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Avalanche Forecast

Archived

Jan 20th, 2021–Jan 22nd, 2021

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.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

North Rockies.

  

Recent snow and extreme southwesterly winds have created touchy wind slabs in many locations, even open areas below treeline. Use caution when transitioning into wind affected terrain.

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Uncertainty is due to extremely variable snowpack conditions reported through the region.

Weather Forecast

  

WEDNESDAY NIGHT: Clearing / Light, northwest ridgetop wind / alpine low temperature -16 / Freezing level valley bottom.

THURSDAY: Mostly sunny / Light, northwest ridgetop wind / alpine high temperature -10 / Freezing level valley bottom.

FRIDAY: Mix of sun and cloud / Light, northwest ridgetop wind / alpine high temperature -12 / Freezing level valley bottom.

SATURDAY: Cloudy / Moderate, west ridgetop wind / alpine high temperature -12 / Freezing level valley bottom.

Avalanche Summary

On Tuesday, our field team reported widespread wind effect at all elevations, creating fragile cornices and reactive wind slabs on north and east facing slopes below ridgetops. MIN report HERE.

On Monday our field team observed some size 2 natural avalanches on a north facing alpine feature east of Crescent Spur. These likely ran over the weekend.

On Sunday we received some great observations of avalanches running naturally on a crust in the trees in "The Farm" area, more details here.

Snowpack Summary

30-50 cm of recent snow and extreme southwest winds have created large, fragile cornices and widespread wind slabs in many locations, even open areas below treeline.

Underneath the recent snow you are likely to find a hard melt-freeze crust that can be found up to about 1600 m. You may also find a weak layer of buried surface hoar in isolated, sheltered locations below treeline. As the new snow continues to settle and form slab properties, these layers could become reactive to human traffic. 

Around 60 to 130 cm of snow rests above another weak layer buried in early-December. On Jan 14th our field team found it in the Holmes area. This setup has created a persistent slab avalanche problem and a low probability/high consequence scenario. Depending on location, the buried weak layer is composed of surface hoar and/or a crust/facet combination. Reports suggest that this layer has become dormant in much of the region except for perhaps the Pine Pass area.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Avoid lee and cross-loaded terrain.
  • Cornices become weak with daytime heating or solar exposure.
  • Avoid exposure to slopes that have cornices overhead.
  • Cornices often break further back than expected; give them a wide berth when traveling on ridgetops.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.