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Avalanche Forecast

Archived

Feb 27th, 2021–Feb 28th, 2021

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

Regions

Sea To Sky.

Watch for signs of instability in wind affected terrain. You may find reactive wind slabs on a variety of aspects as northwest winds relocate recent snow in a reverse loading pattern.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

Saturday night: Flurries bringing around 3 cm, freezing level 500 m. Light southwest wind below 2000m, strong northwest ridgetop wind.

Sunday: Light flurries bringing a trace of snow. Freezing level 1300 m. Light southwest wind below 2000m, strong west-northwest ridgetop wind.

Monday: 10-20 cm new snow. Freezing level 1300 m. Strong southwesterly winds affecting all elevations.

Tuesday: Flurries. Freezing level 700 m. Light to moderate southwest wind.

Avalanche Summary

Reports from Thursday and Friday show:

  • Evidence of the previous natural storm cycle up to size 3 on north to east aspects 1500 to 2000 m.
  • Good propagation in explosive triggered storm slabs to size 2 (Thursday).
  • Storm/wind slabs have been reactive to human triggers around treeline to size 1.
  • Cornices have been very reactive to explosives as well as remote human triggers, size 2-2.5.

Going forward:

  • Wind loading patterns will likely reverse with northwest winds, forming reactive wind slabs in new features.

Snowpack Summary

20 cm of recent snow overlies a variety of old surfaces including facets, spotty surface hoar in wind sheltered areas around treeline, a sun crust on steep solar aspects and a rain crust below 1500 m. Upper level wind, having recently shifted northwest is likely transporting the recent snow into wind slabs in a reverse-loading pattern in exposed features at upper elevations. Older wind slabs formed by previous southwest winds may also remain reactive to human triggers.

Another weak layer buried in mid-February is composed of a crust, facets or spotty surface hoar depending on elevation/aspect, and is now down 50-80 cm. There have been no recent avalanches reported on this layer. Large triggers, such as an avalanche in motion, cornice fall, or intense loading from snow, wind and/or a rapid rise in temperature could awaken these deeper weak layers.

The mid and lower snowpack is well settled and strong in most areas.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Minimize your exposure time below cornices.

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.