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Avalanche Forecast

Archived

Feb 13th, 2016–Feb 14th, 2016

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

Regions

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Moderate W-SW winds will transport new snow onto lee slopes near and above treeline Sunday, building unstable wind slab on lee aspects. The avalanche danger will increase late Saturday night and on Sunday as snow or rain increases along with a warming trend. 

Detailed Forecast

Increasing moderate rain and snow is expected with a warming trend Saturday night through Sunday. With the snow level expected to rise to the near tree line elevations band by early Sunday, a significant increase in avalanche danger is expected overnight with natural avalanches becoming increasingly likely. If a natural avalanche cycle occurs overnight, this should locally lower the danger during the day Sunday, at least in those areas that experienced avalanches. The main question is about the rate and timing of warming. If the warming is not as great as expected or delayed, a very heightened avalanche danger should persist into Sunday.

On slopes that have not released due to warming and additional load, natural or triggered avalanches should be increasingly likely Sunday.  

Strong W-SW winds will transport new snow onto lee slopes near and above treeline where wind-driven snow may build unstable wind slab on lee aspects. 

Wet loose avalanches will be a concern on steeper slopes at lower elevations.

Avoid cornices along ridges and slopes below cornices since cornices may still be weaker due to the warm weather.

Snowpack Discussion

Weather and Snowpack

Two fair weather periods in January allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on Jan 3rd and 11th, but are no longer considered a persistent slab threat after several warm and wet systems tested this layer and it has been unreactive where it can still be identified in recent snowpit tests. A warm and wet system late last month caused a rain crust to form in most areas and elevations along the east slopes on Jan 28th. 

The warmest weather of the winter occurred this past Sunday through Wednesday with extended temperatures climbing into the 50's in most areas east of the crest. The warm temperatures and solar effects earlier in the week caused loose wet avalanches and overall snowpack consolidation. A warm front brought periods of very light rain and snow to the east slopes Thursday night through Friday, with snow levels oscillating around 5000-6000 ft Friday. Cool snow showers Friday night and Saturday combined to deposit 2-6 inches of storm snow as of Saturday evening.

Recent Observations

Extensive observations in the Washington Pass area over the past few days indicated an active loose-wet avalanche cycle occurred mainly Monday afternoon. NWAC pro-observer Tom Curtis in the Blewett Pass area in the 4000-5900 foot range Tuesday and also saw several small to large wet loose avalanches on solar slopes near and above tree line.

A professional observation from the central-east zone Thursday indicated that a facet/Jan 28th rain crust combo was reactive in snowpit tests, but no avalanches were observed down to this interface. However, a skier triggered avalanche down to the faceted 1/28 crust was observed in the Washington Pass area on a NE aspect around 7000 ft Thursday. While we believe the upper snowpack to generally be well bonded to the 1/28 crust, this interface should get a decent test over the next few days and bears watching.

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.