Avalanche Forecast

Issued: Feb 20th, 2018 4:58PM

The alpine rating is moderate, the treeline rating is moderate, 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 - Wind effect is extremely variable

Weather Forecast

Some cloud Wednesday then mostly clear and cold until light snow arrives on Friday. Overnight lows near -25 Celsius in some locations. WEDNESDAY: Cloudy with isolated flurries possible. Tree line temperatures around -14 Celsius. Winds moderate westerly 20-30 Km/hr.THURSDAY: Sunny with cloudy periods. Tree line temperature around -11 Celsius. Winds light westerly becoming moderate 40 Km/hr.FRIDAY:  Light snow (5-10cm). Tree line temperature around -10 Celsius. Moderate southwest winds 30-50 Km/hr.

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. See also this MIN post for dramatic photos of wind effect. 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 storm snow from last week 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. Also watch out for wind effect at lower elevations caused by last week's very strong winds.
Use ridges or ribs to enter your line lower down, avoiding pockets of wind slab.If triggered the wind slabs may step down to deeper layers resulting in large avalanches.Be careful with wind loaded pockets, especially near ridge crests and roll-overs.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

1 - 2.5

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.
Avoid steep convexities or areas with a thin or variable snowpack.Minimize exposure to overhead avalanche terrain, large avalanches may reach run out zones.Pay attention to overhead hazards like cornices which could easily trigger persistent slabs.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 4

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