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Avalanche Forecast

Archived

Feb 22nd, 2020–Feb 23rd, 2020

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.
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.

Watch for changing conditions with arrival of Sunday's new snow and Strong wind. Be alert for signs of instability - especially if over 10 cm new snow accumulates OR you encounter signs of fresh wind loading under foot / snowmobile.  

Confidence

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

Weather Forecast

Saturday Night: Cloudy clear periods and Isolated flurries. Trace Accumulations. Moderate West winds (15-35 km/hr) will diminish to Light Southwest (15-20 km/hr) into the early morning. Freezing level will drop to valleys. Alpine Low -8 C.  

Sunday: Increasing Clouds with Snow arriving mid-day. Total accumulations of 10-15cm. Moderate Southwest winds (20 km/hr) becoming strong ( 55 km/hr). Alpine High -2 C with Freezing Levels rising to 1400m.

Monday: Mostly cloudy with scattered flurries. 3-10 cm of new snow. Winds diminishing Light to Moderate (15-35 km /h) from the West / Northwest. Alpine Highs around -4 / Low -7 with Freezing Level 1100m.   

Tuesday: Cloudy with continued snow in morning and mix of sun and clouds by afternoon. 3-8 cm new snow. Winds Moderate (20-35 km /h) from the West / Northwest. Alpine Highs around -6 / Low -13 with Freezing Level 1200m.  

Avalanche Summary

No notable avalanche activity has been reported in the region since Tuesday. We forecast this to change with Strong Winds and the arrival of new snow Sunday.  

There were several notable avalanches Sunday to Tuesday. Explosive control work Monday and Tuesday dropped many large cornices (size 2), some triggering slab avalanches on slopes below that failed on the early February crust. A skier triggered large avalanche (size 2) on a southeast aspect, which failed on the early February crust is described in this MIN report from the Lizard Range on Sunday, Feb 16th Explosive control work produced a large avalanche (size 2) with crown depth up to 80 cm on Tuesday, Feb 18th that also failed on the early February crust. Another MIN from Limestone Ridge from Tuesday, Feb 18th reports a large avalanche (size 2) that was either loose wet or cornice triggered.  

Highest precipitation rates of snow are forecast to arrive on Sunday afternoon and into the night. However, even small amounts of new snow earlier in the day combined with Strong winds can lead to the development of wind slabs in the Alpine, the immediate lee of Ridgetop and on cross loaded features at treeline and below.  

Snowpack Summary

New snow will fall on a variety of snow surfaces: melt freeze and sun crusts on steep solar aspects, scoured slopes on windward sides of alpine and near treeline, soft faceted snow in sheltered areas and wind slabs in immediate lees of ridge crests.

A thick rain crust sits 30-60 cm below the surface and can be found up to 2100 m. Avalanche activity has been observed at the faceting interface between this crust and overlying snow a week ago.  

The mid-pack is well settled and strong, but the base of the snowpack contains basal facets that are most prominent in shallow rocky start zones.

Terrain and Travel

  • Fresh wind slabs will likely form throughout the day, diligently watch for changing conditions.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.

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.