Over a foot of new snow has fallen in the last 24 hours under windy and warmer conditions. Upper elevations saw up to 20 inches of new snow beginning New Years Eve Day. High winds accompanied this storm and have created storm and wind slabs on the old snow surface. The snow surface prior to this storm was a combination of Surface Hoar and Near Surface Facets which made a new buried weak layer that can be found between 12 and 21 inches down in the snowpack. Multiple natural avalanches occurred during and after this storm and it remains possible for skiers and sledders to trigger an avalanche on steep terrain today. We also heard reports of a stiff and breakable wind slab in the upper elevations where winds created dangerous wind slabs through the New Years storm.
Our persistent weak layers are doing exactly what they say. They are persisting, or still preserved in the mid and ground level of our snow pack. We now have a new layer of buried surface hoar about 1-2 feet down that is propagating in our snowpit tests. This is going to be a layer to watch for the next few days as the snowpack adjusts to its new load.
The other persistent problem is lower in our snow pack and it could have serious consequences: facets sitting on a crust that are producing planar failures that show propagation potential. While it is deep enough and relatively hard to initiate, triggering a slide in this layer would be fatal. There now exists the possibility of step down avalanches where you could trigger an avalanche in the new or wind blown snow that could overload the weak layer near the old crust which is now buried close to 4 feet down. Click here for a PST video on this layer.
It is easy to get complacent about a persistent avalanche problem but it should be in the back of your mind as you travel right now, The presence of these layers is enough to keep most seasoned backcountry travelers on lower angle slopes and away from committing terrain. Persistent weak layers are unpredictable based on variations in the terrain and snow pack, they can also fail in unpredictable ways across a slope sometimes weeks after being buried.
We traveled up the Lick Creek Canyon yesterday to the Duck Lake Drainage. We saw and had reports of multiple natural avalanches on multiple aspects. The crowns that we were able to see were mid slope, not near ridgetop and had failed at the new snow, old snow interface 19-21 inches down. We also had reports of very stiff windslab in the upper elevations near the ridgetops that were cracking and collapsing under the weight of a skier. We were also able to get a video of the sudden collapse of the facet on crust problem that is now about 41 inches down in the snowpack. Despite the new snow, we still had challenging skining through the down logs and brush close to Lick Creek Road, once we got up to the Duck Lake area, conditions were much more user friendly.
Thanks to everyone that sent in observations of avalanche crowns that they saw yesterday.
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.SHORT TERM...Today through Monday...Light snow showers in the
West Central Mountains will end early this morning. An upper ridge
will build in from the west today providing dry and mild
conditions across southeast Oregon and southwest Idaho.
Temperatures will be 5 to 15 degrees above normal, with the
warmest temperatures approaching 50 degrees in the lower valleys.
A fast-moving frontal system will bring a round of precipitation
to the area on Saturday. Snow levels will range from 3000 to 4000
feet. Up to an inch of snowfall is expected in the mountains, except
1-3 inches central Idaho. Gusty south-southeast winds will
develop ahead of the front tonight, with breezy west-northwest
winds behind the front on Saturday. The winds will diminish and
the precipitation will end Saturday evening as a weak upper ridge
moves into the area. Northwest flow aloft will bring some moisture
in the area on Sunday for a chance of snow showers. Little
accumulation is expected in the valleys with up to an inch in the
mountains. The flow will remain moist across the north for more
snow showers and additional light accumulations Sunday night
through Monday. There will be a cooling trend early next week with
near-normal highs expected on Monday.
.LONG TERM...Monday night through Friday...Northwesterly flow
will bring a series of upper level troughs the region. These
systems will weaken as they move through the upper level ridge
however, they should bring precipitation to the region. Snow
levels remain at valley floors making any and all precipitation
fall in the form of snow. Higher elevations will see the greatest
probabilities of snowfall. The first trough moves through Tuesday
night into Wednesday followed by a second one on Friday.
Temperatures sit near normal for this time of year.
This avalanche advisory is provided through a partnership between the Payette National Forest and the Payette Avalanche Center. This advisory covers the West Central Mountains between Hard Butte on the north and Council Mountain on the south. This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires at midnight on the posted day unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.