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Avalanche Forecast

Archived

Dec 18th, 2020–Dec 19th, 2020

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

Regions

South Rockies.

Wind slabs are likely to be encountered at upper elevations and may be reactive to human triggering, especially in lee features.

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

FRIDAY NIGHT - Mainly cloudy with flurries, 5 cm / southwest wind, 50-80 km/h, gusting to 110 km/h / alpine low temperature near -4 / freezing level 1300 m

SATURDAY - A mix of sun and cloud with scattered flurries / southwest wind, 40-80 km/h / alpine high temperature near -3 / freezing level 1500 m

SUNDAY - A mix of sun and cloud / southwest wind, 30-60 km/h / alpine high temperature near 0 / freezing level 2000 m

MONDAY - A mix of sun and cloud with isolated flurries / southwest wind, 30-40 km/h gusting to 75 km/h / alpine high temperature near 0 / freezing level 2000 m 

Avalanche Summary

There were a few reports of both natural and explosives triggered avalanches up to size 2 on Thursday and Friday. 

Please consider sharing your observations on the Mountain Information Network. Thank you to those that have already submitted this winter!

Snowpack Summary

Up to 20 cm of recent new snow and strong to extreme southwest wind have likely formed reactive wind slabs at treeline and in the alpine.

This recent snow means there is now 30-50 cm above the early December crust which exists up to approximately 2000 m. Use caution if you find cohesive snow above this crust; there may be weak faceted grains above or below the crust. 

The base of the snowpack consists of a hard melt-freeze crust from early-November that may have faceted grains around it. There has not been recent avalanche activity on this layer but it remains on our radar. The most likely spot to trigger it would be on thin and rocky slopes.

Snowpack depths vary substantially in the region with the average snowpack depth at treeline being approximately 100 cm.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid areas where the snow feels stiff and/or slabby.

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.

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.