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Avalanche Forecast

Archived

Dec 20th, 2020–Dec 21st, 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 and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
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 unlikely, human triggered possible.

Regions

South Rockies.

A new storm with intense loading rates is set to hit the region by Monday afternoon and continue overnight. Danger ratings may rise to HIGH by late afternoon. Avoid all avalanche terrain during periods of heavy loading from new snow and wind.

Confidence

Moderate - Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

SUNDAY NIGHT - Trace to 5 cm / moderate west wind, alpine low temperature near -8 

MONDAY DAY - Snow increasing later in the day 10-15 cm, moderate west wind / alpine high temperature near -5 

MONDAY NIGHT - 15-30 cm / south east gusty wind / alpine high temperature near -3 

TUESDAY - Morning flurries up to 5 cm / windy at times / alpine high -7

WEDNESDAY - Clearing, light west winds, alpine high -9

Avalanche Summary

On Sunday the South Rockies Field Team experienced lots of whumpfing and remote triggered a size 2 avalanche in a low snowpack area. The failure plane was facets on the early December persistent weak layer, even though there was no crust at that elevation (2100m). They also saw recent natural avalanche activity up to size 2.5. Alpine and open treeline features most exposed to wind produced the most avalanche activity.

On Saturday a natural size 2.5 avalanche was spotted out of extreme, shallow, rocky, wind affected terrain in the Crowsnest Pass. It was thin, but propagated widely. The date of the event was uncertain.

Please consider sharing your observations on the Mountain Information Network. Thank you to those that have already submitted this winter!

Snowpack Summary

After a break overnight, we will see increasing snowfall amounts on Monday afternoon forming new storm slabs. Slabs are especially reactive in areas where they have been stiffened even slightly by wind.

This recent snow means there is now 40-60 cm above the early December weak layer. There may be weak faceted grains and/or a crust making it susceptible to human triggering and step down avalanches from recent storm snow. 

The base of the snowpack consists of a hard melt-freeze crust from early-November that may also have weak crystals around it. There has not been recent avalanche activity on this layer but it remains on our radar. The most likely spot to trigger it would be on thin and rocky slopes.

Snowpack depths vary substantially in the region with the average snowpack depth at treeline being approximately 90-140 cm.

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow and wind.
  • Shooting cracks, whumphs and recent avalanches are strong indicators of an unstable snowpack.
  • Storm slabs in motion may step down to deeper layers resulting in large 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.