Avalanche Forecast Purcells

Sunday 3rd February 2019

Avalanche Danger Ratings Sun 3rd Feb 4:07PM Danger Ratings Alpine: Considerable Danger Ratings Treeline: Considerable Danger Ratings Below Treeline: Considerable Wind Slabs Wind Slabs Persistent Slabs Persistent Slabs Deep Persistent Slabs Deep Persistent Slabs

Avalanche Canada Forecaster: kdevine

Date Issued:

Valid Until:

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

SUNDAY NIGHT - Mainly cloudy with clear periods / light northeast winds / alpine low temperature near -22MONDAY - Mainly cloudy with sunny periods / northeast winds 10-15 km/h / alpine high temperature near -17TUESDAY - Sunny with cloudy periods / northeast winds 15-25 km/h / alpine high temperature near -14WEDNESDAY - A mix of sun and cloud / northwest winds, 15-35 km/h / alpine high temperature near -14

Avalanche Summary

Several explosives triggered avalanches to size 2.5 were reported on Sunday.Widespread avalanche activity continued on Saturday with reports of numerous natural avalanches to size 3.5, as well as explosives triggered avalanches to size 2.5, and human triggered avalanches to size 1.5. Many of these avalanches failed on a persistent weak layer that was buried in mid January.A widespread natural avalanche cycle to size 3 was reported on Friday. There were also several human triggered avalanches to size 1.5 reported in the region on Friday. Some of these reportedly failed on the persistent weak layer that was buried in mid January. This MIN report illustrates how reactive the new snow was on Friday.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 new snow has fallen in the region since Thursday. This 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.

Wind Slabs Wind Slabs

Likelihood

Likely - Possible

Expected Size

1 - 2

Up to 30-60 cm of new snow has fallen in the region. In many areas, recent winds have formed wind slabs at all elevations and on all aspects due to shifting wind directions.

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

Aspects: All aspects.

Elevations: Alpine, Treeline.

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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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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.

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

Aspects: All aspects.

Elevations: Alpine, Treeline.

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