Avalanche Forecast

Moderate - Uncertainty is due to the limited number of field observations.

TUESDAY NIGHT: Broken cloud cover, moderate northwest wind, alpine temperature near -10 C.

WEDNESDAY: Scattered cloud cover, light to moderate west wind, alpine temperature near -7 C. 

THURSDAY: Mainly sunny, light southerly wind, alpine temperature near -2 C with potential for a light alpine temperature inversion.

FRIDAY: Scattered flurries, light to moderate southwest wind, alpine temperature near -6 C. 

With the the snow falling through the day Monday into the night, I suspect there was widespread avalanche activity throughout most of the region on Tuesday. Early reports from Tuesday in Rogers Pass indicate some explosives controlled avalanches to size 4 running full path from the alpine to the valley floor Monday night. 

On Monday there were several reports of persistent slab avalanches running size 2.5 to 3 at treeline and in the alpine on east and west aspects. 

Over the weekend a prolific natural avalanche cycle took place with avalanches averaging size 1.5 to 2.5, but there were a few size 3 and even size 3.5's in the mix as avalanches ran on the early December crust. Avalanches were running on both the mid December surface hoar and the early December crust/surface hoar/facet combo.

An active storm cycle has produced 60 to 100 cm of storm snow over the last week. Underneath this storm snow there is a sandwich of weak layers which are widespread throughout the region.

Just under the new snow, down about one meter below the surface is the mid December surface hoar. This layer has been failing naturally and has been quite sensitive to human triggering throughout the storm cycle. Large avalanches have been running on this interface connecting big terrain features. 

Below that lies the early December persistent weak layer, it's about 80 to 140 cm below the surface. This weak layer is a mixed bag that often presents as surface hoar and facets sitting on top of a stout supportive crust. This is a nasty weak layer, when avalanches fail on it, they have been large and destructive. Smaller avalanches in motion can step down to this one too. This weak layer is going to haunt us for the foreseeable future. 

• Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
• Remote triggering is a concern, watch out for adjacent and overhead slopes.
• Uncertainty is best managed through conservative terrain choices at this time.
• Be aware of the potential for large avalanches due to the presence of a persistent slab.
• Storm slabs in motion may step down to deeper layers resulting in large avalanches.