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Avalanche Forecast

Archived

Feb 2nd, 2021–Feb 3rd, 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 possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Lizard-Flathead.

New snow accompanied by a strong southwest wind will build touchy and reactive slab avalanches, especially where they sit above a surface hoar, facet or crust interface. Natural avalanche activity may taper but skier and rider triggers are likely.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

Tuesday Night: Snow amounts 5-15 cm. Ridgetop wind generally light from the southwest with strong gusty periods. Alpine temperatures near -7 and freezing levels at valley bottom.

Wednesday: Mix of sun and cloud. Ridgetop wind moderate from the West. Alpine temperatures near -5 and freezing levels 1100 m.

Thursday: Cloudy with flurries. Ridgetop wind light to moderate from the southwest. Alpine temperatures near -10 and freezing levels at valley bottom.

Friday: Light snow up to 5 cm. Ridgetop wind generally light with some strong gusts from the West and freezing levels at valley bottom.

Avalanche Summary

On Tuesday, a widespread natural avalanche cycle occurred up to size 2.5. 

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, skier and rider triggered avalanches will be likely on Wednesday. 

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, this slab of recent snow sits on top of a plethora of old snow surfaces comprising of hard wind slab, scoured areas, sastrugi, and isolated pockets of soft snow. Below 1800 m a hard melt-freeze crust underneath the new snow is found. Moist surface snow now exists below 1600 m. Once the freezing levels drop Tuesday evening the surface snow at lower elevations will likely form a crust.

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

  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Persistent slabs have potential to pull back to lower angle terrain.
  • 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.