Avalanche Forecast

Archived

Dec 16th, 2020–Dec 17th, 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 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 avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Regions

North Columbia.

A pulse of flurries is forecasted to drop up to 20 cm. Expect to find a developing slab problem as wind impacts new flurries and older loose, dry snow. Choose conservative terrain and watch for signs of instability such as whumphing, cracking, and recent avalanches.

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

WEDNESDAY NIGHT - Mainly cloudy with isolated flurries, 10-25 cm / southwest wind, 20-40 km/h / alpine high temperature near -7

THURSDAY - Flurries, 10-15 cm / southwest wind, 20-40 km/h / alpine high temperature near -7

FRIDAY - Periods of snow, 10-20 cm / southwest wind, 30-60 km/h / alpine high temperature near -6

SATURDAY - Snow, 10-20 cm / southwest wind, 25-55 km/h / alpine high temperature near -3

Avalanche Summary

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.

With forecasted new snow and moderate to strong southwest winds, slabs will become more reactive to human triggers, especially in wind loaded areas.

Snowpack Summary

20-40 cm of snow has fallen in the region since Wednesday morning. This new snow has buried a weak layer of surface hoar in many areas. This new snow combined with southwest winds will form reactive slabs, especially at higher elevations.

A weak layer that was buried about a week ago 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.
Look for signs of instability: whumphing, hollow sounds, shooting cracks, and recent avalanches.
Be careful with wind loaded pockets, especially near ridge crests and roll-overs.

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.