Avalanche Forecast Purcells

Wednesday 6th February 2019

Avalanche Danger Ratings Wed 6th Feb 5:00PM Danger Ratings Alpine: Moderate Danger Ratings Treeline: Considerable Danger Ratings Below Treeline: Considerable Persistent Slabs Persistent Slabs Wind Slabs Wind Slabs Deep Persistent Slabs Deep Persistent Slabs

Avalanche Canada Forecaster: mbender

Date Issued:

Valid Until:

There is a potential for triggering large avalanches at lower elevations due to the presence of a buried weak layer that exists mainly at treeline and below.

Confidence

High - The weather pattern is stable

Weather Forecast

THURSDAY - Mainly cloudy with light flurries / light southwest winds / alpine high temperature near -16FRIDAY - A mix of sun and cloud / moderate northeast wind / alpine high temperature near -17SATURDAY - A mix of sun and cloud / moderate east wind / alpine high temperature near -18

Avalanche Summary

Natural avalanche activity began to slow down on Sunday, but a persistent weak layer that was buried in mid January continues to be reactive to human triggers. This layer is sensitive enough avalanches to be triggered remotely (from a distance). Human triggered avalanches up to size 2 were reported on Sunday and Monday. This MIN report from Sunday illustrates the potential for humans to remotely trigger the mid January layer.Widespread avalanche activity was reported on Friday and Saturday. Numerous natural avalanches to size 3.5, explosives triggered avalanches to size 2.5, and human triggered avalanches to size 1.5 were reported. Many of these avalanches were triggered remotely (from a distance) and failed on the mid January layer. A few of the recent explosives triggered avalanches stepped down to ground.

Snowpack Summary

Approximately 30-60 cm of recent new snow sits on wind slab, surface hoar (feathery crystals), facets (sugary snow) and a crust on sun-exposed slopes. A very notable feature in the snowpack at this time is a persistent weak layer that was buried in mid January, which is now buried 50-90 cm. This layer consists primarily of surface hoar, however there is also a crust associated with it on sun-exposed slopes. This layer has been most reactive at treeline and below.The base of the snowpack has a deep persistent weak layer near the ground. This layer consists of facets over a crust. This weak interface continues to produce large and destructive avalanches that are sporadic in nature, and very difficult to predict. This layer is most likely to be triggered from areas where the snowpack is shallow and weak. Rocky alpine bowls, ridge crests and rocky outcroppings are some examples of the kind of terrain to be wary of.

Persistent Slabs Persistent Slabs

Likelihood

Likely - Possible

Expected Size

1 - 2.5

50-90 cm of snow now sits on a weak layer of surface hoar and crust that was buried in mid January. This layer has been most reactive at lower elevations.

Choose low angled terrain.Any steep opening in the trees should be treated as suspect right now.

Aspects: All aspects.

Elevations: Treeline, Below Treeline.

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Wind Slabs Wind Slabs

Likelihood

Possible

Expected Size

1 - 2

30-60 cm of recent new snow has been redistributed by recent north and east winds.

Watch for whumpfing, hollow sounds, shooting cracks or recent avalanches.Be careful with wind loaded pockets, especially near ridge crests and roll-overs.If triggered, wind slabs may step down to deeper layers and result in large avalanches.

Aspects: South East, South, South West, West.

Elevations: Alpine.

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Deep Persistent Slabs Deep Persistent Slabs

Likelihood

Possible

Expected Size

3 - 4

The base of the snowpack has a very weak layer that continues to produce very large avalanches from time to time. The probability of triggering this layer is somewhat low, but the consequences are very high.

Avoid making assumptions about this layer based on aggressive tracks on adjacent slopes.Use caution on alpine slopes, especially around thin areas that may propagate to deep instabilities.Minimize overhead exposure; avalanches triggered by cornice fall may be large and destructive.

Aspects: All aspects.

Elevations: Alpine, Treeline.

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