Avalanche Forecast
Kootenay Boundary
It's getting trickier as the recent storm snow settles into a cohesive slab and the mid-December surface hoar becomes active. With all this uncertainty conservative terrain selection remains prudent.
Avalanche Problem 1: Storm Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- Good day to choose conservative lines and carefully watch for clues of instability.
- Avoid wind loaded terrain at and above treeline, especially near ridge crests and roll-overs.
- Fresh wind slabs will likely form throughout the day, diligently watch for changing conditions.
Avalanche Problem 2: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Surface hoar is now buried up to 100 cm deep and has recently produced large avalanches. Increased settlement and cohesion of the overlying slab will likely make this interface more sensitive to human triggering.
Travel and Terrain Advice
- Storm slab avalanches in motion could step down and trigger larger persistent slab avalanches.
- Use conservative route selection, choose moderate angled and supported terrain with low consequence.
- Avoid steep slopes and convexities at and below treeline where buried surface hoar may be preserved.
Avalanche Summary
On Saturday explosive control work produced what might be the first large avalanche failing on the mid-December interface. The size 2.5 avalanche ran on a 35 to 40 degree slope that was southeast through southwest facing at treeline. Control work produced numerous other storm slabs to size 2 on northwest, north, northeast, southeast, south and southwest aspects. Natural avalanches to size 2 were reported on north facing features at 2100 m. Last Tuesday a skier triggered a size 1.5 wind slab on a north-facing alpine slope in Kokanee Glacier Provincial Park. The skier went for a short ride, but was uninjured in the event.
Snowpack Summary
Two successive storms produced 25 to 50 cm of low density storm snow that was accompanied by moderate to strong winds out of the east, southeast, south and southwest. The new snow overlies 3 to 5 mm surface hoar, but we don't know much about the distribution of this weak layer yet.
Between 60 and 100 cm below the surface you'll find the December 15th interface which consists of a melt-freeze crust on steep, solar, higher elevation slopes and well-developed surface hoar which seems most pronounced in sheltered terrain at and below treeline. The bond at this variable interface is of critical importance, especially in areas where the overlying slab is deep and well-consolidated.
The lower snowpack is thought to be strong and well-settled.
Weather Forecast
After a succession of snowstorms over southern BC, the weather over the province will settle into a dry cold period as 2017 comes to an end and 2018 begins. The culprit for the cold is our old friend the Arctic high that has been waiting patiently over northern BC for an opportunity to push south and envelope the entire province in cold, dry Arctic air. All indications point to the cold dry spell lasting until mid-week.
SUNDAY: A few clouds, freezing level at valley bottom, moderate to strong northwest wind, no snow expected.
MONDAY: A few clouds, freezing level at valley bottom, light to moderate west/northwest wind, no snow expected.
TUESDAY: Clear skies at dawn, cloud cover increasing in the afternoon, freezing level at valley bottom, with an Above Freezing Layer (AFL) between 2000 m and 3000 m, light variable wind, no snow expected.
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25 to 50 cm of new snow has fallen in the last few days accompanied by wind mainly out of the south. Human triggered avalanches are becoming more likely as time and wind loading allow the snow to settle into a more cohesive slab.