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Avalanche Forecast

Archived

Dec 19th, 2013–Dec 20th, 2013

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Snoqualmie Pass.

Bottom Line: Strong storm building through the day Friday with heavy precipitation at warming temperatures and strong winds creating very dangerous avalanche conditions by afternoon. 

Detailed Forecast

The strongest storm in some time should begin to deposit shallow snow Thursday night and increase and become moderate to heavy through the day Friday along with significant warming and strong crest level winds.  

This storm will likely test the strength and bonding of the near surface weak layers as well as introduce new storm slab instabilities.

The storm should begin with very cold temperatures Friday morning and this should deposit a layer of low density snow over a firm crust. Temperatures should rise through the afternoon along with heavier precipitation rates. This should build increasingly unstable storm slabs through the day causing a significant increase in the danger. By midday and through the afternoon there will be an increasing likelihood of triggered or natural slides, especially near and above treeline.

The strong west to northwest crest level winds will also build unstable dense wind slab on lee slopes near ridges. 

This storm should cause very dangerous avalanche conditions near and above treeline where travel in avalanche terrain is not recommended by later Friday. Dangerous avalanche conditions should also develop below treeline by afternoon. 

 

Snowpack Discussion

Early December storms ranged from rain to high elevations followed by cooling with 1-2 feet of snow. No significant storms have occurred since early December. However there have been significant variability in temperature regimes ranging from very cold temperatures to recent high freezing levels and warm temperatures early this week.

Very warm temperatures over the weekend through this Tuesday produced some wet snow avalanches on steeper slopes. Initial news from an off-duty patroller at Crystal Mountain Thursday saw evidence of a recent wet loose snow avalanche on a south facing slope that stepped down to the ground Monday.  The slide likely stepped down on a pocket of depth-hoar possibly found near rocks and shallower snow cover. 

Significant cooling Wednesday and Thursday with some light snow has now formed a new near surface crust in most areas with some shallow trace to few inches of new snow on the surface. 

Shallow snow cover is limiting the avalanche danger at lower elevations.

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.