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Avalanche Forecast

Archived

Dec 28th, 2020–Dec 29th, 2020

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Northwest Inland.

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.

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

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.