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Avalanche Forecast

Archived

Feb 12th, 2016–Feb 13th, 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 possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

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Moderate W-SW winds will transport new snow onto lee slopes near and above treeline Saturday. New snow is generally expected to bond well to moist snow surfaces and various crusts, but wind-driven snow may build unstable wind slab on lee aspects. The avalanche danger will increase late Saturday night and on Sunday as snowfall increases along with a slow warming trend. 

Detailed Forecast

Light snowfall accumulations are expected Friday night through Saturday above 3500 feet along the east slopes.

Moderate W-SW winds will transport new snow onto lee slopes near and above treeline. New snow is generally expected to bond well to moist snow surfaces and various crusts, but wind-driven snow may build unstable wind slab on lee aspects. 

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

The avalanche danger will increase rapidly late Saturday night as snowfall increases along with a slow warming trend. This unstable combination of moderate precipitation near the Cascade crest, strong alpine winds and warming may continue on Sunday again elevating the avalanche danger along the east slopes.

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. 

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. 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.

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.

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.