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Avalanche Forecast

Archived

Feb 10th, 2017–Feb 11th, 2017

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

Regions

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Strong winds Friday and Friday night will have built fresh wind slab on lee aspects mainly near and above treeline. Watch for new wind slab in the upper portion of the below treeline band. Easterly aspects will be highlighted in the elevation/aspect diagram but watch for firmer wind transported snow on a variety of aspects. Recent observations have been limited along the east slopes of the Cascades, increasing the amount of uncertainty in the forecast. 

Detailed Forecast

Light showers Friday night, should taper off after midnight with only isolated light snow showers expected on Saturday near the Cascade crest. The same trend is expected for the moderate to occasionally strong W-SW winds forecast for Friday night with winds easing considerably on Saturday.

Strong winds Friday and Friday night will have built fresh wind slab on lee aspects mainly near and above treeline. Watch for new wind slab in the upper portion of the below treeline band. Easterly aspects will be highlighted in the elevation/aspect diagram but watch for firmer wind transported snow on a variety of aspects.   

Despite the cooling trend, new and shallow storm slab instabilities may be locally sensitive. Storm slab size will range from small to large in the Washington Pass area where more snowfall has been received over this storm cycle. 

Recent observations have been limited along the east slopes of the Cascades, increasing the amount of uncertainty in the forecast. 

Potential persistent slab layers should have received a good test during this storm cycle. We will wait for more information regarding PWL sensitivity and distribution in areas with a shallower snowpack and in the Washington Pass area before re-listing persistent slab.   

Snowpack Discussion

Weather and Snowpack

A strong storm cycle Friday 2/3 through Monday 2/6 produced 2-3 feet of snow along the east slopes of the Cascades with lesser amounts at lower elevations and in the Mission Ridge/Blewett area. 

A strong plume of moisture brought heavy snow to the east slopes of the Cascades Wednesday night. A mix of snow, freezing rain, and rain was seen Thursday for the central and southeast Cascades while the northeast Cascades experienced a warming trend aloft but likely stayed mostly snow throughout the event. A strong cold front swept through the Cascades mid-day Thursday with bands of light to moderate showers in SW flow following the front. Peak warming occurred for the lower east slopes Thursday night as cold air was scoured out.   

Friday was windy with a slow cooling trend during the day. Light showers increased in the afternoon. 

Recent Observations

North

A 5 cm layer of facets roughly 10 cm below the 1/17 crust may still be found in the Washington Pass zone and in areas further east. This reactive PWL has only been found in isolated locations in cold non-wind affected north facing terrain. No avalanche activity has been observed on this layer. 

No recent observations since Tuesday. 

Central

A large avalanche off of Dirty Face Peak knocked in a garage door of a house on the north shore of L. Wenatchee around 1130 am Thursday. 

South

No recent observations since Tuesday. 

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.

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.