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Avalanche Forecast

Archived

Jan 12th, 2020–Jan 13th, 2020

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 unlikely, human triggered possible.
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

Northwest Inland.

Be on alert for newly formed wind slabs if you're braving the cold on Monday.

Confidence

High -

Weather Forecast

Sunday night: Clear. Moderate to strong northeast winds, decreasing.

Monday: Sunny. Light to moderate east winds. Alpine temperatures around -27.

Tuesday: Sunny. Moderate east winds. Alpine temperatures around -28.

Wednesday: Sunny. Moderate east winds, increasing over the day. Alpine temperatures around -27.

Avalanche Summary

Reports from the region are limited, however widespread avalanche cycles occurred over the past weekend and again on Friday as a result of rapid loading during each storm. A MIN report detailing activity on Friday describes numerous 30 cm-deep storm slabs releasing from 35+ degree slopes and reaching size 2.5 (large) in many parts of the region. 

Before the last storm, reports of persistent slab avalanches associated with buried surface hoar were trickling in, from Ashman, Houston Telkwas, and the Howsons. They included natural and artificially triggered avalanches ranging in size from 1.5-3, and activity was noted on all aspects, mainly at treeline. Loading from recent snowfall has increased the size and consequences of avalanches that may occur on these layers.

Looking forward, wind-redistributed snow from the recent storm will remain our primary concern. Under forecast cool temperatures the new snow may continue to react to human triggers as dry loose sluffs. Greater danger should be expected in areas where winds have redistributed snow into deeper and more reactive slabs. 

Snowpack Summary

About 30 cm of new snow accumulated in the region on Friday. In many areas this recent snow is expected to be heavily wind-affected by recent outflow winds. This snow mainly buried wind-affected surfaces at alpine and upper treeline elevations, but it may cover a new layer of surface hoar at sheltered lower elevations or a thin sun crust on steeper south-facing slopes.

Below the new snow interface, 40-60 cm of older, wind-affected storm snow is expected to be forming a good bond with the interface below it.

A concerning pair of surface hoar layers are currently buried about 80-120 cm below the surface in sheltered areas around treeline. There is uncertainty about where this layer remains a problem, but reports of avalanche activity on these layers before the last storm suggest they may be active. The new load delivered by Friday's storm increased this potential.

Terrain and Travel

  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Keep your guard up at lower elevations. Wind slab formation has been extensive.
  • Be aware of the potential for loose avalanches in steep terrain where snow hasn't formed a slab.

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.