Avalanche Forecast
Issued: Dec 21st, 2020 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Persistent Slabs.
ghelgeson,
Avalanche Canada
A significant winter storm will lambaste the region Monday night further destabilizing the smorgasbord of weak layers in our upper snowpack. Avoid all avalanche terrain at this time, large to very large natural avalanches are expected as the snowpack adjusts to this new load.
Summary
Confidence
Weather Forecast
One last pulse tonight, and then it looks like we’re moving into a bit of a clearing trend this week.
MONDAY NIGHT: Freezing level around 1000 m, strong northwest wind, 15 to 40 cm of snow expected.
TUESDAY: Broken cloud cover, freezing level near valley bottom, moderate northwest wind, 1 to 5 cm of snow possible.
WEDNESDAY: Scattered cloud cover, freezing level at valley bottom, light southwest wind, no new snow expected.
THURSDAY: A few clouds, freezing level at valley bottom with potential for a light alpine temperature inversion, light southerly wind, no new snow expected.
Avalanche Summary
Over the weekend a prolific natural avalanche cycle took place with avalanches averaging size 1.5 to 2.5, but there were a few size 3 and even size 3.5's in the mix as avalanches ran on the early December crust. Avalanches were running on both the mid December surface hoar and the early December crust/surface hoar/facet combo.
Snowpack Summary
An active storm cycle has produced 60 to 100 cm of storm snow over the last week. Underneath this storm snow there is a sandwich of weak layers which are widespread throughout the region.
Just under the new snow, down about one meter below the surface is the mid December surface hoar. This layer has been failing naturally and has been quite sensitive to human triggering throughout the storm cycle. Large avalanches have been running on this interface connecting big terrain features.
Below that lies the early December persistent weak layer, it's about 80 to 140 cm below the surface. This weak layer is a mixed bag that often presents as surface hoar and facets sitting on top of a stout supportive crust. There are facets below the crust too. This is a nasty weak layer, when avalanches fail on it, they have been large and destructive. Smaller avalanches in motion can step down to this one too. This weak layer is going to haunt us for the foreseeable future.
Terrain and Travel
- Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.
- Only the most simple non-avalanche terrain free of overhead hazard is appropriate at this time.
- Remote triggering is a concern, watch out for adjacent and overhead slopes.
- Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
Problems
Storm Slabs
We're expecting 15 to 40 cm of new snow Monday night into Tuesday with strong north wind. Watch for the formation of fresh sensitive slabs in the new snow that will likely be sensitive to human triggering all day Tuesday.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Persistent Slabs
The last storm pushed the persistent slab problem over the tipping point resulting in very large avalanches Saturday & Sunday. Monday night's storm is certain to add increased vigor to this problem. The slab rests on a persistent weak layer which has the capacity to propagate avalanches across terrain features. Wind exposed features are expected to be particularly volatile.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 22nd, 2020 4:00PM
