numerous storm slab and persistent slab avalanches were reported up to size 1.5 primarily on NE aspects above 1700 m. These avalanches were easily triggered by the weight of a human either directly or remotely (from a distance away). Check out these reports that show how reactive the slab is: MIN Report, MIN report, or this MIN report. 

With additional forecast snow and strong wind, the trend will likely continue on Tuesday resulting in natural avalanche activity and slabs primed for human triggers.

Snowpack Summary

Up to 45 cm of recent snow has formed a cohesive and reactive slab that sits above a weak interface of surface hoar and surface facets (weak sugar or feather-like snow crystals). In the alpine the recent snow sits on top of layers of hard wind slab, scoured areas, sastrugi and isolated pockets of soft snow. A hard melt-freeze crust underneath the new snow is found up to 1800 m. 

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

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow and wind.
  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Avoid terrain traps where the consequence of any avalanche could be serious.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.