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Avalanche Forecast

Archived

Mar 5th, 2019–Mar 6th, 2019

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

Regions

Purcells.

Wind slabs continue to be reactive at upper elevations. A persistent slab problem still exists at lower elevations and has been responsible for occasional human-triggered avalanches.

Confidence

Moderate -

Weather Forecast

TUESDAY NIGHT - Clear periods / southeast winds 10-30 km/h / alpine low temperature near -12WEDNESDAY - Cloudy with sunny periods and isolated flurries / southeast winds 10-25 km/h / alpine high temperature near -10THURSDAY - Mainly cloudy with flurries, 5 cm / southwest winds 10-30 km/h / alpine high temperature near -8FRIDAY - Cloudy with isolated flurries / west winds, 15-30 km/h / alpine high temperature near -9

Avalanche Summary

On Monday, there were reports of a few human triggered avalanches, size 1-1.5. These were mainly wind slab avalanches, but there were two reports of persistent slab avalanches in the southwest of the region that failed on the persistent weak layer that was buried in early February.Here is a link to a video of an avalanche that occurred last week in the Purcells that illustrates the current wind slab problem in the region.Reports of persistent slab avalanches are becoming less frequent, suggesting our January/February weak layers have evolved into a lower likelihood/high consequence avalanche problem that still requires a measure of discipline to manage effectively.

Snowpack Summary

In most areas, the surface is primarily wind slabs, and a crust on sun exposed slopes. The wind slabs continue to be reactive, likely due to the fact that they are sitting on facets (sugary snow).Up to 25 cm of snow fell at the end of last week in the southwest of the region. This snow likely sits on wind slabs, and a crust on sun-exposed aspects. The snow surface is likely a crust on sun exposed slopes.Lower down there are up to three layers of surface hoar (weak, feathery crystals) that were buried in mid and late January, and early February. These layers are around 30 to 80 cm deep and are most prominent at lower elevations - especially below treeline. The surface hoar may sit on a crust on south facing slopes.The base of the snowpack is composed of weak and sugary faceted grains that sit on a crust. This weak layer has produced large and destructive avalanches that are sporadic in nature and difficult to predict. See the Forecasters' Blog here for more information on this problem. Continued cold temperatures have been weakening the lower snowpack.

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.