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Avalanche Forecast

Archived

Dec 18th, 2020–Dec 19th, 2020

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

Regions

Cariboos.

Recent loose snow is forming slab properties over a weak layer of surface hoar. Scattered reports document increasing reactivity, expect to find an increasingly touchy slab with more snow and wind on the way. 

Confidence

Low - Uncertainty is due to the limited number of field observations.

Weather Forecast

FRIDAY NIGHT- Snow, 10-20 cm / south-southwest wind, 45-70 km/h / alpine high temperature near -5 / Freezing level rising to 1200 m, weak inversion

SATURDAY - Flurries, 5-10 cm / southwest wind, 45-60 km/h / alpine high temperature near -6 

SUNDAY - Snow, 10-20 cm / southwest wind, 20-40 km/h / alpine high temperature near -3 / Freezing level rising to 1300 m

MONDAY - Snow and flurries, 10-15 cm / northwest wind, 20-40 km/h / alpine high temperature near -10

Avalanche Summary

Snowmobilers remotely triggered a storm slab over surface hoar Wednesday near Allen Creek (MIN report). We have limited field observations from this region, however expect slabs to become increasingly reactive as fresh snow gains cohesion and bonds over a weak and touchy surface hoar layer.

Several large natural avalanches were reported last Friday at Chappel Creek (see this MIN report). These avalanches occurred on south aspects at treeline elevations. Based on the depth of these avalanches, they likely failed on recently buried surface hoar and/or sun crust layers. 

Snowpack Summary

Incremental loading has stacked 30-60 cm (with the higher numbers surrounding Blue River) low density snow around the region. Southwest winds have built slabs in lee features at upper elevations.

A weak layer of surface hoar that was buried about a week ago is now down 40-60 cm. In many areas the weak layer exists as a crust, and in other areas, surface hoar. The bond at this interface is poor, slabs will become increasingly reactive as fresh snow gains cohesion and slab properties.

Near the base of the snowpack is a crust that was buried in early November. This crust likely has weak facets associated with it. There have been no recent avalanches reported on this layer, though it may be possible to trigger from shallow, rocky terrain.

Terrain and Travel

  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Avoid freshly wind loaded terrain features.
  • As the storm slab problem gets trickier, the easy solution is to choose more conservative terrain.
  • Investigate the bond of the recent snow before committing to your line.

Problems

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.

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.