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Avalanche Forecast

Archived

Feb 21st, 2018–Feb 22nd, 2018

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

Regions

North Columbia.

Previous strong winds have formed wind slabs in wind-exposed areas, and have been reactive to human triggers.  Keep in mind that large avalanches on buried weak layers may still be possible and choose terrain carefully.

Confidence

Moderate -

Weather Forecast

THURSDAY: Mix of sun and cloud. Ridge wind light, northeast. Temperature -12. Freezing level valley bottom.FRIDAY: Flurries. Accumulation 5 cm. Ridge wind light, southwest. Temperature -12. Freezing level valley bottom.SATURDAY: Mix of sun and cloud, light flurries. Accumulation trace. Ridge wind light, west. Temperature -10. Freezing level valley bottom.

Avalanche Summary

Tuesday there were several reports of skier triggered wind slab avalanches up to size 1.5 on a variety of aspects between 1800-2100 m, including a remotely triggered (from a distance) pocket of hard wind slab near ridge top. 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. Although deeper, persistent avalanche activity has become less frequent over the past week, light triggers in shallow rocky areas, as well as large triggers such as a cornice collapse or step down from a wind slab release, still have the potential to result in large destructive avalanches.

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, have redistributed these accumulations into deeper, reactive slabs in wind-exposed terrain. The recent snow sits on the old interface the was buried mid February consisting of; 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 complex snowpack which is slowly beginning to show signs of improving but still remains suspect. 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.