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Avalanche Forecast

Archived

Feb 5th, 2021–Feb 6th, 2021

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

Regions

Purcells.

Continued discipline and careful route finding will be required on Saturday to avoid triggering large avalanches on a buried persistent weak layer of surface hoar. This layer has been especially reactive at treeline elevations.

Confidence

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

Weather Forecast

  

FRIDAY NIGHT: Cloudy / Strong, northwest ridgetop wind / alpine low temperature -16 / Freezing level valley bottom.

SATURDAY: Cloudy with flurries; 3-10 cm. / Moderate, west ridgetop wind / alpine high temperature -7 / Freezing level valley bottom.

SUNDAY: Mix of sun and cloud / Moderate, northwest ridgetop wind / alpine high temperature -12 / Freezing level valley bottom.

MONDAY: Sunny / Light, northwest ridgetop wind / alpine high temperature -13 / Freezing level valley bottom.

Avalanche Summary

There was a recent report of skier remotely triggering a size 2.5 persistent slab from a ridgetop at treeline near Quartz Creek on Wednesday. This avalanche failed on the very weak layer of surface below the recent storm snow. It was reported that light wind effect had stiffened the recent storm snow just enough to be highly reactive to skier triggering. Min report HERE.

Touchy storm slabs are sitting on a weak layer of surface hoar which is likely to be reactive to human triggers. Avalanches may be larger than expected due to the presence of this weak layer.

Snowpack Summary

Southwest switching to northwesterly winds may form reactive wind slabs on a variety of aspects at treeline and above.

40-60 cm. of recent snow is sitting on a very weak layer of surface hoar which has potential to surprise backcountry users by how easily it may be triggered and how wide the persistent slabs propagate across slopes. Recent avalanches on this layer have primarily been reported at treeline and below.

60-100 cm. down is an older surface hoar and/or a thin melt crust from mid January. This layer was more prominent in the north Purcells in sheltered, open slopes at treeline.

Deep persistent weak layers can still be found in the lower snowpack. These weak layers are most likely to be triggered from rocky areas with a shallow or thin to thick snowpack.

Terrain and Travel

  • Be aware of the potential for larger than expected storm slabs due to the presence of buried surface hoar.
  • Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
  • Avoid open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.
  • Be careful as you transition into wind affected terrain.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

Problems

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.

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.