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Avalanche Forecast

Archived

Dec 8th, 2020–Dec 9th, 2020

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

Regions

North Columbia.

20-40cm of new snow has formed a dense slab that is poorly bonded to a significant surface hoar layer at tree line and lower elevations & sits on suncrust in the Alpine. Make conservative decisions & reduce your exposure to avalanche terrain. Watch for signs of instability! 

Confidence

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

Weather Forecast

Cooler and more settled conditions in the forecast area for the week as the weather maker shifts south of the forecast region.  

Tuesday Night: Mostly cloudy with isolated flurries and 2-8cm snow, winds Moderate NW, freezing level drops to 1000 m, Alpine low -5C.

Wednesday: Cloudy with Sunny Periods and isolated flurries, light to moderate Westerly wind, freezing level 1000 m. (Higher FL in the Southern regions up to 1300m)

Thursday: Cloudy with sunny periods, light northwest wind, freezing level 800 m.

Friday: Mainly Cloudy, light and variable winds, freezing level 900 m. Alpine Low -12C / High -6C

Avalanche Summary

An avalanche cycle is ongoing with observations trickling in at the time of this report. Large avalanches (Size 2) were reported from skier controlled and explosive control work on Tuesday. A few Very Large (Size 3) Natural avalanches were reported running to valley bottom.  

The natural cycle should being to taper with precipitation easing and cooler temperatures on Wednesday, but natural triggered avalanches remain possible and human triggered avalanches will still remain likely.  

Furthermore, smaller avalanches may trigger larger avalanches where they step down to the deeper persistent weak layer of the November crust.  

Our eyes and ears in the mountains are limited this winter due to fewer professional observations. If you go out in the mountains, please share your observations on the Mountain Information Network (MIN). Photos are especially helpful! Thank you so much for all the great MINs submitted so far!

Snowpack Summary

The recent warm windy storm brought 20-40cm of dense storm snow that is settling into a denser storm slab. At treeline and below this storm slab sits atop widespread large surface hoar and up to 20cm of facets. In the Alpine, the storm slab sits atop suncrust on solar aspects and isolated smaller Surface Hoar in sheltered locations. Generally speaking, new snow does not bond well to these types of surfaces.  

The lower snowpack is characterized by a couple of crusts, the most notables from early November rains (Nov 5 Crust), which is now sitting near the base of the snowpack and is likely surrounded by a weak layer of sugary facets. It is most likely to be reactive to human triggers on south facing slopes and in shallow, rocky areas in the alpine.

Terrain and Travel

  • Choose conservative terrain and watch for clues of instability.
  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Don't let the desire for deep powder pull you into high consequence terrain.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.