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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.

Regions

Northwest Coastal.

Start (and finish) your day early and avoid exposure to sun-exposed avalanche terrain. Steer clear of cornices overhead, and plan your egress route carefully. 

Confidence

Moderate - The snowpack structure is generally well understood.

Weather Forecast

Saturday night: Clear, light variable wind, alpine temperatures dropping to +2 C, freezing level around 2000 m.

Sunday: Mainly sunny, strong east ridgetop wind, alpine high temperature +5 C, freezing level rising to 2800 m.

Monday: Sunny, light variable wind, alpine high temperature +7 C, freezing level 2400 m. 

Tuesday: Mix of sun and cloud, light northwest wind, alpine high temperature +5 C, freezing level 2000 m. 

Avalanche Summary

Numerous large to very large (size 2-3) natural and explosive-triggered wet loose and wet slab avalanches have been reported in the last few days on all elevations and aspects. These conditions are described in this MIN report from Oscar Peak on Friday. 

On Friday, a natural cornice failure triggered a very large (size 3) slab avalanche, breaking 70 cm deep on a crust layer on a northeast aspect near 2000 m. Glide avalanches have also been reported.

Snowpack Summary

Lingering wind slabs may still be reactive in isolated locations on steep northerly aspects in the alpine that still have dry snow. All other terrain has been undergoing a melt-freeze cycle with successive days of warm sunny weather. 

Strong solar radiation and warming will likely initiate wet loose avalanches as the snow loses cohesion. Cornices are large and fragile. Cornice falls could trigger large avalanches on the slopes below. Check out the Forecaster's Blog for information on spring conditions. 

The snowpack is overall strong and settled in most areas, however, sustained warming can increase the likelihood of large avalanches failing on deeply buried weak layers. This is most likely to occur on steep, rocky alpine slopes with a shallow or thin to thick snowpack.

Glide cracks releasing as full depth glide slab avalanches become more common in the spring and are extremely difficult to predict. Best practice is to avoid slopes with glide cracks.

Terrain and Travel

  • Minimize overhead exposure; avalanches triggered by warming or cornice fall may be large and destructive.
  • Back off slopes as the surface becomes moist or wet with rising temperatures.
  • As surface loses cohesion due to melting, loose wet avalanches become common in steeper terrain.
  • Extra caution is needed around cornices under the current conditions.

Problems

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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.