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Avalanche Forecast

Archived

Apr 19th, 2021–Apr 20th, 2021

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

Regions

Northwest Coastal.

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

Confidence

Moderate - Uncertainty is due to difficult to forecast freezing levels.

Weather Forecast

Monday night: Clear, light wind becoming northwest, alpine temperature -2 C, freezing level 2400 m. 

Tuesday: Sunny with cloud in the afternoon, light northwest wind, alpine high temperature +2 C, freezing level 2200 m. 

Wednesday: Mix of sun and cloud, moderate northwest wind with strong ridgetop gusts, alpine high temperature +1 C, freezing level 1600 m. 

Thursday: Mainly sunny, moderate northeast wind, alpine high temperature near 0 C, freezing level 1500 m.

Avalanche Summary

Numerous large to very large (size 2-3) natural and explosive-triggered wet loose and wet slab avalanches were reported over the weekend across elevations. A couple of natural cornice failures on northeast alpine slopes triggered very large (size 3) avalanches on slopes below. Glide avalanches have also been reported.

Snowpack Summary

The snowpack 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. As the heat penetrates deeper into the snowpack, operators have reported larger wet slab avalanches. These avalanches are dense, destructive, and can run far, reaching otherwise snow-free valley bottoms. Cornices are large and looming along ridgelines. Sun and warm temperatures will increase the chances of cornice failures, especially when temperatures remain above zero overnight. Check out the Forecaster's Blog for information on how to manage these 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

  • Avalanche hazard is expected to to increase througout the day, think carefully about your egress.
  • 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.
  • Minimize overhead exposure; avalanches triggered by warming or cornice fall may be large and destructive.

Problems

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.

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.