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Avalanche Forecast

Archived

Mar 30th, 2017–Mar 31st, 2017

Alpine
Natural avalanches unlikely, human triggered possible.
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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Loose wet avalanches will be increasingly likely Friday, where small wet snow avalanches may entrain deeper wet snow layers and become large. Use caution in steep sun exposed terrain during the warmest part of the day, avoiding steep slopes above terrain hazards such as trees or cliffs. Loose wet slides may trigger wet slabs in steep isolated terrain.

Detailed Forecast

High pressure is expected to rebuild over the area Thursday night through Friday. This will cause clearing overnight and allow for wet surface snow to begin refreezing and strengthening.

Partly to mostly sunny conditions Friday and warming temperatures should cause a gradual increase in danger from wet snow avalanches, especially on slopes receiving direct sun and during the warmest part of the day.

Recent shallow wind slabs should continue to settle and stabilize where formed on lee slopes, mainly above treeline and on NW-SE aspects.

The expected clearing and cooling overnight Thursday should cause a strengthening surface crust by Friday. However, 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. Pay particular attention to steep solar facing slopes above terrain traps such as trees, cliffs or gullies where being caught and carried, even in a small slide, could prove consequential.

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. 

Cooling and showery weather Thursday allowed wet snow to begin refreezing with an additional 1-3 inches accumulating in most areas. The winds diminished by Thursday and combined with daytime warming, this has allowed for recent wind slabs to begin stabilizing.   

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

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.

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.