Avalanche Forecast

Issued: Dec 8th, 2020 5:00PM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Storm Slabs and Deep Persistent Slabs.

Avalanche Canada cmortenson, Avalanche Canada

Areas with total storm snow amounts of 30+cm will be HIGH danger. Elsewhere, 10-25cm of storm slab will not bond well to large Surface Hoar at & below tree line & sits on suncrust in the Alpine. Limit exposure to avalanche terrain & watch for signs of instability!

Summary

Confidence

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

Weather Forecast

The "atmospheric river" storm shifts south and is forecast to deliver more snowfall to regions south of Nakusp. Cooler weather with dropping freezing levels are forecast for the following few days.

Tuesday night: 5-20 cm new snow (more seen south of Nakusp), moderate southwest wind, freezing level 1400 m.

Wednesday: Trace-10cm new snow (more seen south of Nakusp), light west wind, freezing level 1400 m.

Thursday: Flurries, light northwest wind, 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 from areas that already received over 30+cm 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.  

In the northern regions, recent 20-40cm of snow fell warm (or is falling) and with moderate to strong Southwesterly winds and settled into a dense storm slab. This storm slab is not bonding well with a widespread surface hoar layer and sun crusts. 

The southern region (south of Nakusp) may experience this avalanche cycle Wednesday wherever snowfall amounts exceed 30cm. Wide propagations are possible, especially on convexities around treeline and below where new snow falls onto of a widespread large surface hoar layer. 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

Snowfall amounts from Tuesday are highly variable across the forecast region as the "atmospheric river" shifts south for Wednesday. The northern regions have already seen 10-25cm of recent storm snow while the southern areas are forecast to get 10-40cm overnight Tuesday and into Wednesday. The snow has arrived with warm windy conditions perfect for the formation of a dense storm slab.  

At tree line and in lower elevations this 10-40cm of storm slab sits atop widespread large surface hoar. In the Alpine, the storm slab sits atop suncrust on steep solar aspects. 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 early November rains (Nov 5 crust) which is now sitting near the base of the snowpack and is 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.
  • 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.

Problems

Storm Slabs

An icon showing 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. 15-40cm 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. This doesn't mean the Surface hoar is no longer a problem & 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 formed 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 in low elevations, where snow is soggy or moist, loose wet stuffing may also be an issue.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Very Likely

Expected Size

1.5 - 2.5

Deep Persistent Slabs

An icon showing 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

Possible

Expected Size

2 - 3

Valid until: Dec 9th, 2020 5:00PM