THIS AVALANCHE ADVISORY EXPIRED ON December 18, 2016 @ 6:40 am
Avalanche Advisory published on December 17, 2016 @ 6:40 am
Issued by Dave Bingaman - Payette Avalanche Center
bottom line

The Avalanche hazard today is Moderate.  Wind Slabs near ridge tops and areas of crossloading lower down on exposed slopes will be encountered across the compass today due to moderate but steady North winds throughout the day yesterday.  Additionally, it may be possible to trigger avalanches up to 1.5 feet deep on steep, wind protected slopes at the interface between the old snow and the newer snow from our last storm event.

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
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Wind slabs deposited during the storm are probably mostly eroded out now with a dense wind board replacing them on the Northerly slopes. With steady and mostly northerly winds dominating throughout the day yesterday and overnight you should expect to find newly formed wind slabs scattered throughout the upper elevation terrain on several aspects and on the leeward side of any exposed features.

 

Avalanche Problem 2: Storm Slab
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The interface between this week's new snow and the older snow below will likely continue to be our layer of concern until we have another significant storm. In the northern portion of the advisory area we were unable to find the thick grauple layer that we found on West Mountain earlier this week. The density change is enough to create the hazard of storm slabs up to 1.5 feet thick in some areas.  Where the grauple layer is present it is more likely to propagate making it easier to trigger the storm slab above.  

recent observations

We toured out to Fisher Saddle area yesterday and found an amazing drying out of the warm wet snow that fell Wednesday and Thursday.  IT WAS COLD and got colder through the day. North winds near the upper end of the Fisher Road were in the 10-20 range and made it feel even colder.  We found that the instabilities at the interface between the new and old snow in this area was less reactive than we saw near Tamarack Resort on Thursday. We did not find the widespread grauple layer above 7000 feet and our test results were much better with no propagation on an Extended Column Test and Compression Test scores in the CTH range.  We did see clean but resistant shear faces at the bottom of the new snow layer and found some grauple mixed in throughout.  Winds were the big issue yesterday with the north wind scouring and sculpting the upper elevation slopes into a firm, mostly supportable and featured wind board with active wind transport creating soft, thin wind slabs on a variety of slopes.

Worth noting is the widespread presence of a layer cake of rounding but faceted snow resting above and below an ice crust near the bottom of the snowpack in many areas where the October snow was preserved. This is likely the layer that has been producing the random collapsing or whumphing of the snowpack in some areas. This layer has been producing sudden planar shears when loaded in deep tests but is likely below the stress bubble of a skier or snowmobile in all but very shallow areas.

From a recreational standpoint the skiing was better than expected, travel was easy with about 30 cm ski penetration on protected slopes. Sleds are still digging through the snowpack to the ground in many places.  We could not believe what we saw that had been "uncovered" by sleds off trail yesterday. Coverage is still thin with just enough new snow to camouflage many of the buried obstacles.  Lower elevation slopes are a mine field of stumps and downed trees. Be careful out there!

North aspect near 8300 feet above Fisher Saddle.

weather

Today and tomorrow will mark near record low temperatures throughout Idaho. Expect partly sunny skies with increasing clouds on Sunday with single digit highs temps.  NW winds will make mountain temperatures feel closer to -20 to -30 with the wind chill factored in. Next week will bring a gradual warming with 2 different storm systems bringing in additional snowfall starting  Monday.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Saturday Saturday Night Sunday
Weather: Partly sunny and cold Partly cloudy Partly sunny
Temperatures: 9 deg. F. -13 deg. F. 11 deg. F.
Wind direction: S/SW calm light and variable
Wind speed: 5 calm light
Expected snowfall: 0 in. 0 in. 0 in.
Granite Mountain at 7700 feet.
  Saturday Saturday Night Sunday
Weather: Partly sunny and cold, wind chills between -20 and -30. Partly cloudy, wind chills between -17 and -27 Increasing clouds, wind chills between -8 and -18.
Temperatures: 6 deg. F. -7 deg. F. 10 deg. F.
Wind direction: NW then W later in the day W/NW W/SW
Wind speed: 8-13 8-14 6-15
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.