Avalanche Forecast

Archived

Dec 23rd, 2018–Dec 24th, 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 possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

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As the storm clears out, remnants of a very large, and recent avalanche cycle have been observed. Deep, and large avalanches are still possible. Conservative terrain selection will be necessary to stay safe in avalanche country.

Discussion

Snowpack and Avalanche Discussion:

Areas in the East North Zone received 5" to 8” of new snow on the 23rd. Temperatures were cold initially, and winds were light, but strong enough to move snow near and above treeline.

Between the Pacific Crest and the eastern slope there is a large difference between snowpack depths. This is nothing unusual, and more often than not the west side of the Cascades and the passes get more snow than areas further east. Over time, this adds up. Right now, we are starting to see that difference grow. For example, at 4600ft just west of the crest, the Park Creek Ridge Snotel site is reporting 80” on the ground. At 5400ft on Washington Pass the snowpack is 58” deep, and at similar elevations on the 21st I was finding 45” on the ground.

The persistent weak layer that we have been tracking since December 9th has been observed to be rounding out, gaining strength, and sintering west of the crest. This layer is in the forefront of our minds further east, as we have a more shallow snowpack. A very large natural avalanche cycle occurred between December 18th and 20th on these layers in the East Central and East North Zones.

Snowpack Discussion

Coming December 24th.

Problems

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.

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.