Avalanche Forecast Purcells

Monday 4th February 2019

Avalanche Danger Ratings Mon 4th Feb 4:44PM Danger Ratings Alpine: Considerable 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: kdevine

Date Issued:

Valid Until:

Natural avalanche activity has slowed down, but human triggered avalanches remain likely, especially in wind loaded areas and at lower elevations where a buried surface hoar layer has produced avalanches recently.

Confidence

Moderate -

Weather Forecast

MONDAY NIGHT - Clear periods / northeast winds 10-15 km/h / alpine low temperature near -18TUESDAY - A mix of sun and cloud / northeast winds 10-15 km/h / alpine high temperature near -13WEDNESDAY - Mainly sunny / northwest winds 15-25 km/h / alpine high temperature near -14THURSDAY - A mix of sun and cloud with isolated flurries / southwest winds, 15-35 km/h / alpine high temperature near -12

Avalanche Summary

Natural avalanche activity tapered to some degree on Sunday. Explosives triggered avalanches were reported to size 3, with one size 4 reported as well. Several human triggered avalanches to size 2 were reported, these failed on a persistent weak layer that was buried in mid January.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.There is a great MIN report from January 22nd that shows a natural avalanche (size 3.5) in International Basin, on the deep persistent layer. Check it out here. While this is an old observation, it is relevant as this layer is still lurking, and may catch people off guard.

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 persistent weak layer that was buried in mid January is now buried approximately 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.

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

Up to 30-60 cm of recent new snow has been redistributed by recent winds in many areas, forming wind slabs on all aspects due to a shift in wind direction.

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: All aspects.

Elevations: Alpine, Treeline.

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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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