Avalanche Forecast
North Columbia
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
- The new snow could use a bit of time to settle and stabilize.
- Don't be too cavalier with decision making, storm slabs may remain sensitive to human triggering.
- Caution with wind slabs, especially in steep unsupported and/or convex terrain features.
Avalanche Problem 2: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
As we move out of a major 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
- Uncertainty is best managed through conservative terrain choices at this time.
- Storm slab avalanches have potential to step down and produce very large destructive avalanches.
- If you are increasing your exposure to avalanche terrain, do it gradually as you gather information.
Avalanche Summary
On Tuesday natural avalanche activity was widespread, running on all aspects and elevations. Storm and wind slabs were reported to size 3.0, persistent slab avalanches failing on the mid-December interface ran to size 3.5. A size 2.5 avalanche on a northeast facing slope at 1920 m resulted in a single fatality. More details are available here.
On Monday a widespread natural avalanche cycle was driven by warming temperatures and up to 100 cm of storm snow. Avalanches to size 3 were reported from a variety of aspects and elevations. Of particular interest was a size 3 avalanche that was remote triggered by a skier on a south facing slope at 1900 m, failing on the mid-December interface.
On Saturday, a human-triggered size 2.5 avalanche was reported near 2300 m on a south west aspect in Glacier National Park. See the MIN report for incident details.
Snowpack Summary
60 to 120 cm of snow fell between Sunday and Tuesday night. On Monday temperatures warmed up to -1 C at tree line. Winds have been strong to extreme from the south which has built cornices and formed wind slabs on lee (down wind) slopes.
The new snow sits on a complex snowpack and there are three main weak layers that professionals are monitoring:
1) 80 to 110 cm of storm snow sits on a crust and/or surface hoar layer from mid-January. The crust is reportedly widespread, with the possible exception of high elevation north aspects. The mid-January surface hoar is 5 to 20 mm in size and was reported at tree line elevations and possibly higher.
2) Deeper in the snowpack, the early-January persistent weak layer is 110 to 140 cm below the surface. It is composed of surface hoar on sheltered slopes as well as sun crust on steep solar aspects and is found at all elevation bands. Snowpack tests show sudden fracture characters and signs of instability such as whumpfs, cracking and recent avalanches.
3) Another weak layer buried mid-December consisting of a facet/surface hoar/crust combination is buried 120 to 170 cm deep. 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 20 to 40 cm by Sunday evening. Stay tuned for more details.
THURSDAY: Increasing cloud cover, freezing level at valley bottom, light west/southwest wind, trace of snow possible.
FRIDAY: Overcast skies, freezing level at valley bottom, moderate west/southwest wind, 5 to 15 cm of snow possible.
SATURDAY: Overcast skies, freezing level at valley bottom, moderate to strong northwest wind, 5 to 15 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 snow storm, avalanches in motion could entrain significant mass, especially in bigger terrain features and avalanche paths.