Avalanche Forecast

Issued: Jan 21st, 2018 4:23PM

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

Avalanche Canada mconlan, Avalanche Canada

The snowpack is complex and being loaded by new snow and strong winds. Three weak layers are reactive to natural and human triggers. The easy solution is to choose simple terrain free of overhead hazard as the snowpack adjusts.

Summary

Confidence

Moderate - Timing, track, or intensity of incoming weather system is uncertain

Weather Forecast

MONDAY: Cloudy with intermittent snowfall, accumulation 5-10 cm, light to moderate southwesterly winds, alpine temperature near -9 C, freezing level near 900 m.TUESDAY: Mostly cloudy with intermittent snowfall, accumulation 2-5 cm, light to moderate southerly winds, alpine temperature near -9 C, freezing level near 700 m.WEDNESDAY: Mostly cloudy with intermittent snowfall, accumulation 5-10 cm, light to moderate southerly winds, alpine temperature -5 C, freezing level near 1000 m.

Avalanche Summary

On Saturday, small to large (size 1 to 2.5) wind slabs and storm slabs were noted on all aspects at treeline and in the alpine, generally releasing on the mid-January weak layer.  Numerous small to large persistent slab avalanches on the early-January weak layer were also observed from skiers and explosives in the southwest of the region.On Friday, there were several reports of small to large (size 1 to 3) storm slab, wind slab, and persistent slab avalanches, triggered naturally and by skiers.  Similar avalanches were reported on Wednesday and Thursday.  These avalanches have been releasing on all three persistent weak layers described in the Snowpack Discussion.

Snowpack Summary

The current snowpack is complex, we now have three active weak layers that we are monitoring.40-60 cm of storm snow now sits on a newly formed crust and/or surface hoar layer (mid-January). Prior to the storm, the crust was reportedly widespread, except for possibly at high elevations on north aspects. The buried mid-January surface hoar is 5 to 20 mm in size and was reportedly present at all elevations before the storm. Deeper in the snowpack, a persistent weak layer known as the early-January layer is 50 to 90 cm below the surface. It is composed of surface hoar on sheltered slopes as well as sun crust on steep solar aspects and is present to at least 2100 m, possibly higher. Recent snowpack tests have shown sudden fracture characters with moderate loads and high propagation potential, as well as other signs of instability such as whumpfs and cracking. Yet another persistent weak layer buried mid-December consisting of a facet/surface hoar/crust combination, is buried 90 to 150 cm deep. It is most problematic at and below tree line. This layer is not thought to be present in the alpine.A rain crust buried in November is 100 to 200 cm deep and is likely dormant for the time being.

Problems

Storm Slabs

An icon showing Storm Slabs
Around 60 cm of new snow has formed a slab that sits on a weak layer composed of feathery surface hoar and/or a crust. Expect the slab to be deeper in lee features due to strong south winds. Be cautious in open cutblocks and gullies below treeline.
If triggered, the storm/wind slabs may step down to deeper layers, resulting in large avalanches.Be very cautious as you transition into wind affected terrain, wind slabs may be deep and touchy.Avoid exposure to terrain traps where the consequence of a small avalanche could be serious.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 2.5

Persistent Slabs

An icon showing Persistent Slabs
Three persistent weak layers are lurking in our snowpack, which remain capable of producing very large and destructive avalanches. Keep the terrain choices reined in as the snowpack adjusts to the new load.
Avoid open slopes and convex rolls where buried surface hoar may be preserved.Numerous large avalanches have run on these layers in well supported, treed terrain.Good day to make conservative terrain choices and watch for clues of instability.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

2 - 3.5

Valid until: Jan 22nd, 2018 2:00PM

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