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Avalanche Forecast

Archived

Feb 22nd, 2021–Feb 23rd, 2021

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

Regions

Purcells.

Human triggered avalanches are likely at upper elevations on Tuesday. Be especially wary of recently wind loaded features. Fresh slabs sit on a thick layer of weak, sugary facets and bonding at this interface may take some time.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength. Recent weather patterns have resulted in a high degree of snowpack variability within the region.

Weather Forecast

MONDAY NIGHT: Up to 10 cm of snow, moderate to strong northwest wind, freezing level dropping from 1500 m to valley bottom.

TUESDAY: Scattered flurries with sunny periods, light northwest wind, freezing level 1000 m.

WEDNESDAY: Sunny, light northwest wind, freezing level 1000 m.

THURSDAY: Scattered flurries, moderate southwest wind easing to light west, freezing level 1000 m.

Avalanche Summary

Preliminary reports from Monday include several size 1-1.5 natural and explosive triggered wind slabs up to 40 cm deep in one of the lowest snowfall areas of the region.

Earlier in February, there were a few reports of large avalanches (size 2.5-3) on southeast aspects in the alpine, likely releasing on a layer of facets on a crust (see a photo here), and a large human-triggered avalanche (size 2.5) on surface hoar near Quartz Creek.

Snowpack Summary

10 to 40 cm of new snow sits overtop of the old, faceted and/or wind affected snow that sat on the surface during the extended cold, windy drought period. At upper elevations, the new snow has been blown around by strong to extreme westerly winds, forming reactive slabs in lee features.

A weak layer that formed in late January is now 30-70 cm deep. In the northern Purcells this layer has been reported as a surface hoar layer at treeline and below treeline elevations, but a combination of facets on crusts could exist at all elevations throughout the region. 

The northern Purcells also have an older surface hoar layer that can still be found 60-100 cm deep at treeline. Additional weak layers may exist near the bottom of the snowpack and steep rocky areas with a shallow or thin to thick snowpack should be considered suspect trigger points.

Terrain and Travel

  • Fresh snow rests on a problematic persistent slab, don't let good riding lure you into complacency.
  • Stay off recently wind loaded slopes until they have had a chance to stabilize.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.