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Avalanche Forecast

Archived

Jan 30th, 2020–Jan 31st, 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 and human triggered avalanches likely.
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.

Regions

Lizard-Flathead.

A wet and warm weather system is headed for the region. Expect to find increasingly reactive storm slabs as snow falls with strong winds and rising temperatures.

Confidence

Moderate - Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

Thursday night: Cloudy with isolated flurries, up to 10 cm. Alpine low temperature -7 C. Moderate southwest winds.

Friday: Mix of sun, cloud and flurries, 10-20 cm. Alpine high temperature +2 C. Moderate to strong southwest winds. Freezing level rising to 2000 m.

Saturday: Flurries and snow, 10-30 cm. Alpine high temperature 0 C. Moderate southwest wind gusting to extreme. Freezing level 1900 m.

Sunday: Mix of sun, cloud, and isolated flurries. Alpine high temperature -9 C. Light southwest wind gusting to strong. Freezing level dropping to valley bottom.

Avalanche Summary

On Wednesday, explosives triggered avalanches to size 2 within the recent storm snow.

Last weekend and early this week, there were reports of natural, human-triggered, and explosive triggered avalanches up to size 2.5 breaking in the new storm snow, specifically where it was 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 have impacted 20-40 cm recent snow producing slabs in the alpine and exposed tree line areas and building reactive cornices. The recent snow covers a temperature crust which formed Friday up to 1600 m and higher elevations 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.
  • Be aware of the potential for large avalanches due to the presence of a deep persistent slab.
  • Extra caution for areas experiencing rapidly warming temperatures for the first time.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.