Avalanche Forecast

Archived

Dec 31st, 2018–Jan 1st, 2019

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 possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

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Spooky and dangerous conditions exist where enough recent snow and wind have created thick slabs that sit over a layer of buried surface hoar. If venturing out into the mountains, beware of any steep, open slope. Careful navigation will be necessary for a safe day out.

Discussion

Snow and Avalanche Discussion:

On the 30th skiers were able to intentionally trigger avalanches in Portly Bowl. These occurred on East to Southeast aspects above 6,500ft. An observer reported a very large slide off Pt. 8183. On the 31st an observer in the Icicle Creek drainage found the snowpack to be very reactive. A supportable rain crust has been observed up to about 5,500ft. As soon as he got above 6,000ft, where the trees were more sparse, cracks began shooting out from his skis as he walked along the ridge. Avalanches were triggered from a distance while walking on this ridge. These were on average 18" deep, and easily up to 400ft wide. As the slabs slid downhill, light reflected off the shiny bedsurface, it was large, feathery, surface hoar. A widespread natural avalanche cycle had occurred during the tail end of the storm, but some slides may have released well after the snow stopped falling. We do not know much about the distribution of this layer across the ranges of the eastern slope. However, we need to treat our upper elevation terrain as guilty until proven innocent.

This past storm brought about 8" with 0.8" of water equivalent to Washington Pass, with moderate winds. This may have been just enough to create a slab over a layer of surface hoar from around Christmas. Other areas further east in the zone have been observed to be lacking a slab structure.

Snowpack Discussion

Coming soon.

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.