Avalanche Forecast

Issued: Jan 20th, 2018 4:11PM

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

mconlan, Avalanche Canada

Purcells.

Sunday's storm will bring snow and strong winds, which will load a highly complex snowpack. Three active weak layers remain sensitive to human triggering. 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

SUNDAY: Cloudy with snowfall, accumulation 5-10 cm, strong to extreme southwesterly winds, alpine temperature -8 C, freezing level near 1000 m.MONDAY: Mostly cloudy with intermittent snowfall, light to moderate southwesterly winds, alpine temperature near -9 C, freezing level near 1000 m.TUESDAY: Partly cloudy with intermittent snowfall, light southwesterly winds, alpine temperature near -9 C, freezing level near 800 m.

Avalanche Summary

On Friday, there were several reports of small to large (size 1 to 2.5) storm slab, wind slab, and persistent slab avalanches, triggered naturally, by skiers, and by explosives. Â Similar avalanches were reported on Wednesday and Thursday.Â 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.20-40 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 now buried surface hoar is 10 to 30 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 40 to 80 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 at all elevation bands. 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 60 to 110 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 90 to 150 cm deep and is likely dormant for the time being.

Problems

Storm Slabs

Up to 30 cm of recent snow sits on a combination of weak, feathery surface hoar and a crust. Expect the slab to be deeper in lee features and to be sensitive to human triggering.

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.If triggered, the storm slabs may step down to deeper layers, resulting in large avalanches.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 2

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.

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.Avoid open slopes and convex rolls where buried surface hoar may be preserved.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

2 - 3.5

Valid until: Jan 21st, 2018 2:00PM