Avalanche Forecast

Issued: Jan 28th, 2019 3:00PM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Deep Persistent Slabs, Persistent Slabs and Wind Slabs.

ghelgeson, Avalanche Canada

Purcells.

The snowpack in the Purcells this season is exceptional, and not in a good way. We've summarized our thoughts on this precarious snowpack in the latest forecasters blog here. Take it easy out there, this is not the time for the big lines.

Summary

Confidence

Moderate - Wind effect is extremely variable

Weather Forecast

High pressure, cold temperatures and clear skies should allow for great travel and visibility Tuesday. Cloud cover starts to build back in Wednesday ahead of two storm pulses that should deliver a bit of snow beginning on Friday.MONDAY NIGHT: Freezing level at valley bottom, light northerly wind at most elevations with moderate northwest wind at ridgetop, no precipitation expected.TUESDAY: Clear skies, freezing level at valley bottom, light variable wind, no precipitation expected.WEDNESDAY: Broken cloud cover, freezing level at valley bottom, mostly light variable wind with moderate northwest wind at ridgetop, no precipitation expected.THURSDAY: Scattered cloud cover, freezing level at valley bottom creeping up towards 1000 m in the afternoon, light variable wind, trace of snow possible.

Avalanche Summary

Wind slabs to size 2.5 were observed Sunday on north, northeast, south and southwest facing terrain features between 1900 and 2800 m. These are likely the wind slabs that formed Saturday night.On Thursday explosive control work released another slide on the deep persistent layer at the base of the snowpack. There is a great MIN report from January 22nd that shows a natural avalanche (size 3.5) in International Basin, also on the deep persistent layer. Check it out here.

Snowpack Summary

Saturday delivered warmth and strong to extreme wind out of the south, southwest and northwest. This left a robust crust (up to 4 cm thick) on steep south facing aspects and redistributed snow into potentially deep wind slabs in lee alpine features. The warmth also allowed 10 to 50 cm of snow to settle into a well consolidated slab above the mid-January persistent weak layer (PWL). This PWL is made up of large surface hoar that is most prevalent at and below treeline. On steep solar aspects this interface presents as a crust, there may be locations where the surface hoar actually sits on the crust.The snowpack is also harboring a nasty weak layer near the ground that is made up of weak faceted grains above a crust. This interface continues to produce sporadic avalanches that are very large and destructive. Avalanches triggered on this weak layer take the entire seasons snowpack with it and have resulted in numerous close calls and serious incidents. This layer is most likely to be triggered from zones where the snowpack is thin and weak. Rock outcroppings and ridge crests around large open slopes and bowls in the alpine are particularly suspect

Problems

Deep Persistent Slabs

The last avalanche observed on the weak facet/crust interface at the ground was Thursday, but human triggered avalanches remain possible. Is it a 1 in 100 slope, or a 1 in 1000 slope problem now? We're not sure and this is what makes it so tricky.

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.

Likelihood

Possible

Expected Size

2.5 - 4

Persistent Slabs

10 to 50 cm of snow sits above a weak interface of surface hoar and crust. The recent warm to cold temperature transition has allowed a more cohesive slab to form above the surface hoar which is most prevalent between at and below treeline.

Take heed of obvious signs of instability such as whumpfing, shooting cracks and recent avalanches.Avoid convex rolls at and below treeline where buried surface hoar may be preserved.Use extra caution around steep open terrain features, such as cutblocks, gullies and cutbanks.

Aspects: All aspects.

Elevations: Treeline, Below Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2

Wind Slabs

Wind over the weekend was out of the south, southwest, west and northwest with more than enough oomph to form wind slabs. These wind slabs may rest on a crust, or perhaps even buried surface hoar near treeline.

Avoid areas where the snow feels stiff and/or slabby.Wind slabs in motion may step down to deeper layers resulting in very large avalanches.Be careful around freshly wind loaded features, especially near ridge crests and roll-overs.

Aspects: All aspects.

Elevations: Alpine.

Likelihood

Unlikely - Possible

Expected Size

1.5 - 2.5

Valid until: Jan 29th, 2019 2:00PM