Avalanche forecast

Kootenay Boundary

Feb 19 – 20, 2021Avalanche Canada
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Avalanche danger

Sun
Mon
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Highlights

Watch for fresh slabs building with flurries and increasing wind. Dig down and investigate the snowpack before stepping into bigger terrain.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible - Likely
    Size
    1–2

    Increasing southwesterly winds will redistribute flurries and loose snow building new wind slabs and covering older, stiffer slabs. Now hidden under fresh snow, these older, stiffer wind slabs have become increasingly unreactive in the past week. However, you may still find (now buried 5-15cm) old, hard wind slabs which remain sensitive to human triggering in the steeper more complex terrain features. 

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely - Possible
    Size
    1.5–2.5

    60-80 cm of snow sits on a persistent weak layer that was buried near the end of January. This surface hoar/crust interface is slowly gaining strength but may remain human triggerable in steep convex features at treeline and potentially steep alpine features where it presents as a crust.

Confidence

Low - Uncertainty is due to the extreme variability of wind effect on the snowpack. Uncertainty is due to the limited number of field observations.

Weather Forecast

THURSDAY NIGHT: Isolated flurries, up to 5 cm / light to moderate southwesterly wind / alpine high temperature -6

FRIDAY: Flurries, up to 5 cm / moderate southwesterly wind / alpine high temperature -8

SATURDAY: Isolated flurries, up to 5 cm / moderate gusty southwesterly wind / alpine high temperature -4 / freezing level climbing to 900 m

SUNDAY: Snow, 10-20 cm / moderate gusting to strong southwesterly wind / alpine high temperature +1 / freezing level rising above 1500 m

Avalanche Summary

Small (size 1) wind slabs were reactive to skiers on Tuesday and Wednesday, along with whumphing and cracking.

On Sunday natural and explosive triggered wind slabs size 1 to 2 were reported from northwest, north, northeast and east facing features between 1900 and 2300 m. MIN reports from Sunday indicate that there was quite a bit of whumphing which is a clear sign of instability.

A bout of sustained strong east and northeast winds prompted widespread wind slab formation and reactivity on Friday and Saturday. Many 20-50 cm deep slabs released naturally, with skier triggering and with explosives, some with remarkably wide propagation. 

We also had many reports last week of persistent slab avalanches triggered by skiers at elevations around treeline (1800-2000 m) where a persistent weak layer exists as surface hoar. Persistent slab avalanche activity has been on the decline more recently, but subtle factors like diminishing slab properties and savvy terrain use may account for this trend. Professionals in the region continue to highlight persistent slabs as a lingering concern.

Another explosives-triggered persistent slab in the Bonningtons on February 9th stepped down to our early December crust layer to produce a 200 cm deep, size 3.5 (very large!) avalanche. A similar avalanche (size 3.5, step down to December crust) occurred naturally in the same area in the first week of February.

Snowpack Summary

5-15 cm of fresh snow now covers widespread wind slabs formed during previous strong east/northeast winds. Time and warm temperatures are reducing the sensitivity of these older wind slabs to human triggering, but the slabs rest on facets formed during the cold snap, so these wind slabs may persist longer than we're accustomed to.

The upper snowpack continues to settle over another weak layer buried 50-80 cm deep (from January 24). This persistent weak layer may present as facets or a crust. Recent test results show that this weak layer is starting to gain a bit of strength.    

There are additional weak layers in the mid and lower snowpack including a surface hoar interface buried 70-120 cm in depth and a crust from early December that is surrounded by weak faceted grains, but these appear to have gone dormant for the time being. A sudden warm up in the form of a Pineapple Express (Atmospheric River), or a large load in a shallow rocky start zone may be able to wake them up.

Terrain and Travel

  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Avoid open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.
  • Use caution on large alpine slopes, especially around thin areas that may propagate to deeper instabilities.

Regions covered