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

Regions

Northwest Inland.

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

Weather Forecast

Saturday night: Mostly clear, winds becoming east and increasing to moderate, alpine temperatures dropping to -2 C, freezing level dropping to 1300 m.

Sunday: Mainly sunny, strong east ridgetop wind, alpine high temperatures near 0 C, freezing level rising to 1600 m.

Monday: Sunny, light southeast ridgetop wind, alpine high temperature +2 C, freezing level 2100 m. 

Tuesday: Sunny, light northwest wind, alpine high temperature +2 C, freezing level 2100 m. 

Avalanche Summary

As the spring diurnal cycle continues, there have been reports of numerous large (size 2) wet loose avalanches on south-facing slopes at all elevations and a few large (size 2-2.5) cornice failures on all aspects. 

One wet slab was suspected to have released on the November crust layer on a shallow west aspect in the alpine, triggered by a loose wet avalanche from above. Triggers, such as cornice fall, loose wet avalanches, or a rider hitting a thin spot in the snowpack, are particularly concerning on steep, rocky slopes with a shallow or thin to thick snowpack as they are the most likely ways to trigger deeply buried weak layers.

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. Check out the Forecaster's Blog for information on spring conditions. 

Strong solar radiation and warming will trigger more wet loose avalanches on Saturday as the snow loses cohesion. Cornices are large and fragile. Cornice falls could trigger large avalanches on the slopes below.

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.

Terrain and Travel

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

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.

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.