Avalanche Forecast Purcells

Monday 11th February 2019

Avalanche Danger Ratings Mon 11th Feb 3:24PM Danger Ratings Alpine: Moderate Danger Ratings Treeline: Moderate Danger Ratings Below Treeline: Moderate Wind Slabs Wind Slabs Persistent Slabs Persistent Slabs Deep Persistent Slabs Deep Persistent Slabs

Avalanche Canada Forecaster: jfloyer

Date Issued:

Valid Until:

Wind slabs may be touchy on all aspects in exposed terrain. There has also been recent large human-triggered avalanches and other signs of instability on buried surface hoar layers. See our Forecaster Blog for more info here.

Confidence

High -

Weather Forecast

MONDAY NIGHT: Cloudy with intermittent snowfall, accumulation 1 to 3 cm, moderate southwest wind.TUESDAY: Cloudy with light snowfall, accumulation trace to 10 cm with the most in the south of the region, moderate to strong southwest wind, alpine temperature -15 C.WEDNESDAY: Cloudy with light snowfall and afternoon clearing, accumulation 1 to 3 cm, light northwest wind, alpine temperature -14 C.THURSDAY: Increasing clouds over the day, light east wind, alpine temperature -10 C.

Avalanche Summary

A few large wind slab avalanches were observed in the region on Sunday, generally in alpine terrain and up to 60 cm deep. Otherwise, cornices were reactive to explosive control.The January persistent weak layers described in the Snowpack Summary continue to be reactive to human triggers. The most recent activity was on Friday, as skiers triggered large avalanches on east and southeast slopes between 1900 and 2300 m. Check out the MIN reports here, here and here for examples of these weaknesses.

Snowpack Summary

Recent strong winds from all directions have affected the snow surface at all elevation bands. Expect to find wind slabs in terrain depressions, such as adjacent to ridges, and otherwise wind affected snow on all aspects.Beneath this lies two weak layers of surface hoar that were buried at the end of January and mid-January. These layers are around 30 to 60 cm deep. The mid-January layer may be associated with a melt-freeze crust on southerly aspects. These layers are most prominent at treeline and below treeline elevations.The base of the snowpack is composed of weak and sugary faceted grains that overly a melt-freeze crust. This weak layer has produced large and destructive avalanches that are sporadic in nature and very difficult to predict. It 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 terrain features to be wary of. See the Forecaster Blog here for more info on this problem.

Wind Slabs Wind Slabs

Likelihood

Possible

Expected Size

1 - 2

New wind slabs will likely from on Tuesday with strong southwest winds. Wind slabs from the weekend developed with northerly winds. Wind slabs may therefore be touchy on all aspects and at all elevation bands.

If triggered, wind slabs may step down to deeper layers and result in large avalanches.Use caution in freshly wind-loaded features, especially near ridge crests and in steep terrain.Avoid open slopes and steep terrain, since wind slabs may not bond well to underlying layers.

Aspects: All aspects.

Elevations: Alpine, Treeline.

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Persistent Slabs Persistent Slabs

Likelihood

Possible

Expected Size

1.5 - 2.5

Two weak layers of surface hoar crystals are buried between 30 and 60 cm in the snowpack. The layers are most prominent around treeline and below treeline elevations. Skiers have recently triggered large avalanches on these layers.

Any steep opening in the trees should be treated as suspect right now.Avoid steep, open and/or sparsely treed slopes at and below treeline.Be aware of the potential for wide propagation.

Aspects: All aspects.

Elevations: Treeline, Below Treeline.

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Deep Persistent Slabs Deep Persistent Slabs

Likelihood

Possible - Unlikely

Expected Size

2.5 - 3.5

Particularly in thin snowpack areas, the base of the snowpack is composed of weak faceted grains and a crust. This layer has produced sporadic but very large avalanches. The probability of triggering this layer is low, but the consequences 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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