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Avalanche Forecast

Archived

Jan 8th, 2020–Jan 9th, 2020

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

Regions

Northwest Inland.

Analyze slopes for patterns of wind effect to navigate around wind slab problems at higher elevations. Observations from the region are limited but we can expect our persistent slab problem became a bit more serious with the recent snowfall. 

Confidence

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

Weather Forecast

Wednesday night: Cloudy with isolated flurries. Light to moderate northwest winds.

Thursday: Cloudy with scattered flurries bringing a trace to 5 cm of new snow, easing over the day and beginning again overnight. Light to moderate west winds. Alpine temperatures around -10 C.

Friday: Cloudy with continuing flurries and 2-day snow totals of approximately 10 cm by end of day. Light southeast winds. Alpine high temperatures around -12.

Saturday: Mainly cloudy with isolated flurries and a trace of new snow. Light west winds. Alpine high temperatures around -16

Avalanche Summary

Reports from the region are limited, however a widespread avalanche cycle is likely to have occurred throughout the southern part of the region on Friday and Saturday, when more than 40 cm of snow fell. 

A report from Tuesday includes an observation of a large, audible natural avalanche in the Howson range. Details on the event are limited, but the natural trigger suggests touchy conditions still exist in parts of the region.

Before the 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 have occurred on all aspects mainly at treeline but also in the alpine. Loading from recent snowfall has increased the size and consequences of avalanches on these layers.

Snowpack Summary

40-60 cm of recent storm snow has been highly wind affected in the alpine. Expect to find deep wind loaded pockets below ridgetops and the lee sides of exposed features.

A couple of layers of surface hoar are now buried 50-100 cm below the surface in sheltered areas around treeline. Reports of avalanche activity on these layers before the last storm suggest they may be active. Loading from recent snowfall has increased this potential.

Terrain and Travel

  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Avoid freshly wind loaded terrain features.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.

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.