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Avalanche Forecast

Archived

Mar 20th, 2021–Mar 22nd, 2021

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

Regions

North Rockies.

Strong wind is forecast on Sunday, which may form new wind slabs that could be touchy to riders during formation and into Monday. The problem will be most likely where 15+ cm of soft snow is found, so assess for new snow, strong wind, and slab characteristics during your travels.

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

SATURDAY NIGHT: Increasing cloud with overnight snowfall, accumulation 1 to 3 cm, 40 km/h southwest wind, alpine temperature -13 C.

SUNDAY: Cloudy with snowfall, accumulation 10 to 20 cm with localized areas of 30 cm possible, 40 to 60 km/h southwest wind, alpine temperature -10 C.

MONDAY: Early-morning snowfall then a mix of sun and cloud, accumulation 5 to 10 cm, 20 km/h west wind, alpine temperature -9 C.

TUESDAY: Cloudy with snowfall, accumulation 5 to 15 cm, 40 km/h southwest wind, alpine temperature -6 C.

Avalanche Summary

No new avalanches were observed on Friday. On Thursday, some large cornice failures were observed, of which some triggered small slab avalanches on the slopes below (see here). Looking forward, wind slabs are likely to be triggered by riders on Sunday at higher elevations, as new snow and strong wind are forecast.

Snowpack Summary

Around 15 to 30 cm of new snow may be found across the region by Sunday afternoon. Wind slabs are likely forming at high elevations in exposed terrain from strong southwest wind. Storm slabs may be found in steep sheltered terrain. This snow overlies a thick melt-freeze crust up to treeline and to mountain tops on sun-exposed slopes. Cornices are very large along ridgelines and always have the possibility of failing naturally or from the weight of a human.

A weak layer of facets buried mid-February may be found around 100 to 120 cm deep, or shallower in the east of the region. This layer was reactive about a week ago in the Hasler riding area (see here). The most likely place to trigger this layer is where the snowpack is shallow in alpine terrain. Large loads, such a cornice falls or multiple sleds on the same slope, often trigger slopes that single rider couldn't trigger.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Wind slabs are most reactive during their formation.
  • Avoid exposure to slopes that have cornices overhead.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Avoid areas with a thin or variable snowpack.

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.

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.