Avalanche Forecast

Archived

Dec 23rd, 2018–Dec 24th, 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.

Regions

Stevens Pass.

Recent storm snow will need more time to bond to an underlying crust. Above treeline, the low density snow is easily transported into thickened wind slabs. Deep, large avalanches are still possible, especially where the snowpack is more shallow. Conservative terrain selection is recommended.

Discussion

Snowpack and Avalanche Discussion:

Stevens Pass received about 8” of new snow with 0.62” of water equivalent as of 4pm on December 23rd. Temperatures were initially cool and winds were light, but strong enough to move snow near and above treeline. During the day on the 23rd, skiers were able to trigger avalanches about a foot deep and 100ft wide on the new/old interface at 6,200ft on a Northeast aspect of Jim Hill. Out of any wind effected zone, loose avalanches were possible on very steep slopes.

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 3950ft on Stevens Pass there is 55” of snow on the ground. At 4,100ft on Blewett Pass the snowpack is only 20” deep.

The persistent weak layer that we have been tracking since December 9th has been observed to be rounding out, gaining strength, and sintering near the pass. Further east in the zone we are still gathering information, but this layer is still in the forefront of our minds in areas with 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. It appeared that the north side of Jim Hill may have ran during this time as well.

Snowpack Discussion

Coming December 24th, 2018

Problems

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.