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Avalanche Forecast

Archived

Dec 28th, 2019–Dec 29th, 2019

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

Regions

Northwest Inland.

Triggering slab avalanches is possible on steep terrain features in the alpine and at treeline.

Confidence

Low - Uncertainty is due to the limited number of field observations.

Weather Forecast

SATURDAY NIGHT: Light flurries delivering a few cm of snow, 20-40 km/h wind from the southwest, alpine temperatures around -5 C.

SUNDAY: Mostly cloudy with isolated flurries and up to 5 cm of new snow, 20-30 km/h wind from the southwest, alpine high temperatures around -3 C.

MONDAY: Next frontal system arrives bringing 5-15 cm of snow by the afternoon, 40-60 km/h wind from the southwest, freezing level up to 1000 m with alpine high temperatures around -2 C.

TUESDAY: Stormy weather continues with another 5-15 cm of snow, 30-50 km/h wind from the southwest, alpine high temperatures around -2 C.

Avalanche Summary

A size 1.5 skier-triggered avalanche was reported on the MIN on Friday (see full report and photos here). The avalanche occurred on a convex north-facing slope at treeline and failed on a surface hoar layer 50 cm below the surface. This is the first avalanche reported in the region the past few days, but highlights a potential persistent slab problem that could produce larger avalanches on bigger terrain features.

Snowpack Summary

Sheltered terrain has 10-30 cm of low density snow, while exposed terrain has been affected by recent wind from the southwest. Reports suggest there are two layers of surface hoar buried 25-50 cm below the surface. A skier triggered avalanche on one of these layers on Friday suggests the snow above this layer has likely developed into a reactive slab in certain parts of the region. There is uncertainty about the distribution of this layer, but the sheltered slopes around treeline elevations are the most suspect for having preserved surface hoar.

Terrain and Travel

  • Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
  • Use ridges or ribs to avoid areas of wind loaded snow.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.

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.