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Avalanche Forecast

Archived

Jan 11th, 2020–Jan 12th, 2020

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

Regions

Northwest Inland.

Cold temperatures and strong winds are the story for Sunday. Expect danger to increase over the day as snow is redistributed into reactive new wind slabs. This is expected to occur in exposed areas at all elevations.

Confidence

High -

Weather Forecast

Saturday night: Possible light flurries followed by clearing. Light northwest winds shifting northeast and increasing.

Sunday: Sunny. Moderate to strong northeast winds. Alpine temperatures around -27.

Monday: Sunny. Light to moderate east winds. Alpine temperatures around -28.

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

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 Friday's 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. The new snow mainly buried wind-affected surfaces at alpine and upper treeline elevations. 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 75-120 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

  • Avoid freshly wind loaded terrain features.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Approach steep open slopes at and below treeline cautiously, buried surface hoar may exist.

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.