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Avalanche Forecast

Archived

Feb 11th, 2016–Feb 12th, 2016

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
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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Light rainfall will renew the loose wet avalanche potential on all aspects of steeper slopes below treeline. New snowfall should bond well to a moist snow surface in most locations, but wind-driven snow may build unstable wind slab on lee aspects of higher terrain by late Friday afternoon. 

Detailed Forecast

Mild snow levels should keep precipitation in liquid form below 6000 feet until a cold front passes through the Cascades Friday mid-afternoon with snow levels lowering to 4500 feet by late afternoon. Precipitation should be light, except moderate Friday afternoon with the frontal passage. 

Light rainfall will renew the loose wet potential on all aspects of steeper slopes below treeline. New snowfall should bond well to a moist snow surface in most locations, but wind-driven snow may build unstable wind slab on lee aspects of higher terrain by late Friday afternoon. 

Continue to watch for loose wet avalanche potential above terrain traps such as above cliffs, near gullies or where avalanche debris would deeply accumulate. These are the types of places that even small wet avalanches can have serious unintended consequences.

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 large upper ridge and warm air mass aloft over the West Coast from this past Sunday through Wednesday led to the warmest weather of the winter. During this stretch temperatures were generally above freezing. Freezing levels came down a few notches Thursday, but mild conditions continued with spotty light rain or snow increasing Thursday afternoon. 

The warm temperatures and solar effects caused loose wet avalanches, snowpack consolidation, and melt-freeze surface crusts. This will have further stabilized the lower and mid snow pack and turned the most recent storm snow into moist to wet snow in most areas. 

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 Jeff Ward was in the Washington Pass area in the 6000-7600 foot range on Tuesday and saw several size 1 natural loose wet avalanches on steep solar slopes.

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. 

Problems

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.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.