Avalanche Forecast

Issued: Dec 9th, 2020 5:00PM

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

Avalanche Canada cmortenson, Avalanche Canada

Highly variable conditions: Hazard highest where recent snowfall amounts exceed 20cm. New storm snow is not bonding well to rain-crusts and buried surface hoar. Watch for signs of instability and chose conservative terrain.

Summary

Confidence

Low - Recent weather patterns have resulted in a high degree of snowpack variability within the region. Uncertainty is due to the limited number of field observations.

Weather Forecast

Cooler, a mix of sun and clouds for the remainder of the week as high pressure builds. Higher Freezing levels to the south.

Wednesday Night: A mix of clear and cloudy with isolated flurries, light Southwesterly wind, freezing level valley bottom (600m in the north and 1000m near the Kootneys). Alpine Low -7C.

Thursday: Cloudy with sunny periods, light Southeast wind, freezing level 1100 m. Alpine High -7C.

Friday: A mix of sun and clouds, isolated fluries, light and variable winds, freezing level 700 m. Alpine Low -10C / High -6C

Saturday: A mix of sun and clouds, light and variable winds, freezing level 700 m. Alpine Low -10C / High -7C

Avalanche Summary

Avalanche hazard remains quite variable and dependent on snowfall amounts and freezing lines experienced during this recent storm. Certain regions received 40cm of snow while others barely saw 10cm. Lower hazard may exist in these areas with less snow fall.  

Numerous reports from Wednesday have come in describing the poor bond of new snow to the recent rain crusts producing small to large avalanches 5-25cm deep. Skier controlled small avalanches are being reported in addition to explosive control small and large avalanches at tree line and in the Alpine.  

On Tuesday, at the peak of the cycle, reports of large avalanches (Size 2) naturally and from skier controlled and explosive control work on Tuesday. A few Very Large (Size 3) Natural avalanches were reported running to valley bottom along the highway corridor.  

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

As things cool after high freezing levels, anticipate refrozen surfaces below treeline up to 1800m. Freezing rain with the recent storm has created a thick rain crust which thins with elevation up to 2300m in many regions of the forecast area. Its distribution and extent remains unclear but we believe it is widespread. In some areas there are two rain crusts: a thin crust near the base of the recent storm snow and a second thicker (1-5cm) crust near the surface. New snow is bonding poorly to these layers. See this MIN report from near Kokanee Glacier & this MIN report from the South Columbia near Hall Mountain.  

Recent storm snowfall amounts of 20-40cm overlie a widespread large surface hoar. We suspect this layer may still be preserved in many tree line locations which were not affected by high freezing levels and rain events. In other words, if there is no rain crust at and below tree line, be on the look out for buried surface hoar.  

In the Alpine, recent winds will have redistributed snow. These storm slabs sit atop suncrust on solar aspects.  

The most notable layer in the lower snowpack is 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 shallow rocky slopes in the alpine.

Terrain and Travel

  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • In areas where deep persistent slabs may exist, avoid shallow or variable depth snowpacks and unsupported terrain features.

Problems

Storm Slabs

An icon showing Storm Slabs

Below treeline (& possibly up to 2400m): High freezing levels and rain events created a rain crust (or two). New snow is not bonding well to these near surface rain crusts.

At and near tree line, the 20-40cm of new snow rests on widespread layer of large surface hoar crystals. Shooting cracks, remote triggering and settlements (whumphing) are classic signs of instability that buried surface hoar is present and potentially reactive. 

At upper elevations where recent winds transported new snow, storm slabs formed in lee features (Northerly and Easterly slopes) and will be especially touchy where they sit over a crust.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 2

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

Unlikely

Expected Size

2 - 3

Valid until: Dec 10th, 2020 5:00PM

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