Avalanche Forecast

Issued: Dec 29th, 2020 4:00PM

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

Avalanche Canada cmortenson, Avalanche Canada

Stick to conservative terrain during stormy weather and watch for signs of instability. New snow and winds will add load to existing weak layers, making human triggered avalanches likely. Caution in known hot spots near the Nass Valley / Sterling and Beaupre riding areas.  

Summary

Confidence

Moderate - Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather. Uncertainty is due to field data and reports showing a wide variation in conditions throughout the region.

Weather Forecast

Tuesday night: Scattered flurries, trace-5 cm new snow, alpine low temperatures - 3C, freezing levels 300m winds moderate south. 

Wednesday: Snow flurries, 5-20 cm new snow, alpine high temperatures -3C, freezing levels 500m & ridgetop winds moderate from variable directions. Southern regions are forecast to receive higher snowfall amounts.

Thursday: Mainly cloudy with scattered flurries, trace-5 cm new snow, alpine high temperatures -4C, freezing levels 300m & ridgetop winds light to moderate west. 

Friday: Snow flurries, 10-20 cm new snow, alpine high temperatures -3C, freezing levels 700m & ridgetop winds moderate to strong southeast. 

Avalanche Summary

Fresh wind slabs continue to produce avalanches (size 1-2) when tested with explosive and by skis. There was also a large natural avalanche (size 2.5) reported on the 27th east of Terrace. Accidentally triggered avalanches illustrate continued reactivity in the riding areas north of Terrace. We are glad everyone appears to be ok. 

On the 28th, there was an accidentally triggered large avalanche (size 2) reported in the Sterling area. On the 27th an accidentally triggered very large (size 3) avalanche was reported from the Beaupre riding area with a crown of 1-1.5 m. While neither reports a failure plane, buried surface hoar is likely since touchy conditions were reported in this region since the 21st on surface hoar (see this MIN & this MIN).  

At the height of the Christmas Eve storm and for the following days, widespread large and very large avalanches were reported. One avalanche among these that stands out was a very large (size 3.5) wind slab was reported to have released naturally on a southwest aspect and technicians suspect this deep wind slab failed on surface hoar following east winds.  

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Snowpack Summary

Forecast 10-25 cm snow by the end of the day Wednesday will arrive with moderate to strong south/southwest winds which will build fresh wind slabs in exposed treeline and above. This new snow will bury recently formed surface hoar as well as add load to existing weak layers buried deeper in the snowpack. Buried older wind slabs(100-150 cm) may remain reactive, especially where they sit on top of surface hoar.

Significant variability exists with buried weak layers. Beneath recent 10-25 cm is a surface hoar layer that has been reported in the Shames areas and appears quite widespread treeline and below. Last week's 40-60 cm of snow may sit atop a crust or surface hoar layer as was reported in Shames. A zone of reactivity has been noted in riding areas along highway 113 north of Terrace, which may be due to a deeper surface soar layer in this zone coupled with recent wind slabs. A crust that was buried in early December is now down 90-200 cm plus and has facets and potentially the above mentioned surface hoar in sheltered areas. 

The early-November crust is buried around 170-200 cm at treeline and deeper in the alpine. This crust may have weak and sugary facets above it in parts of the region. This layer is currently dormant but remains on our radar.

Terrain and Travel

  • Fresh wind slabs will likely form throughout the day, diligently watch for changing conditions.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Be aware of the potential for large avalanches due to the presence of a deep persistent slab.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.

Problems

Wind Slabs

An icon showing Wind Slabs

10-25cm new snow by Wednesday evening will fall with moderate to strong south/southwest winds. This new snow may form reactive wind slabs in immediate lee of ridgecrests at treeline and above, especially where a buried surface hoar layer is present.   

New snow will bury older wind slabs (up to 1.5 meters thick) from strong winds from the Christmas Eve storms, which have been reactive in recent days. Winds from this Christmas period were highly variable and included periods of strong east winds that led to reverse loading and cross-loading.   

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 3

Persistent Slabs

An icon showing Persistent Slabs

Significant variability exists with buried weak layers. A weak layer of surface hoar is buried 50-90 cm below recent storm snow and has been found in the Shames area up to ridgecrest & elsewhere. A deeper surface hoar and crust /facet layer was buried in earlier in December and is down 90-200 cm. Wide propagation and step-down avalanches are a significant possibility with these layers, as was evident in the recent Beaupre avalanche.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

1.5 - 3

Valid until: Dec 30th, 2020 4:00PM