Special Public Avalanche Warning has been extended until January 4th for this region. Considerable avalanche danger is expected to continue due to the presence of a buried weak layer of surface hoar and/or a crust.
Confidence
Poor - Timing, track, or intensity of incoming weather system is uncertain on Friday
Weather Forecast
Mostly clear and cold overnight with moderate Northwest winds and alpine temperatures dropping to about -24. Clear with light Northwest winds on Wednesday with alpine temperatures rising up to about -16. Increasing cloud and a warming trend are expected for Thursday as the winds shift to the Southwest. Snow overnight and during the day on Friday combined with Northwest winds. Confidence is poor for intensity of the Friday storm, hopefully models will agree by tomorrow.
Avalanche Summary
Natural avalanches have been reported up to size 2.0 where the recent storm snow has been transported into a windslab; some of these avalanches have released or stepped down to the mid-December persistent weak layer of surface hoar and crust. Human triggering continues to be likely to very likely due to the storm slab sitting on a hard sliding surface with a weak layer of surface hoar at the interface.
Snowpack Summary
Extensive new windslab development over the last two days in the alpine, at treeline, and also in open areas below treeline as a result of strong Northerly winds. The new windslab may be easy to trigger, and may step down to the persistent weak layer of mid-December surface hoar. The touchy mid-December surface hoar layer is now buried below a 50-90 cm consolidated slab that developed during last weeks storm. Below 2100 m this slab sits on a thick, solid crust/ surface hoar combination and has been acting as a perfect sliding layer. A hard rain crust with facets from early November is buried more than 100 cm down and is currently unreactive, however; triggering from shallow rocky and unsupported terrain remains a concern.
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.
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.