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Avalanche Forecast

Archived

Dec 11th, 2021–Dec 12th, 2021

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

Regions

North Columbia.

Storm slabs are most likely to remain reactive where strong winds have loaded deeper deposits onto leeward slopes. Continue to make conservative terrain choices while the snowpack adjusts to the new load.

Confidence

Moderate - A small change in the upper snowpack could dramatically change avalanche conditions.

Weather Forecast

Saturday night: Around 5 cm of new snow. Moderate southwest wind. Treeline temperatures around -5 C. Freezing level dropping 1000 m to valley bottom.

Sunday: Up to 5 cm of new snow. Light to moderate southwest wind. Treeline temperatures around -10 C.

Monday: Flurries with sunny periods. Light southeast wind. Treeline temperatures around -8 C.

Tuesday: A mix of sun and cloud. Light wind. Treeline temperatures around -10 C.

Avalanche Summary

We are still awaiting observations of avalanche activity during the storm. Neighboring Glacier National Park reported widespread natural activity averaging size 2 and up to size 3.

Reports from Wednesday and Thursday indicate an increase in slab reactivity prior to burial by the current storm. There were several reports of natural size 2 avalanches in alpine terrain as well as several human triggered size 1 wind slabs around treeline. The most reactive slabs were on convex wind-affected slopes. Most avalanches were in the top 20-30 cm of snow.

Snowpack Summary

The convective nature of yesterday's storm has resulted in highly variable snowfall amounts throughout the region. Generally, 15-25 cm of new snow has seen redistribution by strong wind at upper elevations. 

A prominent and widespread crust that reaches as high as 2400 m in the alpine, now sits 50-90 cm below the surface. In many places, overlying snow is well-bonded to the crust but in others, weak faceted grains have been observed growing above it around treeline. Snowpack models show the faceting process progressing quickly at this elevation, likely due to the amount of heat and moisture trapped by the crust. We will be closely monitoring this layer going forward. The snowpack structure is relatively simple beneath the crust, with treeline snow depths around 100-200 cm.

Terrain and Travel

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Be alert to conditions that change with elevation and wind exposure.
  • Stay off recently wind loaded slopes until they have had a chance to stabilize.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.

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.