Dashboard Regions Weather Stations Radar Alerts Glossary
Contact About
Log In

Register for an account and never miss a forecast again!

Register

Avalanche Forecast

Archived

Dec 17th, 2020–Dec 18th, 2020

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

Regions

South Rockies.

Watch out for wind slabs they're likely reactive to human triggers at upper elevations. Choose conservative terrain and watch for signs of instability such as whumphing, cracking and recent avalanches.

Confidence

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

Weather Forecast

Friday: Snow amounts 5cm with alpine temperatures near -7. Ridgetop wind strong-extreme from the southwest and freezing levels valley bottom.

Saturday: Snow 5-10 cm with alpine temperatures near -7. Strong-extreme wind from the West and freezing levels 1000-1500 m. 

Sunday: Snow amounts 10-20 cm. Alpine temperatures -1 and freezing levels 2000 m. Extreme 65-105Km/hr wind from the southwest.

Avalanche Summary

No new reports on Thursday. Avalanche control using explosives in the Lizard range produced several slabs up to size 2 in alpine terrain and a size 1.5 in treeline terrain. This may indicate that slabs may be sensitive to human triggers on 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 new snow and strong to extreme southwest wind have likely formed reactive wind slabs at treeline and in the alpine. The recent snow brings 30-50 cm above the early December crust which exists in the snowpack up to 2000 m. Use caution if you find cohesive snow above the crust, particularly if there are weak, sugary faceted grains immediately above or below the crust. With slab cohesion adding stress and load on that interface we may see this persistent slab avalanche problem come to life.

The snowpack depth varies substantially in the region and has been described as thin, wind-hammered, variable, and tapering rapidly at lower elevations. The average snowpack depth at treeline is approximately 100 cm. 

The middle of the snowpack has been reported as being well-consolidated and may host another hard melt-freeze 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.

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.