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

Regions

South 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 -14 Celsius. Winds moderate westerly 20-30 Km/hr.THURSDAY: Sunny with cloudy periods. Tree line temperature around -12 Celsius. Winds light northerly 15 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 3 were reported on a wide range of aspects above 2100m. On Sunday we received reports of several wind slab avalanches to size 2, on a wide range of aspects. See here for a good example in the MIN report. On Saturday explosives control work produced several storm slab avalanches to size 2.5 on a wide variety of aspects near tree line. 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. On the same day, a helicopter remotely triggered a size 3 persistent slab avalanche on a similar aspect / elevation in the Selkirks a little closer to Revelstoke. Although deeper, persistent avalanche activity is becoming less frequent, these recent avalanches point to the continued reactivity and destructive potential of these layers.

Snowpack Summary

40-60 cm of recent storm snow has settled 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. These accumulations overlie 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/ crust layers buried in January can be found. Expect to find one or the 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.All of these layers have produced large avalanches, with a higher frequency in the south east of this region.

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.