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Avalanche Forecast

Archived

Dec 12th, 2019–Dec 13th, 2019

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

Regions

South Columbia.

New snow sitting on a layer of surface hoar may be a recipe for thin but reactive storm slabs over the next few days.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength. Uncertainty is due to field data and reports showing a wide variation in conditions throughout the region.

Weather Forecast

Thursday Night: Scattered flurries accumulating up to 5 cm in some parts of the region. Alpine low -6, light northwest wind.

Friday: Cloudy with isolated flurries accumulating up to 5 cm in some parts of the region. Alpine high -4, light west wind.

Saturday: Mostly cloudy with isolated flurries accumulating 5 cm, alpine high -5, light northwest wind.

Sunday: Cloudy with sunny periods and isolated flurries, alpine high -7, northwest wind increasing to moderate.

Avalanche Summary

On Thursday, we received many reports of sloughing and small loose snow avalanches up to size 1. Reports of soft storm slabs propagating at ridgetops but breaking up quickly as they ran, were also limited to size 1.

Prior to the new snow, natural and skier triggered windslab and persistant slab avalanches averaging size 2 were widely reported in the north of the region and in neighboring Glacier National Park. Natural events have been reported on a range of aspects and are suspected to have been triggered by wind loading or solar radiation. Keep in mind that the new snow, or small storm slab avalanches could also act as triggers to these deeper avalanches.

Snowpack Summary

Thursday saw 15-20 cm of snowfall in the Monashees, with 5-10 cm elsewhere in the region. The new snow covered a widespread layer of large, feathery surface hoar crystals.

The new snow also buried soft wind slab found in the lee of features such as ridge tops, ribs and gulleys.

A weak layer formed in late November is now buried around 1 m below the surface. This is the layer of concern relating to the persistent slab avalanche problem. The weak layer may present as surface hoar, a crust, facets or a combination, depending on elevation and aspect. It can most likely be found in areas sheltered from the wind, but open to a view of the sky, such as near treeline. Below this, variety of crusts from late October are buried deeper in the snowpack.

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.

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.