Avalanche Forecast

Archived

Feb 23rd, 2021–Feb 24th, 2021

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

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, human triggered possible.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

Kootenay Boundary.

Lingering wind slabs may remain reactive to human triggers at treeline and above on Wednesday. Avoid lee and cross-loaded terrain.

Confidence

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

Weather Forecast

TUESDAY NIGHT: Partly cloudy / Light, northwest ridgetop wind / Alpine low -14 / Freezing level valley bottom.

WEDNESDAY: Sunny / Light, northwest ridgetop wind / Alpine high -3 / Freezing level 1200 m.

THURSDAY: Snow; 5-10 cm. / Strong, southwest ridgetop wind / Alpine high -3 / Freezing level 1200 m.

FRIDAY: Cloudy with isolated flurries; 0-3 cm. / Light, northwest ridgetop wind / Alpine high -3 / Freezing level 1100 m.

Avalanche Summary

On Tuesday, explosive control work on Kootenay Pass produced numerous wind slabs up to size 2.5 on primarily south aspects, with crowns up to 100 cm. in wind loaded features.

There were many reports of human triggered persistent slab avalanches during the first two weeks of February. These avalanches failed on a surface hoar layer that is now 50-100 cm deep and is most prevalent at treeline elevations. There were also two very large explosive triggered avalanches in the Bonningtons in the past month that stepped down to the early December crust (most recently on Feb 9). These deeper instabilities have been trending towards being unreactive, however occasional snowpack tests suggest they may still be possible to trigger in isolated areas.

Snowpack Summary

25-50 cm of recent snow and strong southwest winds have formed reactive wind slabs on lee and cross loaded features at treeline and above.

The main persistent layer of concern is a combination of surface hoar, facet, and crust layers that formed in January that are now buried 50-100 cm deep. No avalanches have been reported on this layer in the past week. Steep, open slopes at treeline elevations are the most likely spots to trigger a large avalanche on this layer.

Terrain and Travel

Be careful as you transition into wind affected terrain.
Avoid lee and cross-loaded terrain.
Approach steep open slopes at and below treeline cautiously, buried surface hoar may exist.
Be aware of the potential for large avalanches due to the presence of a persistent slab.

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.