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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

South 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 - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Saturday night: 5-10 cm of new snow. Strong southwest wind. Treeline temperatures around -5 C. Freezing level 1300 m to valley bottom.

Sunday: Up to 5 cm of new snow. Moderate southwest wind. Treeline temperatures around -8 C. Freezing level 600 m.

Monday: Flurries with sunny breaks. Light to moderate southeast wind. Treeline temperatures around -8 C. Freezing level 500 m.

Tuesday: Flurries with sunny breaks. Light wind. Treeline temperatures around -8 C. Freezing level valley bottom.

Avalanche Summary

Preliminary observations from Saturday include natural, explosive and skier triggered storm and wind slabs up to size 1.5. 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, 20-30 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 30-70 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.
  • Avoid freshly wind loaded terrain features.
  • 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.