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Avalanche Forecast

Archived

Apr 17th, 2021–Apr 18th, 2021

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells.

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.

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.

Problems

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

Wet Slabs

Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.