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

North 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

No Rating - 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: Flurries accumulating up to 5 cm. Alpine low -8, light west wind.

Friday: Flurries accumulating up to 5 cm overnight, alpine high -5, light west wind.

Saturday: Mainly cloudy, isolated flurries with trace accumulation, alpine high -6, light northwest wind.

Sunday: Mainly cloudy, isolated flurries with trace accumulation, alpine high -10, moderate northwest wind.

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, numerous natural, explosive and skier triggered avalanches size 2-2.5 on a variety of aspects have been reported recently throughout the region and in neighboring Glacier National Park. Some of these avalanches were triggered in areas of shallow snowcover or wind slab and stepped down to the persistent November layer resulting in avalanches large enough to injure or bury a person.

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.