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Avalanche Forecast

Archived

Jan 11th, 2021–Jan 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 possible, human triggered probable.
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.

Regions

North Rockies.

Stormy conditions are expected, forming new slabs. The persistent slab problem shouldn't be forgotten yet, particularly around Pine Pass.

Confidence

Moderate - Uncertainty is due to limitations in the field data. Uncertainty is due to extremely variable snowpack conditions reported through the region.

Weather Forecast

MONDAY NIGHT: Clear skies, 40 to 60 km/h southwest wind, alpine temperature -5 C.

TUESDAY: Cloudy with snowfall, accumulation 5 to 15 cm with the most in the west of the region, 40 to 60 km/h southwest wind, alpine temperature -4 C, freezing level 1300 m.

WEDNESDAY: Cloudy with snowfall, accumulation 5 to 15 cm with the most in the west of the region, 40 km/h west wind, alpine temperature -10 C.

THURSDAY: Mix of sun and cloud, 20 km/h west wind, alpine temperature -9 C.

Avalanche Summary

No recent avalanche activity has been noted. Avalanche activity may increase on Tuesday and Wednesday due to new snow, strong wind, and relatively warm air temperature. Be on your guard during this stormy period.

Snowpack Summary

New snow will accumulate on Tuesday and Wednesday, with the most expected in the west of the region. Storm slabs are expected to rapidly form with relatively mild air temperature. The snow will fall with strong southwest wind, which may form new wind slabs in exposed terrain. This snow will accumulate on previously wind-affected snow in exposed areas, soft snow and perhaps surface hoar in sheltered areas, and a hard melt-freeze crust below about 1600 m.

You may find a couple weak layers of surface hoar in the top 50 cm of the snowpack, as suggested by numerous MINs in the region. This layer is expected to be found in terrain features sheltered from the wind, for example in openings below treeline or at treeline elevations. As the new snow forms slab properties, these layers could become reactive to human traffic.

Around 80 to 100 cm of snow rests above a weak layer buried in early-December which has created a persistent slab avalanche problem and a low probability/high consequence scenario. Depending on location, the buried weak layer is composed of surface hoar and/or a crust/facet combination. Reports suggest that this layer has become dormant in much of the region except for perhaps the Pine Pass area.

Terrain and Travel

  • Watch for fresh storm slabs building throughout the day.
  • Be aware of the potential for larger than expected storm slabs due to the presence of buried surface hoar.
  • Expect slab conditions to change drastically as you move into wind exposed terrain.
  • Use small low consequence slopes to test the bond of the new snow.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.