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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

Purcells.

Hazard will increase to HIGH in areas where over 30 cm of total new snow fell. Limit exposure to avalanche terrain: New snow will not bond well to widespread surface hoar at tree line and below & fresh wind slabs lurk in the alpine. Be Careful & Watch for signs of instability!

Confidence

Low - Uncertainty is due to the track & intensity of the incoming weather system. Forecast snowfall amounts are uncertain.

Weather Forecast

Mountainous regions of the Purcells will see higher overall snowfall amounts. Additionally, the "atmospheric river-like" storm will shift south overnight.

Tuesday Night: 10-20cm of snow overnight (more in the south), winds light to moderate Southwesterly, freezing level 1400m.

Wednesday: Cloudy with sunny periods, Nil to 15cm over the day (more in the south), winds light to moderate west wind (with strong NW gusts), freezing level 1400 m.

Thursday: Cloudy with sunny periods, isolated flurries, winds light to moderate Southwesterly, freezing level 700 m.

Friday: A mix of sun and clouds, isolated flurries, winds light to moderate Southwesterly, freezing level 700 m.

Avalanche Summary

An avalanche cycle is ongoing in adjacent regions where 30+cm of snow has already fallen. Large avalanches (Size 2) were reported from skier controlled and explosive control work on Tuesday in the storm snow.  

We expect heightened natural and skier triggered small and large avalanches in the Purcells with forecast snowfall amounts. 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

Around 5-15 cm of new snow fell Monday night in the northern regions with another 5-20cm forecast for Tuesday night. This 10-30cm of new snow will sit on top of a melt-freeze crust on south aspects and widespread surface hoar tree line and below. 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 notable is from rains in early November (Nov 5 Crust). This crust is sitting near the base of the snowpack, 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.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • 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.