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Avalanche Forecast

Archived

Feb 3rd, 2021–Feb 4th, 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 unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Lizard-Flathead.

Natural avalanche activity will likely taper but slabs remain primed for skier and rider triggering. Conservative terrain choices are a great way to handle a persistent slab avalanche problem.

Confidence

Moderate - Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather. Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

Thursday: Cloudy with some flurries 3-5 cm. Alpine temperatures near -7 and freezing levels at the valley bottom. Ridgetop winds are strong from the southwest.

Friday: Mix of sun and cloud. Alpine temperatures are low of -20 and high of -5 with strong ridgetop wind from the West.

Saturday: Mix of sun and cloud with snow up to 5 cm. Alpine temperatures low -22 and high -15. Ridgetop wind light from the West.

Avalanche Summary

On Wednesday, numerous natural storm slab avalanches were reported up to size 2, and several more slabs were triggered by explosives up to size 1.5. Loose dry avalanches were seen up to size 1. On Tuesday, a widespread natural avalanche cycle occurred up to size 2.5. 

Last weekend numerous 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. 

As natural avalanche activity tapers, wind and persistent slabs may still be primed for triggering on Thusday.

Snowpack Summary

Up to 60 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) and a crust. 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 and a surface crust now exists up to 1700 m from the high freezing levels on Tuesday.

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.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Look for signs of instability: whumphing, hollow sounds, shooting cracks, and recent avalanches.
  • 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

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.