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Avalanche Forecast

Archived

Apr 17th, 2014–Apr 18th, 2014

Alpine
Below Threshold.
Treeline
Below Threshold.
Below Treeline
Below Threshold.
Alpine
Below Threshold.
Treeline
Below Threshold.
Below Treeline
Below Threshold.

Regions

Snoqualmie Pass.

A mix of increasing winter and spring avalanche conditions should be seen in the above treeline zone especially in the Washington Cascades near and west of the crest and the Mt Hood area on Friday.

Detailed Forecast

A cold front should move east of the region Thursday night.  This should cause a change to showers overnight with lowering freezing levels.  Showers should diminish or end by early Friday along with further cooling.  Cool temperatures and gradually clearing skies are expected Friday.   Only a few additional inches of snow are expected Thursday night through early Friday.  

This should mainly affect lee slopes above treeline where stronger winds and new snow should build some shallow wind slab conditions, mainly NE through SE facing slopes. Watch for signs of firmer wind transported snow or cracking snow.

The sun continues to get stronger as we get further into spring. Even with mostly cloudy conditions Friday, the sun could do its work mainly on solar slopes in areas where there is more than a few inches of recent snowfall. Wet loose avalanches are possible on such slopes, mainly above treeline in the Washington Cascades near and west of the crest and the Mt Hood area but may extend down into the near treeline zone as well. Watch for increasing pinwheels and natural or triggered wet loose avalanches that increase in size or extent during the daylight hours.

Rain or much less new snow should continue in the near and below treeline areas through early Friday, especially east of the crest. Less change in snow and avalanche conditions should be seen in those areas.

Note that the NWAC will issue a complete mountain weather forecast Friday afternoon and again Saturday as well as complete avalanche forecasts valid for Saturday and Sunday.

Snowpack Discussion

A warm front spread increasing light precipitation over the region late Wednesday with a cold frontal passage moving through the region late Thursday.  As of Thursday morning, areas above 5000 feet had received about 4-8 inches of snow with less along the east slopes and at Hurricane Ridge.  Daytime warming and slightly rising freezing levels Thursday has pushed snow lines above 5500 feet and closer to 6000 feet.  This has wet recent snow and added some shallow additional new wet snow above tree line, mainly near and west of the Cascade crest.

Overall, this should not create a significant increase in the current avalanche danger, however, areas of new wind or storm slab may have built by late Thursday, mainly at the highest elevations above tree line. 

Rain or much less new snow is expected in areas such as Hurricane, near and below treeline in the Washington Cascades near and west of the crest, east of the crest and at Mt Hood, where some shallow wet snow conditions should be expected.

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.

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.