THIS AVALANCHE ADVISORY EXPIRED ON January 9, 2020 @ 7:17 am
Avalanche Advisory published on January 8, 2020 @ 7:17 am
Issued by George Halcom - Payette Avalanche Center
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Human triggered avalanches remain possible today. New snow and Southwest winds may create small natural avalanches, and wind slabs that are sensitive to the weight of a person. Human triggered avalanches on persistent weak layers at the mid and ground level of the snow pack are getting harder to trigger, but have large danger potential if triggered.  Avoid steep, wind loaded terrain, and steep, rocky 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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Natural wind slab avalanches are going to be possible this afternoon as we get 2-6 inches of snow in the upper elevations along with strong winds over 20 MPH. They may produce slabs up to a foot deep by the end of today, and up to 2 feet in areas that are teetering from the last event. Given the temperatures and our observations the older slabs will likely stabilize soon.

 

Avalanche Problem 2: Persistent Slab
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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. The depth is variable and structure is overall poor, and should definately stick in your mind for your decision making. The layer of buried surface hoar about 1-2 feet down that is still preserved, and has gained strength recently.

The other persistent problem (in some areas it is becoming a deep persistent problem) is lower in our snow pack and it could have serious consequences given its depth: 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. 

You will be more likely to trigger this crust layer 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. Larger triggers like snowmobiles are going to effect this weak layer much easier.

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

Yesterday near Fisher Creek saddle, North of McCall, under warmer temperatures, we observed a stronger snow pack that lacked results from earlier in the week. The poor structure is still present, and the persistent weak layers are still giving planar surfaces when they break. Check out this propagation saw test video. It collapsed suddenly on facets above a stout crust. This layer is deep here, but where the snow pack is shallower we believe a person could still effect it. The variable depth of the snow pack from a given place in the mountains will keep this on our radar for a while as it holds a lot of snow above poor structure.

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.

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: 25 deg. F.
Average wind direction during the last 24 hours: W
Average wind speed during the last 24 hours: 1 mph
Maximum wind gust in the last 24 hours: 7 mph
New snowfall in the last 24 hours: NA inches
Total snow depth: 37 inches
weather

 

Area Forecast Discussion
National Weather Service Boise ID
301 AM MST Wed Jan 8 2020

.SHORT TERM...Today through Saturday...Major pattern shift is
underway as a large scale trough sets up over the Pacific
northwest. Weak troughs embedded in the northwesterly flow will
bring periods of showers through Thursday. Snow levels around 3500
feet for mostly rain showers in the valleys will lower to the
valley floors tonight. Mountains will see continue snow through
Thursday favoring westerly facing slopes. Best chance for snow in
the lower valleys appears to be Thursday morning with a weak
trough passage. Mountains will see 4 to 8 inches of snow through
Thursday. Valleys could see up to half inch of snow. A brief break
in the precipitation is expected Friday as the next significant
winter storm system approaches the region. This will be the first
of a few strong winter storms that will bring winter weather
conditions across the region and significant snowfall to the
mountains beginning Saturday morning. Mountains could see 10 to 14
inches of new snow on Saturday. The fast moving cold front will
limit valley snowfall to around an inch or so on Saturday.
Temperatures will gradually cool falling below normal by Friday.
Stay tuned for the latest updates on this evolving storm for the
weekend.

.LONG TERM...Saturday night through Wednesday...An upper level
trough remains parked over the the Pacific Northwest through the
period, with a moist northwest flow across the forecast area.
Periods of snow likely, with temperatures cold enough to be snow,
even at the lowest valley elevations nearly throughout. Significant
snow accumulations likely over the mountains. Larger differences
start to creep into model solutions by Late Tue/Wed, with some
solutions bringing in much colder arctic air, while others are a bit
warmer in more zonal flow. Until more consistency is seen in models,
trended temperatures a bit colder for Tue/Wed, but not as cold as
some solutions would support.

&&

.AVIATION...MVFR/IFR conditions with widespread snow showers over
the mountains, mainly over Idaho today, and then another wave
shifting showers more over Oregon tonight, and toward the Nevada
border by early Thursday. Surface winds generally from the west 10-
20 kts. Winds aloft to 10k feet MSL, west to northwest 30-40kts.

 

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Wednesday Wednesday Night Thursday
Weather: Snow showers. High near 30. South wind 3 to 7 mph. Chance of precipitation is 90%. Total daytime snow accumulation of around an inch possible. Snow showers likely. Cloudy, with a low around 19. Southwest wind around 5 mph becoming calm in the evening. Chance of precipitation is 70%. New snow accumulation of less than one inch possible. Snow showers, mainly after noon. High near 26. Light west southwest wind. Chance of precipitation is 80%. New snow accumulation of less than one inch possible.
Temperatures: 30 deg. F. 19 deg. F. 26 deg. F.
Wind direction: South South West South West
Wind speed: 3-7 mph 5 mph Light
Expected snowfall: 1 in. Less than one in. Less than one in.
Granite Mountain at 7700 feet.
  Wednesday Wednesday Night Thursday
Weather: Snow showers. High near 20. Southwest wind around 11 mph, with gusts as high as 22 mph. Chance of precipitation is 100%. Total daytime snow accumulation of 2 to 4 inches possible. Snow showers. Low around 14. West southwest wind 6 to 10 mph. Chance of precipitation is 90%. New snow accumulation of 2 to 4 inches possible. Snow showers. High near 14. West wind 5 to 7 mph. Chance of precipitation is 90%. New snow accumulation of 1 to 3 inches possible.
Temperatures: 20 deg. F. 14 deg. F. 14 deg. F.
Wind direction: Southwest West Southwest West
Wind speed: 11-15 mph Gusting to 22 mph 6-10 mph 5-7 mph
Expected snowfall: 2-4 in. 2-4 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.