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Avalanche Forecast

Archived

Dec 18th, 2020–Dec 19th, 2020

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

Regions

North Columbia.

We're in an active weather pattern with a complex upper snowpack, avalanche hazard will remain elevated through the weekend.

Confidence

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

Weather Forecast

FRIDAY NIGHT- Snow, 10-20 cm / southwest wind, 30-60 km/h / alpine high temperature near -6

SATURDAY - Snow and flurries, 5-10 cm / southwest wind, 25-55 km/h / alpine high temperature near -5 / Freezing level rising to 1000 m

SUNDAY - Snow, 15-30 cm / southwest wind, 20-50 km/h / alpine high temperature near -3 / Freezing level 1500 m

MONDAY - Snow and flurries, 5-15 cm / southwest wind, 20-40 km/h / alpine high temperature near -10

Avalanche Summary

Storm and wind slabs were reactive underfoot/sled on Thursday. Wind slabs to size 2 were observed at upper elevations in lee terrain, with depths 40-60 cm. A size 1.5 storm slab avalanche was reported failing naturally on the early December crust, with good propagation. Explosives triggered avalanches to size 2 along ridgelines, again being reported with good propagation. 

A handful of MIN reports (thank you!!) from the Gorge found a highly reactive snowpack including a remotely triggered avalanche with surprising propagation from about 20 m away (here).

On Wednesday, small storm and wind slabs showed reactivity on predictable, loaded features and failing on the Dec 7 surface hoar/crust. Explosives and skiers triggered size 1-1.5 slab avalanches on leeward aspects above 2000 m. And 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. 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

  • Choose conservative terrain and watch for clues of instability.
  • Avoid freshly wind loaded terrain features.
  • 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.