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Avalanche Forecast

Archived

Jan 29th, 2020–Jan 30th, 2020

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

Regions

Lizard-Flathead.

Steady southwest winds continue to redistribute loose snow and build wind slabs. Watch for reactive pockets around steep rolls and ridge features.

Confidence

Moderate - Confidence is due to a stable weather pattern with little change expected.

Weather Forecast

Wednesday night: Mostly cloudy with isolated flurries, trace to 5 cm. Alpine low temperature -8 C. Moderate southwest winds.

Thursday: Mix of sun and cloud. Alpine high temperature -6 C. Light southwest winds. Freezing level climbing to 1500 m.

Friday: Mix of sun, cloud and flurries, 5 cm. Alpine high temperature +2 C. Moderate southwest winds. Freezing level rising to 2000 m.

Saturday: Flurries and snow, 10-15 cm. Alpine high temperature 0 C. Moderate southwest wind gusting to extreme. Freezing level dropping to 1600 m.

Avalanche Summary

Over the past several days, there have been reports of natural, human-triggered, and explosive triggered avalanches up to size 2.5 breaking in the new storm snow, specifically where it has been drifted by wind into stiffer slabs on northwest through northeast aspects. A couple of these avalanches have been reportedly triggered by cornice fall. See this MIN report for a helpful illustration of these wind slab avalanches.

Snowpack Summary

Southwest winds continue to drift 20-40 cm of recent snow into slabs on lee and cross-loaded terrain features at upper elevations. This combination has also contributed to cornice growth, increasing the need for vigilance with overhead hazard. The recent snow covers a temperature crust which formed Friday up to 1600 m and on higher elevation solar features.

The stout upper snowpack continues to settle in mild temperatures. Several crust layers exist in the mid-pack as a result of previous warming and rain events. These have not been identified as bed surfaces or failure planes in recent avalanche activity.

The bottom 10-20 cm of the snowpack consists of faceted snow and decomposing crusts. Although inherently weak, this basal layer has not been an active avalanche problem in the region since December.

Terrain and Travel

  • Be alert to conditions that change with elevation, aspect and exposure to wind.
  • Watch for wind-loaded pockets especially around ridgecrest and in extreme terrain.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.

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.