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Avalanche Forecast

Archived

Mar 3rd, 2013–Mar 4th, 2013

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

Regions

South Columbia.

Confidence

Fair - Due to variable snowpack conditions

Weather Forecast

Synopsis:  The pineapple express is being replaced with a ridge of high pressure that should remain in place through Monday.  A weak frontal wave makes it’s way inland Tuesday, but confidence in the exact timing and intensity are poor on Tuesday. Monday: Freezing level at 1000m.  No precip.  Increasing cloud cover.  Light South wind.Tuesday: Freezing level remaining near 1000m.  Light SW wind.  Isolated flurries with no significant accumulation.  Wednesday: Freezing level once again at 1000m.  Light SW wind.  Isolated flurries with no significant accumulation.

Avalanche Summary

Our office continues to learn about the wild widespread avalanche cycle that occurred on March 1st.  Avalanche size during the storm averaged size 3 but the occasional size 4 has been reported too.  I suspect more details will trickle in as visibility improves.

Snowpack Summary

The moisture laden storm has brought the two week total up to 120 - 150cm.  The recent storm was very warm and as a result rain fell as high as 1900m.  I suspect a crust now rests on the surface below 1900m as temperatures have dropped back down below freezing.Below the storm snow lies a highly reactive weak layer of surface hoar (and/or a crust) which was buried on February 12th. Strong southwest winds have redistributed the new snow into deep wind slabs on exposed lee slopes and cross-loaded gully features. The February 12 layer is primed for triggering and has been reacting readily to both natural and human triggers on all aspects and a wide range of elevations. Deeper persistent weak layers from early February and late January continue to show planar failures when moderate to hard forces are applied in snow pack tests. In particular, in the Monashees, a rain crust buried down 150 cm has been responsible for a couple of recent large, surprising avalanches. Immediately following the current storm, the presence of this deeper weak layer could further increase the potential size of avalanche events.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

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.