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Avalanche Forecast

Archived

Feb 24th, 2021–Feb 25th, 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

Kootenay Boundary.

New snow and wind are expected to form fresh storm slabs throughout the day that will be most reactive in wind affected terrain. Pay attention to changing conditions with elevation and aspect.

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system. Forecast snowfall amounts are uncertain.

Weather Forecast

  

WEDNESDAY NIGHT: Cloudy with flurries; 3-5 cm. / Strong, southwest ridgetop wind / Alpine low -14 / Freezing level valley bottom.

THURSDAY: Snow; 10-15 cm, with another 3-10 cm. overnight / Strong, west ridgetop wind / Alpine high -4 / Freezing level 1100 m.

FRIDAY: Mostly cloudy / Light, northwest ridgetop wind / Alpine high -3 / Freezing level 1200 m.

SATURDAY: Mix of sun and cloud / Moderate, northwest ridgetop wind / Alpine high -4 / 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

Weak surface hoar sitting on a crust on solar aspects has been reported in the Kootenay Pass area. This layer may also be present in other areas within the region. Storm slabs formed by incoming snow and wind Thursday will be more sensitive to human triggering in areas where this weak layer was preserved before being buried; especially if the surface hoar is sitting on a crust.

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 below the surface is a combination of surface hoar, facet, and crust layers that formed in January that are now buried 60-150 cm. deep. 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

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.