Avalanche Forecast
Issued: Dec 11th, 2021 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Persistent Slabs.
ahanna,
Avalanche Canada
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.
Summary
Confidence
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 slabs will be most reactive where deeper deposits have been wind loaded into lee aspects. Avoid these areas until they have a chance to stabilize.
Aspects: North, North East, East, South East.
Elevations: All elevations.
Likelihood
Expected Size
Persistent Slabs
50-90 cm of snow from the previous week now sits over a weak layer on a thick crust. The grains that form this weak layer are proliferating quickly, most pronounced at mid-elevations at this time. Natural and human triggered slab avalanches were observed on this layer in the days prior to the current storm, so with the additional snow load this weekend we are suspecting that this layer may become increasingly reactive and produce large avalanches.
Aspects: All aspects.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Dec 12th, 2021 4:00PM
