Avalanche Forecast

Issued: Jan 20th, 2018 4:17PM

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

South Columbia.

Sunday's storm will bring snow and strong winds, loading a highly complex snowpack. Three weak layers will be 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

SUNDAY: Cloudy with snowfall, accumulation 5-15 cm, strong to extreme southwesterly winds, alpine temperature -8 C, freezing level near 900 m.MONDAY: Mostly cloudy with light snowfall, accumulation 2-5 cm, light to moderate southwesterly winds, alpine temperature near -9 C, freezing level near 900 m.TUESDAY: Partly cloudy with intermittent snowfall, accumulation 1-3 cm, light to moderate 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 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. Expect similar avalanches to release on all aspects and at all elevations with new load.

Snowpack Summary

The current snowpack is complex, we now have three active weak layers that we are monitoring.30-60 cm of storm snow and new forecasted snow for Sunday sits on a newly formed crust and/or surface hoar layer (mid-January). The crust is reportedly widespread, except for possibly at high elevations on north aspects. The mid-January surface hoar is 5 to 20 mm in size and was reported at all elevations. The new snow is falling with strong southwesterly winds, producing wind slabs in lee features. Deeper in the snowpack, the early-January persistent weak 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 found to at least 2100 m, possibly higher. Snowpack tests show sudden fracture characters with moderate loads and high propagation potential, and signs of instability such as whumpfs and cracking. Another 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.A rain crust buried in November is 100 to 200 cm deep and is likely dormant for the time being.

Problems

Storm Slabs

Around 50 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 winds. The slab is sensitive to both natural and human triggering.

Avoid exposure to terrain traps where the consequence of a small avalanche could be serious.Be very cautious as you transition into wind affected terrain, wind slabs may be deep and touchy.If triggered, the storm/wind slabs may step down to deeper layers, resulting in large avalanches.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

1 - 2.5

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.

Good day to make conservative terrain choices and watch for clues of instability.Numerous large avalanches have run on these layers in well supported, treed terrain.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