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Avalanche Forecast

Archived

Dec 13th, 2021–Dec 14th, 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 primary concern on Tuesday. Due to recently shifting wind directions, slabs should be expected on all aspects in exposed terrain.

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

An Arctic high pressure system is expected to be the dominant feature for most of the week bringing cold and dry conditions to the region.

Monday Overnight: Mostly clear skies, light to moderate NE winds, treeline temperature around -16 °C.

Tuesday: Mostly sunny, light to moderate variable winds, treeline high around -16 °C. 

Wednesday: A mix of sun and cloud, light to moderate variable winds, treeline high around -12 °C. 

Thursday: A mix of sun and cloud, moderate to strong NW winds, treeline high around -10 °C.

Avalanche Summary

On Sunday, only a few explosive triggered slab avalanches were reported in the region up to size 2.5. On Saturday, two natural size 2 wind slab avalanches were reported on NW aspects in the alpine and explosives were triggering storm slab avalanches up to size 3. 

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

Snowpack Summary

Over the weekend, the region typically saw 20-50 cm of new snow the largest 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, strong outflow (NE-E) winds are expected to form new wind slabs on the opposite aspects from the weekend storm and 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 but may still be reactive to human triggers in some areas. 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.