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Avalanche Forecast

Archived

Mar 21st, 2017–Mar 22nd, 2017

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.

Regions

Olympics.

Above treeline, shallow fresh wind slabs should build near ridges. At lower elevations, re-freezing surface snow should maintain mostly stable snow. Watch for overhead hazard, such as recently formed large cornices along many ridges.

Detailed Forecast

Additional showers, mostly light to occasionally moderate, should persist Tuesday night through Wednesday. Significant cooling is expected through the day Wednesday.

The cooling Wednesday should allow for an overall decreasing danger. Previous moist to wet shallow surface snow should begin re-freezing and strengthening.

Only shallow snow from 4-6 inches overlies the strong rain crust from 3/18 in the Hurricane Ridge area.

Moderate winds and showers at lowering snow levels should build some shallow fresh wind slabs at higher terrain along ridges, mainly on NW-NE-E facing terrain. Watch for firmer wind transported snow on all aspects, especially in areas of complex terrain. 

At lower elevations, gradually re-freezing surface snow will form a new strengthening crust by later Wednesday. 

It is always a good plan to travel well back from ridges, suspected of cornice formation, or on steep slopes below cornices.

Snowpack Discussion

Weather and Snowpack 

The first week or so of March was very cool and snowy. NWAC stations along the west slopes of the Cascades and Olympics piled up about 3 to 8 ft of snow with the most at Mt Baker.

The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch, including a very active avalanche cycle in the Hurricane Ridge area 3/9 through 3/10 with large or very large natural and human-triggered avalanches reported. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. 

After a short respite from the active weather pattern on Thursday, another strong low pressure system brought several inches of rain to the west slopes of the Cascades and Olympics. Several inches of snow accumulated early Friday night before continued warming pushed snow levels above Hurricane Ridge with moderate rain seen through early Saturday morning. Rapid cooling later Saturday morning was followed by generally light snow showers with little in the way of new snow accumulation. Sunday was cool with light winds and mostly sunny skies.

More mild weather with periodic light showers occurred Monday and Tuesday, 3/20-21, with very little snow accumulations.

Recent Observations

NPS rangers at Hurricane Ridge reported about 5 inches of wet snow Saturday morning with little sign of recent wind transported snow.  A public observation from the Obstruction Point Road noted ongoing wind transport, a supportable rain crust and lingering giant cornices along N-NE aspects of ridge-lines Saturday. 

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.