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Avalanche Forecast

Archived

Feb 20th, 2018–Feb 21st, 2018

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

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.

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

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.