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Avalanche Forecast

Archived

Dec 18th, 2019–Dec 19th, 2019

Alpine
Natural avalanches possible, human triggered probable.
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

South Columbia.

Human triggered avalanches are likely as a steady pulse of storms has formed touchy slabs and increased the load on buried weak layers.

Confidence

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

Weather Forecast

WEDNESDAY NIGHT: Scattered flurries with up to 5 cm of snow, 40 km/h wind from the southwest, alpine temperatures drop to -8 C.

THURSDAY: Scattered flurries with 5-10 cm of snow, 40 km/h wind from the southwest , alpine high temperatures around -6 C.

FRIDAY: 20-30 cm of snow, 50-80 km/h wind from the southwest, alpine high temperatures around -2 C.

SATURDAY: Another 20-35 cm of snow by the morning then cloudy in the afternoon, 30-50 km/h wind from the southwest, alpine high temperatures around -2 C.

Avalanche Summary

Heavy snowfall resulted in a cycle of natural avalanches on Tuesday (and potentially on Wednesday too). Numerous natural size 2 storm slab avalanches were reported on Tuesday, and both storm slabs and persistent slabs were reactive to human triggering. The human triggered storm slabs were typically 20-30 cm thick. A few larger persistent slab avalanches were triggered remotely on layers that were 30-50 cm deep. Recent persistent slab avalanche activity has occurred on a range of aspects and elevations, suggesting the problem is widespread.

Expect human triggered avalanches to be likely in the coming days as the load continues to build over a recently buried layer of surface hoar.

Snowpack Summary

A continuous pulse of storms has delivered 20-30 cm of fresh snow to the South Columbias over the past few days. 30-60 cm of recent snow now sits above a widespread layer of large, feathery surface hoar crystals. New snow is expected to continue to accumulate above this layer over the next few days and possibly make it more sensitive to human-triggering.

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 listed 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.