Avalanche Forecast

Issued: Dec 28th, 2020 4:00PM

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

Avalanche Canada cmortenson, Avalanche Canada

Email

Remain disciplined in terrain selection: Human triggered avalanches remains possible in shallow rocky snowpacks and / or in steep and convex terrain. Remember buried weak layers can result in large avalanches & the consequence of a large destructive avalanche is high.

Summary

Confidence

Moderate - Confidence is due to a stable weather pattern; little change is expected for several days.

Weather Forecast

Mild cool weather for the forecast period with a mix of sun and clouds with isolated flurries. The next notable precipitation looks like the end of the week.  

Monday night: Mainly cloudy with clear periods. Alpine low temperatures near -5C and freezing levels valley bottom. Ridgetop wind moderate to strong from the west/northwest. Mild alpine temperature inversion with cool cloudy valleys and warmer clear skies above. 

Tuesday: A mix of sun and clouds with with isolated flurries by the afternoon. Alpine high temperatures near -4C and freezing levels valley bottom. Ridgetop wind increase to moderate gusting strong from the southwest. A mild alpine temperature inversion for the morning breaks down mid-day.

Wednesday: Mainly cloudy with sun breaks and isolated flurries. Alpine high temperatures near -6C and freezing levels valley bottom. Ridgetop wind easing to light to moderate from the southwest/southeast. 

Thursday: Mainly cloudy with sun breaks and isolated flurries. Alpine high temperatures near -4C and freezing levels valley bottom. Ridgetop wind moderate to strong from the west/southwest. 

Avalanche Summary

No new natural slab avalanches to report from recent days in the interior region, There have been explosive triggered large (size 2) result in recent wind slabs and loose dry avalanches in steep terrain. A very large avalanche was accidentally triggered in the Nass Area in the NW coast. This release was a deep wind slab that may have failed on a surface hoar layer.

There remains a low probability high consequence situation with buried weak layers. A week ago there were a few large avalanches that involved the December crust. A significant avalanche from the deep basal facets was reported in this MIN report from Dec 22nd showed a deep release from a leeward high alpine feature off the north side of Hudson Mountain. The deep persistent slab problem may be isolated, however it is very high consequence if you find yourself involved in an avalanche of this size. While these large destructive avalanches remain unlikely they remain of high consequence.

Snowpack Summary

Light snow fall amounts (10-30cm) in recent days have buried recent wind affected snow in alpine and exposed tree line regions. Underneath this soft snow you may encounter scouring, buried windslabs, cross loading or reverse loading patterns.  

The December crust layer that featured in several avalanche reports from Dec 19th and Dec 17th appears to slowly be gaining strength. It remains a notable weak layer and we continue to monitor it. This MIN from the 27th found the crust down 65cm.

A crust that was buried in early November sits near the base of the snowpack. This crust has weak facets associated with it. These facets have produced large avalanches in both the south and north of the region. This MIN report from December 22 highlights a deep release avalanche and why we continue to advise caution around shallow rocky areas in the alpine.

Snowpack depths vary substantially with aspect, elevation, and wind exposure.

Terrain and Travel

  • Be alert to conditions that change with elevation, aspect and exposure to wind.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • In areas where deep persistent slabs may exist, avoid shallow or variable depth snowpacks and unsupported terrain features.

Problems

Wind Slabs

An icon showing Wind Slabs

Wind slabs may be reactive to human triggering, especially in areas where the wind has stiffened the slab or if solar inputs warm steep faces. Wind slabs may be found on all aspects at upper elevations because the wind direction has changed. Cornices along ridgecrests represent a hazard underfoot but are most problematic when they collapse above wind loaded avalanche paths and can initiate large avalanches. 

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

1 - 2

Persistent Slabs

An icon showing Persistent Slabs

A 30-70 cm thick persistent slab sits above the early December crust. Around this crust are facets and potentially surface hoar in sheltered areas. Human-triggered avalanches have stepped-down to this layer producing size 2 avalanches in the last week. This bond is gaining strength, but may still be reactive in steep, convex or unsupported terrain or in a step-down avalanche or cornice collapse.    

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

1.5 - 2.5

Deep Persistent Slabs

An icon showing Deep Persistent Slabs

Basal facets have the potential of forming large and destructive avalanches and continues to show up in test pits as reactive. The most likely place to trigger this layer would be on shallow and rocky slopes. Smaller avalanches or cornice collapse may also have the potential of stepping down to this layer. 

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely

Expected Size

2 - 3

Valid until: Dec 29th, 2020 4:00PM