Avalanche Advisory published on December 15, 2018 @ 7:40:
This advisory is valid for 24 hours
Issued by Dave Bingaman -
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How to read the advisory

 Use caution in upper elevation areas today.  Northerly slopes contain deeper snowpacks and multiple weak layers including buried surface hoar and basal facets creating a VERY weak snowpack in some areas.   New windslabs formed in the last 24 hours on NW, N and NE slopes.  With multiple weak layers, widespread variability and newly created wind slabs, avalanche season has begun.  We will be collecting more data over the next few days and begin issuing advisories next week.


weather

SHORT TERM...Today through Monday...Westerly flow in the wake of
last nights precipitation will keep clouds over the area today.
Sunday starts out dry, but an upper trough approaching the West
coast will increase clouds and the chance of precipitation across
southeast Oregon by afternoon. Similar to Friday the approach of a
deep offshore surface low will drive an easterly wind, especially
through the Snake Plain and Baker Valley tonight and through the
day Sunday. Have seen little change in forecast models with the
timing and intensity of this upper trough as it transitions across
the area. Snow levels are 4500-5500 feet Sunday night and early
Monday with accumulations of 1 to 3 inches expected for sites
above 5000 feet. By Monday afternoon the trough is pushing into
eastern Idaho with the back edge of precipitation closely
trailing. Temperatures are around 5 degrees above normal through
Monday.

recent observations

We saw multiple older crowns yesterday on steep Northerly terrain yesterday.  These were likely leftovers from the storm last week.  most of them were relatively shallow but partially filled in with new snow.  In addition, we observed cracking and one small avalanche along the Goose Lake Rd that were likely triggered by the groomer when they went through earlier in the week.  Hand pits and stability tests showed easy failures on isolation in the upper buried surface hoar layer.

Avalanche Problem #1: Persistent Slab

Our snowpack contains multiple persistent weak layers right now.   Buried surface hoar, density changes within the snowpack and a thick layer of basal facets have created the opportunity for shallow or deep avalanches to be possible in some areas.  Yesterday was our first partial day in the field and we are going to need some additional information before we issue a danger rating, BUT you should travel cautiously and select your routes with a very critical mindset especially on shaded, north facing aspects that have been accumulating snow since mid November.  We had easy test results including an ECT that failed on isolation in the basal facets at 37 inches yesterday in this kind of terrain.  With this weak layer on the ground it is possible that you could trigger an avalanche that would run to the ground.  In addition to the layer of weak sugary facets near the ground, we also found at least 2 distinct layers of buried surface hoar in the upper 12 inches of the snowpack on a northerly aspect.

Caution is the name of the game right now, the deepest(and best skiing and riding) areas are exactly where the hazard is highest right now.  Lower angle, East and West aspects are the place to be right now, the snowpack is more dense and will allow you to travel higher in the snowpack even though there is less snow.  

 

Avalanche Problem #2: Wind Slab

Yesterday afternoon a low pressure system entered the area producing high winds.  By early evening, ridgetop winds were out of the SE and SW gusting into the low 40 mph range.  With plenty of low density snow available for transport, wind slabs will be a problem on or near NW, NE and N facing slopes and possible on E and W facing crossloaded slopes.  These are the same areas that are harboring multiple weak layers.  Expect the avalanche danger to have increased overnight on all windloaded slopes.  Additional wind and light snow accumulations will only add to this problem.

advisory discussion

Please submit your observations to the PAC.  It's easy and you don't need to worry about filling in every blank.  Fill out what you know and add a picture.  Watch your bases and A arms our there!

CURRENT CONDITIONS  Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 18 deg. F.
Max. temperature in the last 24 hours: 31 deg. F.
Average wind direction during the last 24 hours: SE
Average wind speed during the last 24 hours: 20 mph
Maximum wind gust in the last 24 hours: 42 mph
New snowfall in the last 24 hours: inches
Total snow depth: na inches
Two-Day Mountain Weather Forecast  Produced in partnership with the Reno NWS
For 7000-8000 ft.
  Saturday Saturday Night Sunday
Weather: Partly sunny, with a high near 30. South southeast wind 3 to 6 mph. Mostly cloudy, with a low around 21. Light and variable wind. Partly sunny, with a high near 36. Calm wind becoming south 5 to 7 mph in the afternoon.
Temperatures: 30 deg. F. 21 deg. F. 36 deg. F.
Wind direction: SE Light S
Wind speed: 3-6 variable 5-7
Expected snowfall: 0 in. 0 in. 0 in.
For 8000-9000 ft.
  Saturday Saturday Night Sunday
Weather: Mostly cloudy, with a steady temperature around 20. South southwest wind 11 to 14 mph. Mostly cloudy, with a low around 22. South wind around 14 mph. A 20 percent chance of snow after 11am. Partly sunny, with a high near 30. Breezy, with a south wind 14 to 21 mph.
Temperatures: 20 deg. F. 22 deg. F. 30 deg. F.
Wind direction: SSW S S
Wind speed: 11-14 14 14-21
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer

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.