Avalanche Forecast

Archived

Dec 29th, 2018–Dec 30th, 2018

Alpine

Natural avalanches possible, human triggered probable.

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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New snow and moderate 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 temperatures warmed up significantly, reaching above freezing at 5400ft on Washington Pass for the first time since December 13th. Meanwhile down in Mazama, temperatures were in the low 20’s. The cold air remained 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 warmed up, and are now cooling down. A thin rain and/or freezing rain crust likely formed up to about 5,500ft during this time. Winds were gusting into the 20’s on Wa Pass.

Photo: Cold air banked up along Tumwater Mountain in Leavenworth 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.