THIS AVALANCHE ADVISORY EXPIRED ON December 25, 2018 @ 6:54 am
Avalanche Advisory published on December 24, 2018 @ 6:54 am
Issued by George Halcom - Payette Avalanche Center
bottom line

The Avalanche danger is Moderate.  Wind slabs can be found on leeward facing terrain, additional snow and wind today will add to the problem.  Within our variable snow pack, multiple weak layers exist including a well preserved layer of buried surface hoar and basal facets.   Give the snow pack a VERY healthy dose of respect right now until conditions change. If triggered, avalanches have the possibility of spreading out over large areas. 

How to read the advisory


  • Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Conditions are prime for creating fresh wind slabs right now, and we have had quite a bit of wind over the last week that have created wind slabs on multiple aspects. Winds have shifted from S to SW and W and to NW.  Wind slabs of varying thickness and strength can be found on upper elevation terrain on E, N and W terrain.  Some of these may be overlying buried weak layers. 

Watch for visual clues such as ripples, pillows or changes in texture of the snow pack as you travel through the wind affected upper elevations.   In addition, the amount of wind transport is allowing a dense and surprisingly deep snow pack to build quickly in the upper elevations. 

As the next storm enters our area later today expect to see prefrontal winds increasing into the lower to mid 20mph range.  With a lot of light density snow available on the ground right now, additional wind loading should be expected on all leeward terrain.

Avalanche Problem 2: Persistent Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Variability is the name of the game right now in our local snow pack.  This variability runs the spectrum from solid and consolidated snow to weak and unconsolidated with extremely weak layers waiting for a trigger. The problems are also varied creating a complex forecasting and decision making process.  In a nutshell:

  •  Buried surface hoar exists in a growing and strengthening snow pack.  The layer of buried surface hoar that we have been watching is now 2-3 feet down in the snow pack below a slab that is gaining density and depth but not getting stronger.  So far the weight of the new snow has not been enough to tip the scale on a widespread natural avalanche cycle.  This layer is also mostly confined to shady, wind protected areas.
  • Basal Facets-leftover snow from early season storms and the cold start that we had to December.  Our entire snow pack is resting on a pile of kitty litter in some places creating the whumps and collapses that are still being reported as this layer fails when weighted by a skier or snowmobile. Shallow rocky areas and north facing terrain are still harboring alarming pockets of loose faceted snow.
  • Wind slabs ranging in thickness and sensitivity near the ridge tops.
  • Incremental loading from a series of small storms that have not provided the weight to trigger the weak layers in a natural avalanche cycle or squash them down enough to take them out of the equation.

 Snow pros and savvy backcountry travelers have a healthy respect for this kind of a snow pack scenario.  A great analogy is that we are building a house of cards right now and it is getting progressively taller and more wobbly.  As we add either ourselves or additional snow weight from storms or wind we know the house of cards is going to fall.  Roulette might be an overstatement but this level of uncertainty in a growing snow pack is enough to keep experienced recreationist out of steep terrain right now. 

If triggered either of our current weak layers have the potential to spread out or propagate over large distances.  Even a shallow avalanche may have the ability to step down into the deeper layers below.  Your best and safest bet right now is to avoid protected slopes steep enough to produce avalanches.  This lingering problem won't go away until we have enough snow load to crush the weak layers or trigger a natural cycle.

 

Saw Propagation Test Video with impressive failure in Buried Surface Hoar Layer 12/20/18

advisory discussion

Due to the partial government shutdown, avalanche forecasting will be limited.  We expect to forecast 3 times a week until the shutdown has ended.

recent observations

The lack of natural avalanches this week means our snow pack has been able to adjust to the weight of the new snow so far.  Observations from different ends of our advisory area have revealed a variable snow pack that is more consolidated in some places more than others. 

Yesterday, near Josephine Lake, we saw around 6 feet on this North aspect Just North of Josephine Lake. The snow pack is holding a couple of strong over weak layers, one being near the ground. Coverage is getting a lot better, but still not inspiring enough to cover up all the rocks and logs to want to get after it on a sled. 

Please take the time to report your observation on the PAC site.  Just click Submit Observations and fill in what you saw.  You don't have to fill in all of the fields, just add what you can.  Aspect, Elevation and photos are great and add valuable content to our site.

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 19 deg. F.
Max. temperature in the last 24 hours: 20 deg. F.
Average wind direction during the last 24 hours: W
Average wind speed during the last 24 hours: 9 mph
Maximum wind gust in the last 24 hours: 15 mph
New snowfall in the last 24 hours: 6 inches
Total snow depth: na inches
weather

 

Area Forecast Discussion
National Weather Service Boise ID
351 AM MST Mon Dec 24 2018

.SHORT TERM...Today through Wednesday...Snow has decreased or
ended in northern areas and the winter weather advisory there
has been cancelled. Light rain and snow were continuing in
southwest Idaho as of 3 AM. Eastern Oregon was generally dry.
Attention turns to a Pacific storm off the Oregon Coast. This
storm will move inland this morning and then turn sharply south
into Nevada tonight and eventually to southern California. This
track will spread several inches of snow across Harney County,
especially the southern half. Part of the system will also
shear east into Owyhee County, and later into southern Twin Falls
County. Snow level will be 4000 to 4500 feet MSL. Precipitation
is southern valleys will therefore be mainly rain. Colder and
drier air will move into Oregon from the northwest early
Christmas morning. The colder air will reach southwest Idaho
later Christmas Day and precipitation will become mainly snow
showers at that time, although little accumulation is expected.
Northwest winds will increase to 15 to 25 mph in the Snake Basin
Christmas afternoon.

.LONG TERM...Wednesday night through Monday...A wet start to the
extended period as a Pacific system pushes south through the area
Wednesday night through early Friday. Models are in fairly decent
agreement on timing and placement on precipitation, but
accumulations vary quite significantly. The ECMWF keeps QPF totals
for the 24 hour period from 00z Thursday through 00z Friday to
around 0.06 inches for lower valleys while the GFS more than doubles
those values. Keep in mind our snow levels during this time are 2k
feet or less meaning this will all be in the form of snow. This
system will be monitored closely over the next few model runs.
Things dry out Friday through much of the weekend as a ridge
offshore begins to build. Models are still having a hard time coming
to agreement after Saturday with the ECMWF dropping another
Pacific storm system south while the GFS holds onto the ridge a bit
longer. The forecast continues to lean towards the wetter solutions,
but confidence remains low.

&&

.AVIATION...MVFR/IFR conditions expected with bands of rain/snow
continuing to press through for the remainder of the morning hours.
After 18z widespread snow/rain pushes in from the west. Snow should
be expected at or above 3500 ft. Surface winds, southeast to
southwest 10-15kts. Winds aloft to 10k ft MSL, west to southwest
20-30kts.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Monday Monday Night Tuesday
Weather: Snow likely, mainly before 11am. Mostly cloudy, with a high near 31. South wind 6 to 8 mph. Chance of precipitation is 70%. New snow accumulation of less than one inch possible. A 20 percent chance of snow. Mostly cloudy, with a low around 24. Light and variable wind becoming north northeast around 5 mph after midnight. A 20 percent chance of snow before 11am. Mostly cloudy, with a high near 30. East northeast wind 5 to 9 mph becoming northwest in the afternoon.
Temperatures: 31 deg. F. 24 deg. F. 30 deg. F.
Wind direction: S NE ENE-NW
Wind speed: 6-8 5 5-9
Expected snowfall: Less than one in. 0 in. 0 in.
Granite Mountain at 7700 feet.
  Monday Monday Night Tuesday
Weather: Snow likely, mainly before 11am. Mostly cloudy, with a high near 22. South wind 9 to 17 mph. Chance of precipitation is 70%. Total daytime snow accumulation of 1 to 3 inches possible. A 30 percent chance of snow, mainly after 11pm. Mostly cloudy, with a low around 17. Southeast wind 5 to 9 mph becoming light and variable in the evening. New snow accumulation of less than one inch possible. A 30 percent chance of snow, mainly before 11am. Mostly cloudy, with a high near 20. Wind chill values between zero and 10. East northeast wind 8 to 17 mph becoming north northwest in the afternoon. New snow accumulation of less than a half inch possible.
Temperatures: 22 deg. F. 17 deg. F. 20 deg. F.
Wind direction: S SE ENE-NNW
Wind speed: 9-17 5-9 8-17
Expected snowfall: 1-3 in. less than one in. Less than one half 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.