Avalanche forecast

Purcells

Jan 16 – 17, 2021Avalanche Canada
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Avalanche danger

Mon
Tue
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

We're rounding the corner of improving stability, but lots of terrain is still suspect. You can analyze the surface to locate wind slabs lurking in leeward features, but a slab settling over surface hoar isn't so obvious. Use conservative terrain to manage your uncertainties.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely - Possible
    Size
    2–3

    Weak layers of surface hoar exist 50-150 cm deep in sheltered treeline terrain (and below). These layers are highly variable in form and distribution. A widespread buried crust also exists near the base of the snowpack. Although these layers have only been associated with isolated recent avalanche activity, they are adjusting to a new load of new snow and potential currently exists for avalanches in surface layers to step down to a deeper layer to create larger, more destructive avalanches.

  • Storm Slabs

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

    Up to 40 cm of storm snow, coupled with strong winds have loaded avalanche terrain in the region with new slab problems. In exposed areas, thick and reactive wind slabs should be expected in leeward features. In sheltered areas, storm slabs are more likely to be reactive where they overlie a fragile layer of surface hoar. (Think shaded, wind protected slopes around treeline)

    Cornices have also been growing. A cornice fall is a large load that can trigger avalanches on the slope below.

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

Friday night: Decreasing cloud with isolated flurries. Light to moderate northwest wind (potentially strong in the alpine), shifting northwest.

Saturday: A mix of sun and cloud with cloud increasing. Light northwest winds (moderate to strong in the alpine) shifting southwest late in the day. Alpine high temperatures around -7 with freezing levels rising to about 1000 metres.

Sunday: Cloudy with isolated flurries and up to 5 cm of new snow continuing through the evening. Light to moderate southwest winds (moderate to strong in the alpine), slowly shifting northwest. Alpine high temperatures around -7 with freezing levels rising to about 1000 metres.

Monday: Clearing. Light to moderate northwest winds, easing over the day. Alpine high temperatures around -8.

Avalanche Summary

Observations of the storm's aftermath in the Golden area on Wednesday showed explosives control yielding many size 2 (large) storm slabs about 40 cm deep, but with a variability in depth that shows good evidence of wind redistribution.

In the Quartz Creek area, a skier triggered a size 2 (large) storm slab on a 35 degree, north-facing slope at 2100 metres, giving a good indication of storm instabilities that certainly didn't require explosives to produce large avalanches. Check out these MIN reports from the Quartz Creek zone again on Thursday. Riders here had several encounters with reactive slabs formed over surface hoar.

On Thursday we also received reports of the natural avalanche cycle during the storm producing avalanches reaching size 3 in the central part of the region, lending support to the idea that some larger releases may have involved persistent weak layers.

Looking forward to Saturday, it remains a very good idea to give newly formed wind slabs a wide berth as you approach wind affected terrain, and to remain suspicious of steeper sheltered slopes where new snow may have slabbed up over a layer of surface hoar or crust. Using small test slopes to investigate the bond of new snow is a good plan, but be cautious about extending your observations to more committing terrain.

Snowpack Summary

15-40 cm of snow fell in the region during Tuesday's storm. It buried wind affected surfaces at upper elevations, as well as surface hoar in wind sheltered areas and a thin crust on solar aspects. In exposed areas, elevated winds have been redistributing this storm snow into reactive wind slabs and adding to cornices. 

In most places, the storm snow is probably forming a good bond with the previous surface. Exceptions are likely in areas harbouring surface hoar. Think of shaded upper below treeline and lower treeline areas, above where surface crust exists and below more wind-affected elevations.

A couple of other persistent weak layers exist in the upper to mid snowpack:

  • The upper layer, found 40-60 cm down, is a spotty layer of surface hoar at treeline and below. 
  • The lower layer, found 90-140 cm down, is widespread and can present as surface hoar and/or a crust.

Avalanche activity on these layers has dwindled since last week, with snowpack tests results showing some variability between sudden and more resistant propagation. With that in mind, these layers should still figure into your terrain selection in the Purcells.

The lower snowpack is characterized by more crusts, the most notable is a rain crust from early November that is near the base of the snowpack, surrounded by a weak layer of sugary facets. It is most likely to be reactive to human triggers in steep, shallow, rocky areas with a thin to thick snowpack.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Use small low consequence slopes to test the bond of the new snow.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • When a thick, melt-freeze surface crust is present, avalanche activity is unlikely.

Regions covered