Avalanche Forecast

Archived

Apr 20th, 2021–Apr 21st, 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.

Treeline

Natural avalanches unlikely.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely.

Treeline

Natural avalanches unlikely.

Below Treeline

Natural avalanches unlikely.

Regions

Northwest Inland.

There is uncertainty around how much increasing cloud and wind may limit wet avalanche concerns. Plan your day around avoiding sun-exposed slopes and cornices at peak daytime warming.

Confidence

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

Weather Forecast

Tuesday night: Partly cloudy, moderate northwest wind with strong ridgetop gusts, alpine temperature near 0 C, freezing level dropping to 1200 m.

Wednesday: Increasing cloud, isolated flurries with trace accumulations, moderate northwest wind with strong ridgetop gusts, alpine high temperature -1 C, freezing level 1500 m.

Thursday: Mix of sun and cloud, light east wind, alpine high temperature -2 C, freezing level 1300 m.

Friday: Mix of sun and cloud, light southeast wind, alpine high temperature -3 C, freezing level 1300 m.

Avalanche Summary

Over the weekend, there were 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 a variety of 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. 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

The snowpack has been undergoing a melt-freeze cycle with successive days of warm sunny weather. There is uncertainty as to how much the increasing cloud and wind on Wednesday will limit wet avalanche concerns. If there is strong solar radiation and warming, it 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. 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.

Terrain and Travel

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.