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Avalanche Forecast

Archived

Jan 7th, 2021–Jan 8th, 2021

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

Regions

South Rockies.

Sporadic large natural persistent slab avalanches have recently been reported in this region. This low likelihood, high consequence scenario is best avoided by choosing conservative terrain away from overhead hazard.

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

  

THURSDAY NIGHT: Clear / Light, southwest ridgetop wind / alpine low temperature -14 / Freezing level valley bottom.

FRIDAY: Mix of sun and cloud / Light, southwest ridgetop wind / alpine high temperature -6 / Freezing level valley bottom.

SATURDAY: Sunny / Light, southwest ridgetop wind / alpine high temperature -8 / Freezing level valley bottom.

SUNDAY: Sunny / Moderate, northwest ridgetop wind / alpine high temperature -8 / Freezing level valley bottom.

Avalanche Summary

A large (size three) naturally triggered persistent slab avalanche was reported on a large alpine feature near Crowsnest Pass on Thursday. This avalanche was triggered by either a small wind slab in motion or a cornice fall. This ia an example of the "low probability; high consequence" scenario that persistent slab problems often create.

Explosive avalanche control produced a few size 2-3 wind slab avalanches on Tuesday. Last weekend there were numerous natural size 2-3 slab avalanches. Most of these were storm slab avalanches in alpine terrain, although some of the reports from the upper Elk Valley included avalanches at treeline elevations that likely failed on 60-100 cm deep surface hoar layers.

Snowpack Summary

Moderate snowfalls and strong southwest wind over the past few days has left hard surfaces and wind slabs in exposed terrain. The upper snowpack consists of several layers of old wind slabs, while the lower snowpack consists of decomposing crusts and weak faceted snow. In the Elk Valley a decomposing surface hoar layer can be found around one of these crusts 50-100 cm below the surface. Avalanche activity on these layers in the lower snowpack have been sporadic, mostly occurring during natural avalanche cycles. However, this snowpack structure is always a concern for human triggering on steep rocky slopes with variable snowpack depths.

Terrain and Travel

  • Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.
  • Be aware of the potential for large avalanches due to the presence of buried weak layers.
  • If triggered loose wet avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid areas with overhead hazard.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.