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Avalanche Forecast

Archived

Dec 12th, 2018–Dec 13th, 2018

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

Regions

Northwest Coastal.

Storm snow totals are edging toward doubling our previous snowpack depth. This is a time to back off to simple terrain that is totally free of overhead hazards while the new snow settles.

Confidence

Moderate - Forecast snowfall amounts are uncertain

Weather Forecast

Wednesday night: Cloudy with the next storm pulse bringing about 20 cm of new snow with strong to extreme southwest winds.Thursday: Cloudy with easing flurries bringing another 10 cm of new snow. Clouds possibly breaking in the afternoon before snowfall increases again overnight. Extreme southwest winds easing to light over the day. Alpine high temperatures around -3.Friday: Cloudy with a continuing storm pulse bringing 15-20 cm of new snow. Light to moderate northeast winds. Alpine high temperatures around -5.Saturday: Cloudy with flurries beginning later in the day and a possible 5 cm of new snow before end of day. Moderate southeast winds increasing to extreme in the evening. Alpine high temperatures around -4.

Avalanche Summary

A social media report from the Shames area on Tuesday described active avalanche conditions with numerous small (size 1) to large (size 2) natural releases observed at treeline elevations. These storm slabs failed at the old snow surface, noted as a crust in this area.  We can expect a continuation of this natural avalanche activity during the barrage of storms hitting the region.

Snowpack Summary

Another 20-30 cm of new snow expected by the end of Thursday should bring storm totals from the past few days up to about 90-120 cm. Strong to extreme southwest and west winds are likely forming the thickest deposits on lee slopes at high elevations.The new snow has been observed bonding poorly to the weak surface hoar and facets that have been sitting on the surface for the past week, especially on sheltered slopes at treeline. Deeper in the snowpack, there are reports of several crusts including an early season crust with facets near the bottom of the snowpack.Prior to the storms, there was only 70-120 cm of snow in the alpine and very little coverage below treeline, but this is finally changing.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.