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Avalanche Forecast

Archived

Nov 27th, 2017–Nov 28th, 2017

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

Mt Hood.

Watch for the development of new storm and wind slab layers, especially if you venture to higher elevations. Make more conservative terrain choices as the day progresses.

Detailed Forecast

On Tuesday a front crosses the Cascades in the afternoon. At Mt. Hood this will cause increasing winds and warming, with the bulk of the rain and snow starting in the afternoon.

This weather is likely to begin to build new denser storm and wind slab on previous shallow snow from Sunday night. Storm snow avalanches should primarily involve snow from Sunday night and new snow. These layers lie on a moist to wet older snowpack, which may provide a bed surface. Underlying older snow should continue to gain strength.

The avalanche danger should increase during the day Wednesday, mainly above treeline. 

Watch for the numerous travel hazards such as open creeks, barely buried rocks and trees, and glide cracks, creating poor and challenging travel conditions, especially below treeline. 

Remember that closed ski areas without avalanche mitigation are equivalent to backcountry terrain!  

 

Snowpack Discussion

Avalanche and Weather Summary

The great start to the 2017/18 PNW Winter took a giant step backwards over the last week. During an extended period of warmth and wet weather in the days leading into Thanksgiving, an initial round of wet snow and glide avalanches occurred, especially on steep unsupported slopes and rock faces. Another round of rain Saturday night through mid-day Sunday likely did not cause widespread wet snow avalanches on an already beat up and saturated snowpack. This warm, wet and windy period melted significant snow with total snow depths decreasing by 50% or more from their mid-November peak depths at many NWAC stations including Mt. Hood.

A front and a splitting upper trough crossed the US west coast Sunday and Sunday night. At Mt. Hood Meadows, the passage of the sharp cold front on Sunday caused W winds in the 30-40 mph range to decrease to decrease to the 10 mph range. Temperatures dropped from 40's into the 20's at NWAC stations in the Mt. Hood zone. New snow amounts were fairly light ending Monday morning, ranging from 1-3". 

Lingering light snow showers were seen on Monday at continued cooler temperatures and lighter winds.

Observations

No recent observations from Mt. Hood. 

Problems

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.

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.