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Avalanche Forecast

Archived

Feb 21st, 2021–Feb 22nd, 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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Purcells.

Fresh wind slabs may be surprisingly sensitive to triggering as they form over old, faceted surfaces. 

For areas along the northwestern border of the region with storm snow totals greater than 30 cm, refer to South Columbias forecast.

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system.

Weather Forecast

SUNDAY NIGHT: 5-15 cm of new snow. Freezing level 1500 m. Strong to extreme westerly ridgetop wind.

MONDAY: 5-10 cm of snow. Freezing level 1500 m. Strong west wind easing slightly.

TUESDAY: Mix of sun and cloud, light northwest wind, freezing level 1000 m.

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

Avalanche Summary

Preliminary reports from Sunday suggest a few small natural wind slabs and loose dry avalanches running in the new snow. Cornices remain large and fragile but there have been no reports of cornice falls since last weekend.

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

10-25 cm of new snow falls 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 will be easily blown around by strong to extreme westerly winds, forming reactive slabs in lee features.

A weak layer that formed in late January is now 30-60 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 wind slabs will likely form throughout the day, diligently watch for changing conditions.
  • 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

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.