Avalanche Forecast

Archived

Dec 27th, 2022–Dec 28th, 2022

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural and human triggered avalanches likely.

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

Lizard-Flathead, South Rockies, Flathead, Lizard, Bull, Elkford East, Elkford West.

Keep it simple ! The warm weather and continued precipitation is beginning to subside. Uncertainty is present with the timing of when the snowpack will stabilize. Give the snowpack time to cool and heal before considering moving into challenging and complex terrain

Confidence

Moderate

Avalanche Summary

On Tuesday numerous storm slab, wet loose and wet slab avalanches have been reported. These avalanches had been explosives triggered, skier controlled and natural. The sizing of theses avalanches varied, a few small (size 1) with many of them large to very large (size 2 to 3). The reports indicate that the avalanches gained mass as they entrained the moist snow below.

On Monday, explosive avalanche control produced a few avalanches with one very large avalanche (size 3) running up to 800 m in length. It was noted this avalanche stepped down to the the late November persistent weak layer and confirms suspicions that in specific terrain this layer of concern is reactive.

Over a week ago a natural size 2.5 persistent slab avalanche was reported. This avalanche occurred on a northeast aspect, 2000 m on a reloaded bed surface.

Snowpack Summary

Monday's warm and wet weather cycle, has moistened the upper snowpack surface all the way up to 2200 m. During the past storm's precipitation amounts of 70 mm that totaled snowfall amounts of 20 to 30 cm where recorded. This new snow was redistributed by strong southerly winds. Larger deposits in east and north facing terrain can now be found and has formed new wind slabs. Slabs have formed over a variety of surfaces including; facets, surface hoar and a crust, Essentially they are moist and unconsolidated and not bonding very well.

Around 50 cm deep a layer of surface hoar at treeline and a thin crust on steep south facing slopes exists buried in early December.

The primary concern in the snowpack is the mid-November layer consisting of facets and surface hoar, buried around 80 to 120 cm deep. This layer is likely to become reactive as temperatures and the weight of snow and rain increase the load

The lower snowpack presents as primarily facetted combined with depth hoar. Ice has been reported to exist at the base in a few different locations.

The snowpack in the forecast region is quite variable with snowpack depths at treeline varying from 100 cm to 200 cm.

Weather Summary

Tuesday Night

Cloudy with light snowfall 5 to 10 cm. Strong southwesterly winds 90 km/h . Freezing levels 1000 m. High temperatures of 1°C.

Wednesday

Cloudy with 5-10 cm of mixed precipitation expected. Moderate southwest wind 40 km/h. Freezing levels 750 m. High temperatures of -1°C.

Thursday

Cloudy with light snow 1 to 3 cm. Moderate southwesterly wind 30 km/h. Freezing levels at valley bottom with a mid day rise excepted to reach 500 m. Alpine temperatures -6°C.

Friday

Cloudy with very light snow trace amounts. Light southwesterly wind. Freezing levels at valley bottom, alpine temperatures dropping to -9°C.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

Storm slab size and sensitivity to triggering will likely increase through the day.
Watch for fresh storm slabs building throughout the day.
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.