Avalanche Forecast

Issued: Feb 19th, 2018 4:23PM

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

Avalanche Canada mgrist, Avalanche Canada

Strong winds formed reactive wind slabs and cornices at ridge crests. The sun is packing a punch these days and can trigger avalanches. Riding preserved powder in sheltered trees lower down is a good option.

Summary

Confidence

Moderate - Due to the number of field observations

Weather Forecast

Clear and cold for the foreseeable future, with overnight lows near -25 Celsius in some locations. TUESDAY: Cloudy with sunny periods. Tree line temperature around -12 Celsius. Light winds becoming moderate northerly 30-40 Km/hr.WEDNESDAY: Cloudy with isolated flurries possible. Tree line temperatures around -12 Celsius. Winds moderate westerly 20-30 Km/hr.THURSDAY: Sunny with cloudy periods. Tree line temperature around -11 Celsius. Winds light westerly.

Avalanche Summary

On Sunday, natural wind slab activity to size 3 was reported on south / east aspects from 1250m right up to 2400m elevation. See this MIN post for a good example of these avalanches. On Saturday we received reports of a large avalanche running on a deeply buried weak layer with 1.5m crown and good propagation. This feature was steep open terrain below tree line on an east facing aspect. On Thursday we received reports of recent cornice-triggered slab avalanches on high lee (down wind) features in the alpine. See this MIN post for more information. Previously, a very large, widespread natural avalanche cycle up to size 4 was observed around February 10th. Slopes of all aspects and elevations ran full path and reached valley floor. In some cases, mature timber was destroyed. Many of these failed on persistent weak layers mentioned in the snowpack discussion. Although natural avalanche activity on these layers has tapered-off, human triggering of very large avalanches remains a very real possibility.

Snowpack Summary

25-40cm of recent storm snow is settling into a slab in the upper snowpack. Winds have been strong from the north west through south west, creating reactive hard wind slabs (10-15 cm thick) in exposed locations on down wind features. See here for a good video summarizing conditions near Valemount. These accumulations overlie a mix of older wind slabs in exposed higher elevation terrain, a sun crust on steep solar aspects and surface hoar on sheltered slopes.Below the snow surface, several persistent weak layers make up a troublesome snowpack. In the top 1-1.5 m of the snowpack, two surface hoar layers buried in January can be found. Expect to find at least one of these layers on all aspects and elevations.Deeper in the snowpack (120-200 cm deep) is a facet/crust/surface hoar layer from December, most prevalent at and below tree line.Near the base of the snowpack is a crust/facet combo, most likely to be triggered from thin spots in the alpine.All of these layers have produced large avalanches recently. The wide distribution and ongoing reactivity of these layers suggests that avoidance through choosing simple terrain is the best strategy.

Problems

Wind Slabs

An icon showing Wind Slabs
Extra caution advised on south aspects due to the sun's effect and wind slab development thanks to 'reverse loading' from recent north winds. Watch out for wind effect at lower elevations caused by very strong winds.
Avoid recent wind loaded areas until the slope has had a chance to stabilize.If triggered the wind slabs may step down to deeper layers resulting in large avalanches.Use ridges or ribs to enter your line lower down, avoiding pockets of wind slab.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 3

Persistent Slabs

An icon showing Persistent Slabs
Several troublesome layers exist in the snowpack and may be reactive to large triggers such as a cornice or wind slab release. Human triggering may also be possible in shallow or thin, rocky areas.
Pay attention to overhead hazards like cornices which could easily trigger persistent slabs.Minimize exposure to overhead avalanche terrain, large avalanches may reach run out zones.Avoid steep convexities or areas with a thin or variable snowpack.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 4

Valid until: Feb 20th, 2018 2:00PM