Avalanche forecast

South Coast Inland

Feb 19 – 20, 2021Avalanche Canada
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Avalanche danger

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

Expect to find increasingly reactive slabs around ridges and lee terrain features as wind increases and flurries accumulate.

Looking forward, a heavy hit of weather is headed for the region, check out the Forecaster Blog here.

Avalanche problems

  • Wind Slabs

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

    Accumulating snowfall will be most reactive where impacted by wind. Expect to find a growing and increasingly reactive slab as new snow accumulates, the deepest and most reactive deposits will be around ridges and lee features.

    Now hidden under fresh snow, older, stiffer wind slabs have become increasingly unreactive in the past week. However, you may still find (now buried 5-15 cm) old, hard wind slabs which remain sensitive to human triggering in the steeper more complex terrain features.

  • Storm Slabs

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

    In areas that have received fresh snow over the past few days, storm snow is settling over a variety of surfaces. More reactive slabs will be found in areas with deeper accumulations or impacted by wind. This problem is more likely in the southern half of the region (ex. Allison Pass and closer to the US border).

Confidence

Low - Uncertainty is due to extremely variable snowpack conditions reported through the region.

Weather Forecast

THURSDAY NIGHT: Flurries, 5 cm accumulating by morning / moderate southwesterly wind / ridgeline temperature low -10 / freezing level valley bottom

FRIDAY: Snow, 5-10 cm through the day / moderate gusting to strong southwesterly wind / ridgeline temperature high -3 / freezing level 900 m

SATURDAY: Flurries, 5-10 cm / strong southwesterly wind / ridgeline temperature high -4 / freezing level 1000 m

SUNDAY: Snow, 20-50 cm / moderate gusting to extreme southerly wind / ridgeline temperature high +1 / freezing level above 1500 m

Avalanche Summary

On Wednesday, loose dry avalanches to size 2 were triggered by solar warming. 

On Tuesday north of the Duffy, skiers were able to trigger reactive wind slabs on north and west aspects. 

A natural storm slab cycle occurred on Tuesday night near Cheam Mountain. Avalanches to size 2 were triggered by wind and slid on the storm snow interface, and did not step down to any weaker layers. Check out this Cheam MIN report. 

More evidence of recent large size 2-3.5 natural wind slab avalanches were reported on Monday. Several dry loose avalanches size 1-2, entraining 10-25 cm recent snow and running on top of wind slab, were observed in steep terrain.

On Sunday a size 3 natural avalanche was observed on a southwest facing alpine feature in the north of the region. MIN reports from Sunday detail a lot of good skiing in wind sheltered terrain.

Recent wind slab formations are widespread in the region, with small (size 1.5) wind slabs triggered with ski cutting on wind loaded slopes in the Coquihalla on Friday and Saturday. Check out this MIN report for a great example of layered/obscured wind slabs reactive to skier triggering in the south of the region. Another MIN detailing an avalanche involvement in the north of the region gives a clear picture of hazards perched on alpine features.

A notable size 3 (very large) persistent slab was remotely triggered (from a distance) by a group of skiers in the McGillivray Pass February 8th. This occurred on a southwest aspect at 2400 m. It was described as a hard wind slab formed over our facet layer from late January.

Snowpack Summary

Up to 25 cm low density snow overlies a widespread variety of surfaces: hard wind slab in exposed and open terrain, 20 cm (north of the region) to 60 cm (south of the region) of faceting older snow on sheltered and north-facing areas, and a crust on solar features.

Last weekend's, region-wide northeast winds formed many wind slabs across exposed higher elevation terrain. Winds have not been consistent, so don’t be surprised to find reactive, potentially layered or buried slabs on a wide range of aspects as you approach wind-exposed terrain.

Persistent weak layers of concern below this storm snow vary by location in the region. In the south, the primary feature, found at treeline and below in the Allison Pass area, is a melt-freeze crust from mid-January, about 40-50 cm deep. Although this layer is present (60-80 cm) in the Coquihalla snowpack, no persistent weak layers are currently considered problematic in that area.

In the north, many of the recent wind slab formations discussed above have been reactive to skier triggering on early February interfaces of surface hoar (think shaded aspects), and may also exist over a thin sun crust from the same period (think solar aspects). Another older (January 24), slightly deeper, widespread layer of faceted snow remains a concern in shallow or variable snowpack depth locations in the alpine. 

Also in the north, a melt-freeze crust from early December may be found 100 to 200 cm deep. Recent reports have suggested that this layer is unreactive and gaining strength, however ongoing cold temperatures may currently be weakening the snow around it in thin snowpack areas.

Terrain and Travel

  • Recent new snow may be hiding windslabs that were easily visible before the snow fell.
  • Avoid freshly wind loaded terrain features.
  • Watch for areas of hard wind slab on alpine features.
  • Be aware of highly variable recent wind loading patterns.
  • Closely monitor how the new snow is bonding to the old surface.

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