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Avalanche Forecast

Archived

Dec 13th, 2021–Dec 14th, 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 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.

Regions

Lizard-Flathead.

Choose simple terrain that is sheltered from the wind. The storms keep stacking up, and a rider's weight may push a storm slab to a tipping point.  

Confidence

Moderate - We are confident about the likelihood of avalanche activity, what is less certain are their possible size.

Weather Forecast

Tonight's storm may bring more snow than originally anticipated. Cloudy weather and moderate snowfall continue, while the winds will ease off a little.

MONDAY NIGHT: Overcast. 10-20 cm of snow expected. Light to moderate southeast wind, trending towards strong or extreme south at higher elevations. Freezing level around 500m.

TUESDAY: Mostly cloudy. 2-12 cm of snow expected. Light southwest wind, trending to moderate at higher elevations. Freezing level rising to around 900m.

WEDNESDAY: Mostly cloudy. 0-2 cm of snow expected. Light southwest wind trending to strong westerly at higher elevations. Warming through the day, alpine temps around -11 C.

THURSDAY: Partial clearing overnight. Mostly cloudy through the day. 1-3 cm of snow expected. Light winds, trending towards moderate southwest, with strong westerlies in the alpine.  

Avalanche Summary

Expect storm slabs to remain reactive to riders. Snowfall continues, and the snowpack still needs time to adjust to the new load. 

This MIN report has details of a natural size 2.5 avalanche in an East facing treeline feature. 

Reports from Saturday and Sunday indicate that the storm slab was touchy. Avalanche control with explosives produced many avalanches size 1.5-2, with one size 2.5 reported. 

On Saturday, there was a report of avalanches being remotely triggered by skiers in the backcountry. 

Another backcountry rider reported whumpfing and shooting cracks in 30-60cm deep slabs around treeline, causing their group to retreat to wind sheltered terrain.

Snowpack Summary

Snowfall from the previous storm is building new slabs on top of the old. There is a lot of snow available for wind transport, and winds are lighter, but still strong enough to encourage slab formation. Expect to find deeper and more reactive slabs in leeward slopes.  

Below 2400m, expect to find a thick (20 cm or more) melt freeze crust on all aspects down 60-100 cm. 

Several early season crusts are breaking down and forming a cohesive lower snowpack.

Snowpack depths range from 40-130 cm at treeline elevations. The snowpack tapers quickly below 1800m.

Terrain and Travel

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

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.