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Avalanche Forecast

Archived

Apr 22nd, 2023–Apr 23rd, 2023

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

Regions

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Numerous very large deep persistent slab avalanches have been reported on a variety of aspects in the alpine.

Avoid steep, rocky, shallow, or thin-to-thick areas in the snowpack where triggering slabs is more likely.

Confidence

Low

Avalanche Summary

Numerous naturally triggered very large (up to size 3.5) deep persistent slab avalanches were reported on a variety of aspects in the alpine on Friday. They failed on either a crust/facet layer 100+cm deep or weak facets at the bottom of the snowpack. Follow the link on this MIN for the photos of these notable avalanches.

Observations are limited at this time of year, please consider sharing any information or photos you have on the Mountain Information Network to help guide our forecasts.

Snowpack Summary

A weak layer of crust/facets down 100+ cm and/or weak facets at the bottom of the snowpack have recently become reactive.

Warm temperatures and/or solar radiation will increase the likelihood of triggering very large avalanches.

Cornices are large and loom over slopes below like gargoyles.

Recent wind has varied in direction so watch for lingering wind slabs on all aspects at upper elevations.

Weather Summary

Saturday Night

Cloudy with isolated flurries; 0-3 cm / 20 km/h southeast ridgetop wind / Temperature at treeline around -3 C / Freezing level 700 m

Sunday

Mostly cloudy with isolated flurries; 5-10 cm / 30 km/h southeast ridgetop wind / Temperature at treeline around 0 C / Freezing level 1000 m

Monday

Mix of sun and cloud with flurries; 5-10 cm / 30 km/h south ridgetop wind / Temperature at treeline around -3 C / Freezing level 700 m

Tuesday

Mix of sun and cloud / 30 km/h south ridgetop wind / Temperature at treeline around -2 C / Freezing level 800 m

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • The likelihood of deep persistent slab avalanches will increase with each day of warm weather.
  • In areas where deep persistent slabs may exist, avoid shallow or variable depth snowpacks and unsupported terrain features.
  • Cornice failure may trigger large avalanches.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.

Problems

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.

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.

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.