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Avalanche Forecast

Archived

Nov 30th, 2016–Dec 1st, 2016

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, human triggered possible.

Regions

Olympics.

Recent wind and isolated storm slabs may linger Thursday, but slowly stabilize. Also, watch for  local cornices along ridges. Be prepared to alter plans if local snow and avalanche conditions warrant.  

Detailed Forecast

Another break between systems is expected Thursday as brief high pressure transits the region. There may be a few light showers early Thursday, but little accumulation is expected. Winds should be light Thursday.

This should allow for recent storm snow and any wind slabs below ridges to begin settling.  The greater concern Thursday should be on steep lee slopes below ridges that may have wind deposited snow deposited Wednesday. These slopes should range from NW-NE facing. 

In addition, watch for recent cornice formations along ridges and give these early season cornices a wide birth as they may be fragile.   

In much of the lower elevation, below treeline band, there is insufficient snow to produce avalanches, but watch for early season hazards such as rocks and creeks.  Avalanche problems should be limited to the upper part of this elevation band and higher elevations.

 

Snowpack Discussion

Most recent storm began about midnight Wednesday, Nov. 30 with new snow at Hurricane Ridge about 12-14 inches as of late Wednesday afternoon. Winds initially were moderate southerly, shifting to more westerly and diminishing greatly by Wednesday afternoon.

A nice report was received via the NWAC website (Observations tab) for Saturday in the Hurricane Ridge area. In summary, pit tests gave only a low quality shears or collapses and an ECT test that did not indicate any propagation. Loose dry avalanches were indicated as the main concern.

On Sunday morning the Hurricane rangers reported 7 inches of new snow. They reported that a snowboarder on Saturday triggered a small loose dry snow avalanche on the Sunrise run, a steep slope that faces northeast at about 5200 feet.

No observations were submitted on Monday, but another 4 inches accumulated above 5000 feet in the Hurricane Ridge area with generally light WNW winds recorded at the NWAC weather station. Tuesday was a break between weather systems. 

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.