Avalanche Forecast
Issued: Dec 7th, 2020 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Deep Persistent Slabs.
ahanna,
Avalanche Canada
Avalanche danger will rise as snowfall continues through the day. The new snow is not expected to bond well to old surfaces. Watch for signs of instability like shooting cracks and be especially wary of wind loaded features where new snow may be sitting on crust or surface hoar.
Summary
Confidence
Weather Forecast
Monday night: 10-15 cm new snow, moderate to strong southwest wind, freezing level 1200 m.
Tuesday: 5-10 cm new snow, moderate southwest, freezing level 1500 m.
Wednesday: Flurries, light west wind, freezing level 900 m.
Thursday: Cloudy, light northwest wind, freezing level 500 m.
Avalanche Summary
On Tuesday, we expect storm slabs to be touchy to natural and human triggers. You may see wide propagation especially on convexities around treeline and below. You'll likely see some loose dry sluffing out of steep terrain and under your skis.
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
5-10 cm of new snow Tuesday brings storm totals to 15-25 cm. The storm snow sits ontop of a melt-freeze crust on south aspects and widespread surface hoar treeline and below. Generally speaking, new snow does not bond well to these types of surfaces.
Snowpack tests in this MIN show moderate to hard results on a couple of layers which are consistent with our November surface hoar depths at 15 and 30 cm below the surface.
The lower snowpack is characterized by a couple of crusts, the most notable of which is sitting near the base of the snowpack, 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
- Storm slab size and sensitivity to triggering will likely increase through the day.
- 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.
- Be aware of the potential for large avalanches due to the presence of a deep persistent slab.
- Choose conservative terrain and watch for clues of instability.
Problems
Storm Slabs
Loose dry sluffing will likely be observed out of steep terrain and under your skis.
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 surface hoar.
At lower elevations and in wind sheltered areas, the new snow rests on widespread layer of large surface hoar crystals. Once a big and cohesive enough slab forms, this is when we start to see that ⚡electric propagation that buried surface hoar is known for. Slab development depends on several factors and can take time so we don't always see slab avalanches right away. Sustained snowfall, warm temperatures and wind all help contribute to slab development. The timing is difficult to forecast with precision but you can look for signs of instability like shooting cracks to identify where the snow is starting to show slab character.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Deep Persistent Slabs
A facet/crust layer 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.
We're curious to see whether the new loading from the current storm will cause it to wake up. 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 8th, 2020 4:00PM
