Avalanche Forecast

Archived

Dec 29th, 2018–Dec 30th, 2018

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

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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New snow and strong winds have created deep drifts to avoid near and above treeline. Very large avalanches can still be triggered in old snow layers near the ground.

Discussion

Snow and Avalanche Discussion:

On the 29th the temperatures warmed up significantly in the mountains. Cold air remained at low elevations until the evening when the cold front came through and pushed this colder air out. The low elevations are going through a thaw, while the upper elevations have already warmed and are now cooling down. A rain and/or freezing rain crust likely formed up to at least 6,000ft during this time. Winds were strong at upper elevations, and a gust of 102mph was recorded at Mission Ridge.

The southwest edge of the zone such as the Salmon la Sac and Teanaway picked up substantial snowfall and precipitation, while the eastern edge (Blewett Pass and Mission Ridge) did not. This will create a pronounced difference in avalanche danger for Sunday.

Photo: Cold air banked up along Tumwater Mountain on December 29th.

Snowpack Discussion

Coming December 30th

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.