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Avalanche Forecast

Archived

Dec 17th, 2020–Dec 18th, 2020

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

Regions

North Columbia.

Avalanche hazard will increase into the weekend as storm snow accumulates. With ongoing slab development over weak surface hoar, conservative terrain choices are recommended.

Confidence

Moderate - Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

THURSDAY NIGHT - Isolated flurries, up to 5 cm / southwest wind, 20-40 km/h / alpine high temperature near -7

FRIDAY - Snow, 10-25 cm / southwest wind, 30-60 km/h / alpine high temperature near -6

SATURDAY - Snow, 15-20 cm / southwest wind, 25-55 km/h / alpine high temperature near -3 

SUNDAY - Snow, 20-30 cm / southwest wind, 20-40 km/h / alpine high temperature near -7 / Freezing level rising to 1500 m

Avalanche Summary

Small storm and wind slabs continued to be reactive underfoot/sled on Thursday. Explosives triggered avalanches to size 2 along ridgelines and with good connectivity.

On Wednesday, small storm and wind slabs showed reactivity on predictable, loaded features on the Dec 7 surface hoar/crust. Explosives and skiers triggered size 1-1.5 slab avalanches on northerly (or leeward) aspects above 2000 m. An loose, dry sluffing was easy to initiate in steep terrain. With the most reactivity, a small (size 1) avalanche was triggered remotely by a vehicle traveling along an exposed ridgeline. 

On Tuesday, several small (size 1-1.5) loose dry avalanches were reactive to skier traffic. Skiers triggered a small (size 1.5) storm slab avalanche in Glacier National Park. And in the South Columbia's, storm slab avalanches were triggered by explosives and skier traffic on East, North, and West aspects above 2000 m; average depth 5-25 cm. 

Snowpack Summary

Ongoing flurries and snow has accumulated 20-50 cm fresh snow around the region. This new snow has buried a weak layer of surface hoar in many areas. This new snow combined with southwest winds has formed reactive slabs, especially at higher elevations. 

A weak layer that was buried in early December is now down 40-60 cm. This layer is variable and has been reported to exist as either a crust or surface hoar. In any case, it is a persistent weak layer with a significant load above it now, and it may react easily to human triggers.

Another crust that was buried in early November can be found near the base of the snowpack. There have been no recent avalanches reported on this layer, however it may be possible to trigger it from shallow, rocky terrain.

Terrain and Travel

  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Choose conservative terrain and watch for clues of instability.
  • Look for signs of instability: whumphing, hollow sounds, shooting cracks, and recent avalanches.
  • As the storm slab problem gets trickier, the easy solution is to choose more conservative terrain.

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.