Avalanche Forecast

Issued: Feb 20th, 2018 5:07PM

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

mgrist, Avalanche Canada

North Columbia.

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 is a good option.

Summary

Confidence

Moderate - Wind effect is extremely variable

Weather Forecast

Cloudy Wednesday then clear and cold until light snow arrives on Friday. Overnight lows near -20 Celsius in some locations. WEDNESDAY: Cloudy with isolated flurries possible. Tree line temperatures around -15 Celsius. Winds moderate westerly 20-30 Km/hr.THURSDAY: Sunny with cloudy periods. Tree line temperature around -14 Celsius. Winds light westerly 10-20 Km/hr.FRIDAY: Light snow (5-10 cm possible) . Tree line temperature around -10 Celsius. Moderate southwest winds 20-30 Km/hr.

Avalanche Summary

On Monday, several natural wind slab avalanches to size 2.5 were reported on a south / west through north / west aspects between 1800m and 2400m.On Sunday, natural wind slab activity was widespread above 1900m on primarily south east through west aspects. We received reports of an initial wind slab (size 2) stepping down to deeper weak layers and triggering a size 3.5 persistent slab, on a south west aspect near 2300m. Also on Sunday, skiers were able to trigger small storm slabs low down on MacPherson on the widespread rain crust that formed February 8th. See the MIN report for more details. In Rogers Pass a week ago, skiers remotely triggered a size 3.5 persistent slab avalanche from 40 m away on a south-facing, cross-loaded slope at 2600 m. Although deeper, persistent avalanche activity is becoming less frequent, there is still potential for these layers to react.

Snowpack Summary

40-60 cm of recent storm snow is settling into a slab in the upper snowpack. Strong winds, most recently from the north / east, shifted these accumulations into deeper, reactive slabs in wind-exposed terrain. The more recent snow sits on a mix of older wind slabs in exposed higher elevation terrain, a sun crust on steep solar aspects, a rain crust below 1600m, and surface hoar on sheltered slopes.Below the snow surface, several persistent weak layers make up a troublesome snowpack. In the top 1.5-2.5 m of the snowpack, two surface hoar layers buried in January can be found. Expect to find one or other of these on all aspects and elevations.Deeper in the snowpack (around 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.

Problems

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.

If triggered, wind slabs may step down to deeper layers resulting in large avalanches.Use ridges or ribs to avoid pockets of wind loaded snow.Be careful with wind loaded pockets, especially near ridge crests and roll-overs.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2.5

Persistent Slabs

Several troublesome layers exist in the snowpack and may be reactive to large triggers such as a cornice or storm slab release. Human triggering may also be possible in shallow or thin, rocky areas.

Minimize exposure to overhead avalanche terrain, large avalanches may reach the end of runout zones.Avoid steep convexities or areas with a thin or variable snowpack.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