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Avalanche Forecast

Archived

Dec 18th, 2019–Dec 19th, 2019

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

Regions

North Columbia.

Dangerous avalanche conditions continue as storm snow continues to accumulate and stress buried weak layers.

Confidence

Moderate - A small change in the upper snowpack could dramatically change avalanche conditions.

Weather Forecast

WEDNESDAY NIGHT: Snow, accumulations of 10-25 cm, 50 km/h wind from the south, alpine temperature drop to -8 C. 

THURSDAY: Scattered flurries with another 5 cm of snow, 40 km/h wind from the south, alpine high temperatures around -5 C.

FRIDAY: 20-30 cm of snow, 40-60 km/h wind from the south, alpine high temperature -3 C.

SATURDAY: Another 15-30 cm of snow by the morning then cloudy in the afternoon, 30 km/h wind from the southwest, alpine high temperatures around -3 C.

Avalanche Summary

Heavy snowfall resulted in a cycle of natural avalanches on Tuesday (and potentially on Wednesday too). Numerous natural storm slab avalanches (size 2) were reported on Tuesday, as well as human triggered storm slab and persistent slab avalanches. The human triggered storm slabs were typically 30-40 cm thick. A few larger persistent slab avalanches were triggered remotely on layers that were 40-60 cm deep. Several persistent slab avalanches were also reported last weekend on a range of aspects and elevations, suggesting the persistent slab problem is widespread.

Similar activity is expected on Thursday with more snow expected overnight and strong wind throughout the day.

Snowpack Summary

A continuous pulse of storms has delivered anywhere from 20-40 cm of snow to the North Columbias over the past few days. 40-80 cm of recent snow now sits above a widespread layer of large, feathery surface hoar crystals. New snow is expected to continue to accumulate and consolidate above this layer over the next few days, maintaining a high likelihood of human-triggering.

A weak layer formed in late November is now buried around 1 m plus 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. Below this, a 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.