The upper snowpack is a mess and the incoming 15 to 25 cm of new snow is expected to be accompanied by strong wind. A natural avalanche cycle is likely overnight & may persist into Sunday. Travel in avalanche terrain is not recommend Sunday.
This storm may well push the persistent slab problem over the tipping point resulting in large and surprisingly touchy avalanches. The slab rests on a persistent weak layer which has the capacity to propagate avalanches across terrain features. Wind exposed features are expected to be particularly volatile.
Look for the formation of fresh, sensitive storm slabs as strong southwest wind and 15 to 25 cm of new snow come together Saturday night into Sunday. This snow will add to the 30 to 40 cm that already rests on the surface hoar.
The storm parade continues for the foreseeable future.
SATURDAY NIGHT: Freezing level around 1000 m, strong southwest wind, 15 to 25 cm of snow expected.
SUNDAY: Overcast, freezing level around 1200 m, strong west/southwest wind, 5 to 10 cm expected through the day.
MONDAY: Overcast, freezing level around 700 m, light south wind with moderate gusts at ridgetop, 10 to 20 cm of snow possible.
TUESDAY: Broken cloud cover, freezing level near valley bottom, light to moderate northerly wind at ridgetop, no precipitation expected.
On Friday natural slab avalanches to size 3 were reported on all aspects. Early reports from Saturday feature similar activity. The new snow and wind Saturday night into Sunday are expected to initiate a natural avalanche cycle that may persist into Sunday.
Storm and wind slabs were reactive underfoot/sled on Thursday. Wind slabs to size 2 were observed at upper elevations in lee terrain, with depths 40-60 cm. A size 1.5 storm slab avalanche was reported failing naturally on the early December crust, with good propagation. Explosives triggered avalanches to size 2 along ridgelines, again being reported with good propagation.
A handful of MIN reports (thank you!!) from the Gorge found a highly reactive snowpack including a remotely triggered avalanche with surprising propagation from about 20 m away (here).
Ongoing flurries and snow has accumulated 20-50 cm fresh snow around the region. This new snow has buried a weak layer of surface hoar in many areas. This new snow combined with southwest winds has formed reactive slabs, especially at higher elevations.
A weak layer that was buried in early December is now down 40-60 cm. This layer is variable and has been reported to exist as either a crust or surface hoar. In any case, it is a persistent weak layer with a significant load above it now, and it may react easily to human triggers.
Another crust that was buried in early November can be found near the base of the snowpack. There have been no recent avalanches reported on this layer, however it may be possible to trigger it from shallow, rocky terrain.
