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Avalanche Forecast

Archived

Mar 29th, 2017–Mar 30th, 2017

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
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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Fresh storm and wind slab formed Wednesday night should generally be shallow and found near and above treeline on Thursday. If previously wet surface snow has not refrozen, be suspicious of loose wet avalanches that may begin small but entrain older snow and become dangerous and difficult to manage. Sunshine on Thursday will likely activate loose wet slides on solar slopes.

Detailed Forecast

Fresh storm and wind slab formed Wednesday night should generally be shallow and found near and above treeline. Due to the cooling trend, only wind slab will be listed in the avalanche problem set and emphasized above treeline. W-SW winds should have primarily built fresh wind slab on NW-SE aspects.

Despite the cooler snow levels forecast for Thursday, if previously wet surface snow has not refrozen, be suspicious of loose wet avalanches that may begin small but entrain older snow and become dangerous and difficult to manage. Sunshine on Thursday will likely activate loose wet slides on solar slopes. Stay off steeper slopes with terrain traps. Moderate avalanche danger allows for large avalanches in isolated areas.

Recent cornices are very large. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give cornices a wide berth if traveling along ridge-lines and avoid slopes directly below large cornices. See a blog post regarding cornices here.

 

Snowpack Discussion

Special Note: For more information on the massive natural cornice triggered avalanche on the north side of Ruby Mountain on Sunday 3/19 and general thoughts about low-likelihood/high consequence avalanches, please see NWAC's blog post issued Sunday, March 26. 

Weather and Snowpack

Let's just say it's been a wet and wild few weeks regarding weather and avalanches in the Cascades. 

This past week has also been active weather-wise, but water amounts/snowfall totals have been slightly lower relative to the extreme wetness of the past few weeks. NWAC and NRCS stations east of the Cascade crest had about 6-12 inches of snow in the 5 days ending Monday morning. 

Another 5-10 inches fell near the Cascade crest along with a warming trend and increasing W-SW alpine winds Tuesday night and into Wednesday. Lesser amounts were seen further east of the crest and at lower elevations. 

Recent Observations

North

The NCMG were in the Cutthroat area on Monday and found 15-20 cm of recent snow on a firm base. Loose wet avalanches and cornices were the main concerns. 

On Tuesday, NCMG found stubborn and shallow storm slab in hand tests and on short slope tests. Very large cornices were still looming along ridge-lines. A public observation from the Washington Pass area on Tuesday reported shallow but sensitive wind slab on steep east aspects in wind affected terrain.  

Central

No recent observations. 

South

No recent observations. 

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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.