Avalanche Forecast

Archived

Mar 13th, 2021–Mar 14th, 2021

Alpine

Natural avalanches possible, human triggered probable.

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.

Treeline

Natural avalanches unlikely.

Below Treeline

Natural avalanches unlikely.

Regions

Purcells.

Overhead hazards such as cornice failures or small wet loose avalanches triggering large persistent slab avalanches will be the primary concern on Sunday. Pay attention to what's above your head and back off of slopes if the snow surface becomes moist.

Confidence

Moderate - Uncertainty is due to whether buried persistent weak layers become active, triggering avalanches, with the arrival of the forecast weather.

Weather Forecast

SATURDAY NIGHT: Clear / Moderate, southwest ridgetop wind / alpine low temperature 0 / Above freezing layer of air 1000-2000m.

SUNDAY: Sunny / Moderate, southwest ridgetop wind / alpine high temperature 4 / Freezing level 2300 m.

MONDAY: Mix of sun and cloud / Light, west ridgetop wind / alpine high temperature 3 / Freezing level 2100 m.

TUESDAY: Sunny / Light, west ridgetop wind / alpine high temperature 2 / Freezing level 2100 m.

Avalanche Summary

Naturally triggered wet loose avalanches were reported up to size 1.5 on solar aspects at all elevations on Friday.

Rapidly rising freezing levels and sunny skies are expected to elevate the avalanche hazard through the weekend.

Snowpack Summary

Dry snow can be found on shaded aspects at upper elevations. On solar aspects and at lower elevations expect crusty snow in the morning and soft, moist snow in the afternoon. Lingering wind slabs and large cornices still pose a hazard close to alpine ridgelines.

A Persistent weak layer of facets 40-60 cm. deep that was buried in mid-February was reactive in the north of the region earlier this month but no avalanches have been reported on this layer since Mar. 3rd. There are several other weak layers deeper in the snowpack composed of old surface hoar, facets and/or crusts, all of which have been recently unreactive.

Big warm ups can awaken dormant weak layers resulting in large avalanches. Steep, rocky, convex terrain with a shallow or thin to thick snowpack are examples of places where triggering these layers are more likely.

Sunshine and warm temperatures often cause large cornice failures which are hazardous on their own and can also trigger large avalanches on deeper weak layers that the weight of a single rider would not trigger.

Terrain and Travel

Avoid exposure to slopes that have cornices overhead.
Cornice failure may trigger large avalanches.
Back off slopes as the surface becomes moist or wet with rising temperatures.
Be aware of the potential for large avalanches due to the presence of a persistent slab.
Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.

Problems

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.

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.