Avalanche Forecast

Archived

Dec 24th, 2019–Dec 25th, 2019

Alpine

Natural avalanches unlikely, human triggered possible.

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

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Happy Holidays! You can trigger a deep and dangerous avalanche where a recently formed thick slab rests over weak, older snow near the ground. Be most cautious in areas with a shallow snowpack, and on steep slopes that face the north half of the compass.

Discussion

Happy Holidays from all of us at NWAC! The recent atmospheric river event left Washington Pass with 3 feet of snow and 3.2" of water equivalent between the 19th and 21st. Observers reported a cycle of large slab avalanches in Hairpin Valley, Silver Star creek, the Delancy Ridge area, on a northwest aspect of Silver Star Mountain, and a northeast aspect of Hinkhouse peak. Some of these were "wall to wall" with very wide crowns. On the 20th, observers watched a natural avalanche, run at noon on a north aspect of Peak 6460ft near Silver Star Creek.

On the 22nd, an observer near Vasiliki Ridge reported multiple collapses on a weak layer buried in late November that covered areas as wide as 30 feet.

On the 23rd, observers reported no signs of instability during their travels on slopes from west to northeast aspects near the Hwy 20 hairpin.

The snowpack is substantially deeper at the top of Washington Pass than further east towards Vasiliki Ridge. Yes, the likelihood of triggering slides on this deeper weak layer is slowly tapering away, but yes, they are still possible. Persistent slabs are more likely further east in the zone where the snowpack is more shallow.

Snowpack Discussion

New Regional Synopsis coming soon. We update the Regional Synopsis every Thursday at 6 pm.

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.