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Avalanche Forecast

Archived

Dec 19th, 2020–Dec 20th, 2020

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.
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.

The upper snowpack is a mess and the incoming 15 to 25 cm of new snow is expected to be accompanied by strong wind. A natural avalanche cycle is likely overnight & may persist into Sunday. Travel in avalanche terrain is not recommend Sunday.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

The storm parade continues for the foreseeable future.

SATURDAY NIGHT: Freezing level around 1000 m, strong southwest wind, 15 to 25 cm of snow expected.

SUNDAY: Overcast, freezing level around 1200 m, strong west/southwest wind, 5 to 10 cm expected through the day.

MONDAY: Overcast, freezing level around 700 m, light south wind with moderate gusts at ridgetop, 10 to 20 cm of snow possible.

TUESDAY: Broken cloud cover, freezing level near valley bottom, light to moderate northerly wind at ridgetop, no precipitation expected. 

Avalanche Summary

On Friday natural slab avalanches to size 3 were reported on all aspects. Early reports from Saturday feature similar activity. The new snow and wind Saturday night into Sunday are expected to initiate a natural avalanche cycle that may persist into Sunday.

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).

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

  • Avoid all avalanche terrain during periods of heavy loading from new snow and wind.
  • Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.

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.