Avalanche Forecast
Issued: Dec 8th, 2020 5:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Deep Persistent Slabs.
cmortenson,
Avalanche Canada
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!
Summary
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. Â
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. Â
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. Â
Furthermore, smaller avalanches may trigger larger avalanches where they step down to the deeper persistent weak layer of the November crust. Â
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. Â
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
At tree line, in lower elevations and in wind sheltered areas, the new snow rests on widespread layer of large surface hoar crystals. 20-40cm storm slabs of denser snow sit atop these domino-like surface hoar and can form ⚡ electric propagation. Shooting cracks, remote triggering and settlements (whumphing) are classic signs of instability that buried surface hoar is becoming reactive.
Slab development depends on several factors and can take time so we don't always see slab avalanches right after a big snowfall. We will need to keep monitoring this surface hoar layer for a while.
At upper elevations where the wind is transporting the new snow, fat storm slabs forming in lee features will be especially touchy where they sit over a crust, facets or isolated surface hoar.
Where snow remained cool, loose dry sluffing will likely be observed out of steep terrain and under your skis. However where snow is soggy or moist loose wet stuffing may also be an issue.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Deep Persistent Slabs
A facet/crust layer (from Nov 5 rains) sits near the base of the snowpack. For weeks it has sat dormant, showing no avalanche activity but producing the occasional hard result in snowpack tests.
This layer may become reactive with the new load of snow or if triggered by a smaller avalanche in a step down. Keep in mind that if a storm slab avalanche triggers/steps down to this layer, the resulting avalanche would be large and destructive.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 9th, 2020 5:00PM
