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Avalanche Forecast

Archived

Dec 14th, 2021–Dec 15th, 2021

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 Coastal.

Wind slabs continue to be the main concern on Wednesday. Due to recently shifting wind directions, slabs should be expected on all aspects in exposed terrain. 

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Arctic high pressure remains the dominant feature for the forecast period but a low pressure system is expected to slide down the coast on Wednesday bringing increased cloud with a chance of flurries along the immediate coastal areas. 

Tuesday Overnight: Partly cloudy, light to moderate S winds, treeline temperature around -16 °C.

Wednesday: A mix of sun and cloud with isolated flurries, light to moderate variable winds, treeline high around -10 °C. 

Thursday: A mix of sun and cloud, moderate to strong NW winds, treeline high around -12 °C.

Friday: Mainly cloudy with light snowfall, light to moderate SW winds, treeline high around -10 °C.

Avalanche Summary

On Monday, a natural size 3 wind slab was reported in the north of the region on a S-SE aspect in the alpine. Explosives were triggering wind slabs up to size 2.5 in the north and south of the region. This MIN Report shows a snow cat triggered avalanche near Terrace. 

On Wednesday, wind slabs are expected to remain the primary concern in the region. Newer wind slabs which formed on Monday from NE outflow winds are expected to be most reactive but the older wind slabs from the weekend may still be reactive, especially in steep, unsupported terrain features. The persistent slab problem appears to generally have stabilized in the south of the region but remains a concern in the north. We are now likely in a low probability/high consequence scenario with the persistent problem. 

Snowpack Summary

Over the weekend, the region typically saw 20-50 cm of new snow with the greatest amounts towards the SW of the region. This new snow was accompanied by strong SW winds have formed reactive wind slabs in exposed terrain. On Monday, outflow (NE-E) winds likely formed new wind slabs on the opposite aspects compared to the weekend storm and as a result, wind slabs should be expected on all aspects. 

The December 7 persistent weak layer is now typically down 40-70 cm except for near the immediate coast where it is closer to 100 cm. This interface may include a crust, facets, and/or surface hoar. This layer appears to be gaining strength through the south of the region but remains a concern for the north of the region. A bit deeper in the snowpack is the December 1 crust/facet layer which generally appears to have stabilized but may still be reactive in isolated areas. 

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • 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 buried weak layers.

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.