Avalanche Forecast

Archived

Dec 10th, 2015–Dec 11th, 2015

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.

Regions

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Recent storm concerns should decrease through the day Friday, but watch for sensitive wind slab, especially on lee N through SE slopes near and above treeline.

Detailed Forecast

Light showers should diminish overnight through Friday morning. A low pressure system should glance the southern Washington Cascades with only light snow Friday afternoon while partly to mostly sunny skies should be seen further north.

Wind slab formed over the past few days on lee N through SE slopes will be the primary avalanche problem near and above treeline. Lingering shallow storm slab problems should slowly diminish during the day.

Snowpack Discussion

Snowfall over the east slopes the first week of December was about 1-4 feet. Then an atmospheric river arrived early in the week with additional snow in the northeast zone, rain in the southeast zone, with a transition in between in the central east zone. Almost 4 inches of water accumulated in 48 hours for Washington Pass and Holden ending early Thursday morning.

Avalanche and Snowpack Observations

We have varied conditions along the Cascade east slopes! The forecast discussion is the same for the east slopes, but the danger ratings and problems differ by zone.

A regionally deep snowpack exists in the northeast Cascades. Recent heavy precipitation along with a warming trend has likely thoroughly tested or buried persistent weak layers from earlier in the season. Avalanche problems are more likely to be storm related in the northeast zone. Persistent slabs now seem unlikely to trigger, but we will wait for more information before dropping this avalanche problem all together.

NWAC observer Tom Curtis was at Blewett Pass in the central east zone on Wednesday and found sudden collapse test results in faceted snow at the November crust 15 cm from the ground. On Thursday Tom found similar sudden collapses with buried surface hoar averaging 50 cm down at 5100 ft on the north side of Mt. Cashmere. More importantly Tom experienced whumpfing as this layer collasped and heard a natural avalanche release far from his observation location. We don't know how this layer fares as one moves further up in elevation, thus we'll forecast conservatively until more information is known about lingering PWLs in the central-east Cascades.

The southeast zone should have a much shallower and more stable snowpack affected by recent rain and warm temperatures. We have not received any observations from this zone.

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.