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Avalanche Forecast

Archived

Mar 12th, 2021–Mar 13th, 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 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, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Purcells.

  Things are looking HOT up high for the next couple days. Overhead hazards such as cornice failures triggering large persistent slab avalanches will be a primary concern. Recently unreactive weak layers in the snowpack may become active with the rapidly rising freezing levels.

Confidence

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

Weather Forecast

  

FRIDAY NIGHT: Clear / Light, northeast ridgetop wind / alpine low temperature -11 / Freezing level valley bottom.

SATURDAY: Sunny / Light, southeast ridgetop wind / alpine high temperature 3 / Freezing level rapidly rising to 2500 m.

SUNDAY: Sunny / Light, southwest ridgetop wind / alpine high temperature 3 / Freezing level 2400 m.

MONDAY: Mix of sun and cloud / Light, northeast ridgetop wind / alpine high temperature 1 / Freezing level 2200 m.

Avalanche Summary

No avalanches were reported in this region on Thursday. However, the rapidly rising freezing level and sunny skies on Saturday are expected to elevate the avalanche hazard through the weekend.

On Tuesday, a size 2.5 cornice failure was reported.

There was a MIN report on Monday of a small wet loose point release triggering a size 2 slab avalanche around 30 cm. deep on a south aspect. See MIN HERE.

Snowpack Summary

Dry soft snow can be found on shaded aspects above around 1500 m. On south-facing slopes 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, like the one headed our way this weekend often 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.