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Avalanche Forecast

Archived

Nov 29th, 2017–Nov 30th, 2017

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
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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Watch for recently and newly forming storm and wind slabs, especially on wind loaded slopes near ridges and if venturing to higher elevations. Watch for early season terrain hazards, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

Detailed Forecast

A weak to moderate front should move across the Cascades near midday Thursday, causing a period of light to briefly moderate snow and moderate ridge level winds. This weather should build new areas of storm and wind slabs, especially in wind affected terrain, such as higher elevations and near ridges.

Watch for storm slabs in areas that received more than a few hours of moderate or heavy snowfall intensities. 

Watch for firmer wind-transported snow on lee slopes near ridges, generally northwest through southeast aspects, but expect wind deposited snow on a variety of aspects in areas of more complex terrain.

Storm or wind slab avalanches may entrain recent snow down to the firmer old refreezing snow from last week.

Although not  listed as a current avalanche problem, loose dry avalanches are possible Thursday, particularly on steep slopes. 

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

The avalanche danger should generally increase for all elevation bands on Thursday.

Snowpack Discussion

Avalanche and Weather Summary

The great start to the 2017/18 PNW Winter hit the pause button last week throughout the Cascades during an extended period of warm and wet weather. Several cycles of wet snow and glide avalanches occurred, especially on steep unsupported slopes and rock faces. 

Over the weekend, the weather system that brought rain to the rest of the Cascades brought several inches of snow Saturday night through Sunday at Washington Pass. Mixed precipitation or a brief period of rain may have occurred Sunday afternoon as temperatures warmed aloft.  

Little to no new snowfall was seen for most of the central-east and southeast Cascades over the weekend due to warmer temperatures and a period of rain. Snow depths are limited in these areas.

On Tuesday, a frontal passage brought a slight bump in snow levels into the 3000 ft range and deposited about 5-10 inches or more of new snow as of Wednesday morning. The greatest recent storm totals were received in the northeast Cascades.

Observations

On Tuesday in the Slate Creek drainage between about 5400-7250 feet, NWAC observer found about 1-2 feet of low density snow bonding well to a thin crust from last week. The new storm snow was not slab like in the areas travelled and no avalanches were triggered or observed. There was potential for wind slabs in specific terrain above treeline, but no avalanches were directly observed.   

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.