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Avalanche Forecast

Archived

Mar 2nd, 2020–Mar 3rd, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Sea To Sky.

Strong winds and incremental snowfall continue to build wind slabs at upper elevations. Deeper instabilities cannot yet be discounted.

Confidence

Moderate - Uncertainty is due to how the snowpack will react to the forecast weather. Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

 Monday night: Cloudy, 5-10 cm of snow, strong west winds, freezing level 1300 m.

Tuesday: Cloudy, up to 5 cm of snow, moderate southwest winds, gusting strong at ridge-top, freezing level 1100 m. 

Wednesday: Partly cloudy, 5-15 cm of snow overnight, moderate west winds decreasing to light, freezing level 900 m.

Thursday: Cloudy, 20-30 cm of snow, strong southwest winds, freezing level 1000 m. 

Avalanche Summary

On Sunday and Monday, there were reports of small, human-triggered wind slabs on lee features at upper elevations. Check out this MIN for a helpful example. 

During the weekend storm, observers reported a natural avalanche cycle of small to large (size 1-2.5) slab avalanches releasing in the recent snow 20-40 cm deep. Recent explosive mitigation has also triggered several large cornices. Periods of strong solar radiation Saturday afternoon initiated pinwheels, rollerballs, and small wet loose avalanches on steep, sunny slopes.

On Wednesday, a large (size 2) human-triggered avalanche releasing on the February 22 surface hoar was reported.  

Snowpack Summary

20-30 cm of snow has accumulated since Friday. Strong winds redistributed the storm snow into reactive wind slabs on lee features at upper elevations and rapidly loaded cornices. 

Buried 40-70 cm deep, a weak layer of sugary faceted snow and feathery surface hoar from February 22 may persist and warrants careful evaluation. Operators observed recent reactivity in snowpack tests and reported whumpfing in terrain near Tricouni. Check out the latest forecaster blog that offers a deeper dive into these conditions.

Faceted snow and crusts exist near the base of the snowpack in much of the region. This layer is has been dormant recently.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.