Avalanche Forecast
Purcells
Take a cautious and curious approach to the mountains Thursday. We're on the heels of a very large natural avalanche cycle, and we're uncertain as to how sensitive our deeply buried persistent weak layers are to human triggering.
Avalanche Problem 1: Storm Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- Caution with wind slabs, especially in steep unsupported and/or convex terrain features.
- Don't be too cavalier with decision making, storm slabs may remain sensitive to human triggering.
- The new snow could use a bit of time to settle and stabilize.
Avalanche Problem 2: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
As we move out of a very large storm and avalanche cycle there is a lot of uncertainty surrounding the triggerability of our deeply buried persistent weak layers. When we don't know, we have to scale our terrain choices accordingly.
Travel and Terrain Advice
- If you are increasing your exposure to avalanche terrain, do it gradually as you gather information.
- Storm slab avalanches have potential to step down and produce very large destructive avalanches.
- Uncertainty is best managed through conservative terrain choices at this time.
Avalanche Summary
A widespread natural avalanche cycle to Size 4.0 was reported on Tuesday, with larger and more frequent avalanches in the north of the region where storm snow totals were higher.
On Saturday near Kimberley, skiers remote-triggered two size 2 avalanches, by a convex roll at 2000 m elevation. Crown height was 60-70 cm, suggesting the snow ran on the mid January layer. See the MIN report for more details.
Snowpack Summary
Large differences in storm snow totals were observed in the region on Monday into Tuesday: values ranged from 2 to 32cm!!! Overnight winds were strong to extreme from the southwest and temperatures spiked to -3 C at tree line.
The current snowpack is complex, with three active weak layers that we are monitoring:
1) 60-100 cm of recent storm snow overlies a crust and/or surface hoar layer (from mid-January). The crust is reportedly widespread, except for possibly at high elevations on north aspects. The surface hoar is 10 to 30 mm in size, at all elevation bands. The recent snow fell with strong south winds, producing wind slabs and cornices in lee features.
2) Deeper in the snowpack (down 70-100 cm), a persistent weak layer known as the early-January layer is present at all elevation bands, and composed of surface hoar on sheltered slopes and sun crust on steep solar aspects. Recent snowpack tests have shown sudden fracture characters with moderate loads and high propagation potential.
3) Another persistent weak layer that was buried mid-December is 80 to 120 cm deep and consists of a facet/surface hoar/crust combination. It is most problematic at and below tree line.
Weather Forecast
Thursday is expected to be rather lackluster, but things get a lot more interesting as we head into the weekend. A rather bullish Arctic front is slowly moving south while a turbulent thrust of warm air is staging in the eastern Pacific. This clash of opposing systems should generate significant snow and wind, but the exact location and intensity is difficult if not impossible to pin down right now. The region could pick up 10 to 20 cm by Sunday evening. Stay tuned for more details.
THURSDAY: Scattered cloud cover, freezing level at valley bottom, light to moderate west/southwest wind, trace of snow possible.
FRIDAY: Broken cloud cover, freezing level at valley bottom, light to moderate west/southwest wind, 1 to 5 cm of snow possible.
SATURDAY: Overcast skies, freezing level at valley bottom, moderate to strong west/northwest wind, 1 to 5 cm of snow possible.
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Natural activity will likely diminish Thursday, but human triggered avalanches remain a concern. With all the recent storm snow, avalanches in motion could entrain significant mass, especially in bigger terrain features and avalanche paths.