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Avalanche Forecast

Archived

Mar 1st, 2020–Mar 2nd, 2020

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

Regions

Sea To Sky.

Incremental snow and wind over a buried weak layer warrants careful evaluation and terrain selection on Monday.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Sunday night: Cloudy, isolated flurries with trace accumulations, moderate west winds, gusting strong at ridge-top, freezing level 500 m. 

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

Tuesday: Mostly cloudy, 5-10 cm of snow, moderate southwest winds, gusting strong at ridge-top, freezing level rising to 1300 m. 

Wednesday: Decreasing cloud, 10-20 cm of snow overnight, moderate southwest winds decreasing to light, freezing level 900 m.

Avalanche Summary

In the aftermath of the 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. Some of this avalanche activity was human triggered (see this MIN for a helpful example). Explosive mitigation on Saturday 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 50-80 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.