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Avalanche Forecast

Archived

Dec 20th, 2021–Dec 21st, 2021

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

North Columbia.

Strong alpine wind will keep blowing all day Tuesday. Watch for changing conditions as wind slabs will become more reactive.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

An Arctic ridge of high pressure will continue to build from the Yukon into the B.C interior bringing clear skies and cold conditions across the province with outflow winds through Tuesday. Abundant moisture from the Pacific will invade the province starting on Wednesday morning.

MONDAY NIGHT: Clear skies / Strong northwest wind / Low of -15

TUESDAY: Increasing cloudiness / Strong southwest wind / High of -15 / Snow starting in the evening 5-15 cm 

WEDNESDAY: Snow heavy at times 20-30 cm / Strong to extreme southwest wind / High -8 

THURSDAY: Flurries 5 cm/ Moderate west wind / High of -12

Avalanche Summary

A widespread natural cycle of storm slabs was observed at all elevations/ aspects after the last storm, including few very large storm slabs avalanches (size 2.5) as well as numerous loose dry avalanches up to size 1.5. 

A naturally triggered size 2.5 persistent slab avalanche was observed in the Monashees on Wednesday on a West aspect at 2150 m. This avalanche failed on the early December facet/crust combo. It was 100 cm deep, a good example of the high consequence of triggering this layer.

If you go out, make sure to report any observations on the Mountain Information Network.  

Snowpack Summary

30-40 cm of low-density snow can be found throughout the region, with significant amounts (up to 50 cm) in the southeast corner. Shifting winds have formed fresh wind slabs in isolated open areas at treeline and above. Many sluffs were skier-triggered in low-density snow on steep slopes. 

Below the new snow, the defining feature of the snowpack is a widespread crust that reaches as high as 2200 m and now sits 80-150 cm below the surface. In some places, overlying snow is well-bonded to the crust, but in others, weak faceted grains have been observed above it. The facets above the crust are most prominent at treeline where the crust is thinner. In areas where the crust is buried over a meter, it starts to decompose and show sporadic test results.

Terrain and Travel

  • Fresh wind slabs will likely form throughout the day, diligently watch for changing conditions.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Avoid shallow snowpack areas, rock outcroppings and steep convex terrain where triggering is most likely.
  • Seek out sheltered terrain where new snow hasn't been wind-affected.

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.