THIS AVALANCHE ADVISORY EXPIRED ON January 5, 2020 @ 7:56 am
Avalanche Advisory published on January 4, 2020 @ 7:56 am
Issued by Dave Bingaman - Payette Avalanche Center
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Human triggered avalanches remain possible today.  You may be able to trigger avalanches on a layer of buried surface hoar 18-24 inches down in the snowpack. Widespread natural activity has been reported this week on multiple aspects with reports of both skier and snowmobile triggered avalanches on this layer.  A deeper layer may be reactive on a layer of facets near the November crust.  Wind loading this morning will add to the problem near ridgetops.  Avoid steep, wind loaded terrain.

How to read the advisory


  • Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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The West Central Mountains added just over an inch of Snow Water Equivalent(SWE) earlier this week to our existing snowpack 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.  Many natural avalanches occurred during and after this storm and it remains possible for skiers and sledders to trigger an avalanche on steep terrain today.  Winds  ramped up last night and will be gusting to 30+mph today.  Additional snowfall this morning combined with the wind will increase the wind slab hazard near ridge tops and on all wind exposed terrain today.  Wind slabs may be solid enough to support your weight and allow you to get well out onto them before they fail.  Changes in the density of the snowpack, cracking or collapsing under your skis or sled are signs you are on a wind slab.

 

This picture is a wind slab on the backside of Hidden Valley near Brundage Resort that failed on a persistent weak layer.  We observed several skiers traversing through this same terrain with and without avalanche gear yesterday.

 

 

Avalanche Problem 2: Persistent Slab
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Our persistent weak layers are doing exactly what they are supposed to do. They are lingering and 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 that was buried by the New Year's storm. This is going to be a layer to watch as we add more storm and wind deposited snow today and through the next few days.

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 recent video of a Propagation Test on this layer.  You will be more likely to trigger this early season problem where the snowpack is shallow and still weak above the buried layers, especially in rocky terrain where the ground is uneven across a slope.

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. 

Persistent weak layers in snowpack on Sargeant's Mt. near Brundage Resort:

recent observations

We toured out the ridge from Brundage Mt Resort yesterday to see some recent avalanches from Thursday and found quite a few parts of the ridgeline had already slid.  Ridgetop conditions were stiff and produced cracking and collapsing in the wind slabs.  Cornices are starting to grow and are brittle to the weight of a skier.  We also had several notable whumphs as we approached our pit sites and noted that the East face of Sarge had slid naturally which I have not seen in 25 years of skiing that area.  Buried surface hoar, near surface facets and wind deposited snow were the culprits and it is also worth noting that some of the areas that slid were mid slope rather than just below the ridgetops which indicates strong winds and low density snow being blown further down slope.  The East side snowpack was surprisingly shallow at only 80-100 cms,  the Northern aspects were 130-150cms.  Basal facets and depth hoar were well preserved in the bottom of the North side snowpack if you are interested in checking them out, more so than in many other areas we have visited recently.   

Tip of the week:  If you are venturing out of bounds at the local resorts, go with someone who knows more than you and make sure everyone in your group is packing a beacon, probe and shovel.  Based on observed behavior and travel decisions, lots of folks would benefit from the upcoming FPAC Intro to Avalanche Classes over the next few weeks.  #KnowBeforeYouGo, #GetTheGear(for the millennial crowd).

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 22 deg. F.
Max. temperature in the last 24 hours: 27 deg. F.
Average wind direction during the last 24 hours: SSW
Average wind speed during the last 24 hours: 4 mph
Maximum wind gust in the last 24 hours: 31 mph
New snowfall in the last 24 hours: NA inches
Total snow depth: NA inches
weather

.SHORT TERM...Today through Tuesday...Expect widespread
precipitation today with passage of an upper level trough. The
trough will reach Southeast Oregon around 13z and quickly move
into Southwest Idaho around 16z. This system weakens as it moves
east but should be strong enough to produce a few inches of snow
above 4000 feet. Lower valleys will see rain showers as the trough
passes. There will be a brief break this evening ahead of the
next trough for Sunday morning. This system is a bit stronger and
colder and should bring light snow down to the valley floors. Snow
amount will be limited, mainly to under half an inch, as this
system lacks moisture. Upper level ridge rebuilds on Monday and
Tuesday ahead of another trough for mid week. Temperatures will
remain at or slightly above normal for the period.

.LONG TERM...Tuesday night through Saturday...The long wave trough
will remain situated over the Western U.S. with a series of storms
moving through the area. Precipitation will mainly be in the form of
snow although a mix of rain and snow is likely in the lower valleys
during the daytime. The system on Friday looks to be the most potent
with a chance of accumulating snow in the Treasure Valley.
Temperatures are expected to be near normal through the period.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Saturday Saturday Night Sunday
Weather: Snow, mainly before 11am. High near 32. South wind around 6 mph becoming west in the morning. Chance of precipitation is 100%. Total daytime snow accumulation of around an inch possible Increasing clouds, with a low around 18. Calm wind. Snow showers likely, mainly after 11am. Cloudy, with a high near 28. Light and variable wind. Chance of precipitation is 70%. New snow accumulation of 1 to 2 inches possible.
Temperatures: 32 deg. F. 18 deg. F. 28 deg. F.
Wind direction: S Calm Variable
Wind speed: 6 Calm Light
Expected snowfall: 1 in. 0 in. 1-2 in.
Granite Mountain at 7700 feet.
  Saturday Saturday Night Sunday
Weather: Snow, mainly before 11am. Temperature falling to around 15 by 5pm. South southwest wind 8 to 18 mph becoming west northwest in the morning. Chance of precipitation is 100%. Total daytime snow accumulation of 2 to 4 inches possible. ncreasing clouds, with a low around 12. Southwest wind around 7 mph becoming south southeast in the evening. Snow showers, mainly after 11am. High near 19. Southwest wind 5 to 8 mph. Chance of precipitation is 80%. New snow accumulation of 1 to 3 inches possible.
Temperatures: Falling to 15 deg. F. 12 deg. F. 19 deg. F.
Wind direction: SSW SW SW
Wind speed: 8-18 7 5-8
Expected snowfall: 2-4 in. 0 in. 1-3 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.