Avalanche forecast

Purcells

Apr 17 – 18, 2021Avalanche Canada
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Avalanche danger

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

Yet another day without a good overnight refreeze. Temperatures will drop during the day, but in areas where the sun comes out conditions can deteriorate rapidly and the hazard may increase to CONSIDERABLE.

Check out the Forecaster's Blog on warming and how to stay safe.

Avalanche problems

  • Cornices

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

    Cornices are large and fragile. If the sun comes out prolonged sun exposure might lead to cornice falls. Cornice falls are dangerous on their own and they can trigger slabs on slopes below. There is a chance that the heavy load of a failed cornice triggers deep persistent slabs. 

    Cornices often break further back than expected. Give them a wide berth when travelling on ridgetops. Avoid slopes with cornices overhead, especially if they are exposed to the sun. Take into account long runout distances when travelling underneath slopes with cornices overhead.

  • Wet Slabs

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

    It is uncertain if and how long the sun will come out. The likelihood of wet slab avalanches might increase in areas with prolonged sun exposure. Freezing levels are forecast to stay at 2800 m overnight dropping to 1700 m during the day. Clear skies during the night might allow for a shallow refreeze but the snow will likely become moist/wet as soon as the sun comes out. The upper snowpack can quickly lose cohesion and become unstable with strong solar radiation. 

    Wet loose avalanche activity can be expected on sun exposed slopes and below treeline.

    Back off slopes when the snow becomes moist or wet. Avalanches might grow larger than expected by entraining snow along the way and reach lower elevations. Avoid exposing yourself to slopes that are exposed to the sun, especially in the afternoon. Be aware what is above you when leaving your skiing or riding area.

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2.5–4

    Each day of warm weather increased the likelihood of reactivating buried weak layers. It is uncertain if and when large slab avalanches may release but the possibility still remains. 

    Deeper releases are most likely in shallow snowpack areas on the eastern slopes of the Purcells. Heavy cornice falls might trigger large slab avalanches. Skiers and riders might be able to trigger buried weak layers in rocky and shallow snowpack areas. 

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength. Uncertainty is due to the timing or intensity of solar radiation and its effect on the snowpack.

Weather Forecast

The high pressure ridge is forecast to break down Sunday morning bringing light precipitation and lowering freezing levels followed by a classic diurnal melt-freeze cycle on Monday for the next couple of days.

Saturday night: Clear, moderate northwest wind, alpine low +3 C, freezing level around 2800 m.

Sunday: Mainly cloudy with isolated flurries, up to 5 cm new snow and rain below treeline, moderate northerly wind, alpine high +2 C, freezing level dropping to 1700 m during the day.

Monday: Mainly sunny, light to moderate northerly wind, alpine high +3 C, freezing level 2000 m.

Tuesday: Mainly sunny, light variable wind, alpine high +5 C, freezing level 2300 m. 

Avalanche Summary

A widespread wet loose avalanche cycle to size 2 was observed on Friday. Increased wet loose and potentially wet slab avalanche activity and cornice failures likely continued on Saturday. On Friday a skier triggered wind slab of size 2 was reported in the south of the region.

A large natural cornice fall of size 3 likely released a slab on the northeast facing slope below and was reported on Wednesday. Small wet loose avalanches to size 1 and a couple size 2 were observed on steep solar slopes. 

Two weeks ago, a significant natural cornice failure triggered a persistent slab on the slope below and resulted in a size 3 avalanche. This occurred on a high alpine steep east-facing slope. This is an important reminder to stay well back from ridgetops that may be corniced and avoid slopes with overhead cornice exposure especially under the current conditions where recent rapid growth and warm temperatures have made them large and fragile. 

Snowpack Summary

The snow surface consists of a melt-freeze crust on solar aspects and shaded aspects up to treeline which can transition into moist/wet snow during the day. Dry snow can still be found on northerly aspects high in the alpine. 10-30 cm of recent snow has formed isolated pockets of wind slab on leeward slopes and behind terrain features. The new snow sits on a series of melt-freeze crusts on all aspects below 1900 m and southerly aspects to mountain top. 

Cornices are large, fragile and primed to fail with warm temperatures. Many natural cornice falls were recently observed. Some triggered slabs on the slopes below.

Widespread crust layers formed over the last month in the mid to upper snowpack. One widespread crust from mid-March found 30-60 cm deep saw isolated instances of large solar-triggered slab avalanches during the last sunny period. The early November crust is buried down 180-200 cm with faceted crystals above and below it. 

Terrain and Travel

  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.
  • Avoid slopes when the solar radiation is strong, especially if they have large cornices overhead.
  • Avoid sun exposed slopes, especially if snow surface is moist or wet.
  • The likelihood of deep persistent slab avalanches will increase with each day of warm weather.

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