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Avalanche Forecast

Archived

Mar 8th, 2023–Mar 9th, 2023

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

Coquihalla, Harrison-Fraser, Manning, Skagit.

Lingering wind slabs may remain reactive to human triggers on a variety of aspects in the alpine.

Avoid travelling on slopes below cornices.

Confidence

Moderate

Avalanche Summary

Naturally triggered size 1 wet loose avalanches were reported on steep sunny aspects on Tuesday.

On Saturday, explosive avalanche control work resulted in numerous very large(size 3-4) persistent slab avalanches. They occurred on a variety of aspects below steep, wind loaded treeline ridgetops and large alpine features.

Snowpack Summary

Cornices are large and may fail with daytime heating.

Sun crust on sunny aspects. Recent southeasterly winds formed wind slabs on a variety of aspects at treeline and above. These slabs are sitting on either hard surfaces or small facets.

There are several crusts in the mid/lower snowpack. Their depth ranges from 150-200 + cm.

A large trigger, such as a cornice fall or smaller avalanche in motion, could trigger a very large avalanche on one of these deeply buried weak layers. A single rider is unlikely to trigger this layer unless they are in a steep, rocky, and shallow snowpack area.

Weather Summary

Wednesday Night

Clear skies / Light southeast ridgetop wind / Low temperature at treeline -11 C / Freezing level valley bottom.

Thursday

Mix of sun and cloud / Light southeast ridgetop wind / High temperature at treeline -3 C / Freezing level 1100 m.

Friday

Cloudy with isolated flurries; 0-3 cm / Moderate southeast ridgetop wind / High temperature at treeline -4 C / Freezing level 900 m.

Saturday

Mix of sun and cloud / Light southwest ridgetop wind / High temperature at treeline -2 C / Freezing level 1100 m.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Be careful with wind slabs, especially in steep, unsupported and/or convex terrain features.
  • Avoid shallow snowpack areas, rock outcroppings and steep convex terrain where triggering is most likely.
  • Avoid exposure to slopes that have cornices overhead.
  • Be alert to conditions that change with elevation and sun exposure.

Problems

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.

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.