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Avalanche Forecast

Archived

Feb 18th, 2020–Feb 19th, 2020

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

Regions

Lizard-Flathead.

Brilliant clear days ahead! The sun is getting higher in the sky at this time of year and can pack a punch. Keep an eye on solar exposed slopes and be ready to back off if they start to show signs of instability like pinwheeling.

Confidence

High -

Weather Forecast

Tuesday night: Clear. Light northwest wind. Alpine low -16 C. Freezing level valley bottom.

Wednesday: Sunny. Light northwest winds. Alpine high -5 C. Freezing level 1000 m.

Thursday: Sunny. Light southwest wind increasing to strong at ridgetop. Alpine high -3 C. Freezing level 1300 m.

Friday: Sunny. Moderate southwest wind, strong at ridgetop. Alpine high -3 C. Freezing level 1400 m.

Avalanche Summary

Observed Monday; natural and explosive triggered cornice falls size 1.5-2, occasionally triggering thin slabs on slopes below. 

There have been two reports of avalanches running on the early February rain crust. Explosive control work produced a size 2 with crown depth up to 80 cm on Tuesday. A skier triggered size 2 on a southeast aspect is described in this MIN report from Sunday.

Snowpack Summary

A thin sun crust may be found at the surface on steep solar aspects. Recent snow has been wind loaded into thick, slabs in immediate lees of ridge crests at alpine and exposed treeline elevations. In sheltered areas, the recent snow is settling and may contain various sun crusts and layers of small surface hoar crystals. 

A thick rain crust sits 30-60 cm below the surface, up to 2100 m. A slab/weak layer/slick bed surface configuration may be imminent as faceting has been observed between this crust and overlying snow.

The bottom 20 cm of the snowpack consists of basal facets and decomposing crusts that have not been an active avalanche problem since December, but could reemerge as a problem after sustained warming.

Terrain and Travel

  • Watch for wind-loaded pockets especially around ridgecrest and in extreme terrain.
  • Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet
  • A moist or wet snow surface, pinwheeling and natural avalanches are all indicators of a weakening snowpack.
  • Avoid exposure to slopes that have cornices overhead.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.