THIS AVALANCHE ADVISORY EXPIRED ON December 29, 2018 @ 6:41 am
Avalanche Advisory published on December 28, 2018 @ 6:41 am
Issued by Dave Bingaman - Payette Avalanche Center
bottom line

Moderate Avalanche hazard exists throughout the West Central Mountains.  Wind Slabs exist in wind exposed, upper elevation terrain.  Human triggered avalanches are also possible in many areas where buried weak layers exist within the snowpack.  Your safest option is to avoid steep terrain, especially areas where the snowpack is still thin enough to trigger the sugary, faceted layer near the ground and pay attention to which slopes are being affected by the wind.

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 ?
    Very Likely
  • Size ?
    Very Large

The combination of light, dry snow and winds from multiple directions has been perfect for creating soft, thin wind slabs on upper elevation, wind exposed slopes.  These slabs may be found on all slopes that have seen winds over the last week.  These relatively thin slabs also have the possibility of triggering or "stepping down" into  the deeper weak layers within the snowpack. 

Wind slabs are relatively easy to spot and recognize, look for cornices or drifted pillows of snow or areas where the snowpack is more firm and supportive to the weight of a skier or snowmobiler.  Additional winds from the Northern portions of the compass will add to the problem through the day today.

Avalanche Problem 2: Persistent Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Very Likely
  • Size ?
    Very Large

Our snowpack is SLOWLY gaining strength and depth and the buried weak layers are starting to become less reactive.  We still have a more complex and varied snowpack than we typically see in the West Central Mountains though and CAUTION is still your best bet to avoid triggering avalanches.  A layer of buried surface hoar can still be identified just about 3 feet down in the snowpack and basal facets(near the ground) are still showing instability in our pit tests.  

What all this means is that we are getting alot of mixed mesages from our snowpack right now.  On one hand, the structure of the snowpack is improving and we have not had any new avalanches reported in the last 10 days.   On the other hand, we still have 2 distinct weak layers that are consistently providing warning signs in stability tests throughout the local area.  In addition, the depth of the snowpack is still variable making it easier to trigger the weak layer near the ground in shallow areas.  Triggering an avalanche in these thin areas at the ground would make for an ugly ride through the rocks, stumps and over other ground cover.  Finally wind slabs are being consistently created on multiple aspects adding to the hazard on upper elevation slopes. 

The signs of caution are still outweighing the signs to go for it to for snow professionals and other cautious backcountry travelers. Your best and safest bet to avoid triggering an avalanche right now remains avoiding steep slopes and paying attention to changing snowpack conditions and depths as you travel through the mountains.

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

Winds speeds increased last night generally out of the N and NW increasing the possibility of fresh wind slabs being created.  An abundance of light density snow available to be moved around by even moderate winds will allow these slabs to continue to build through the day and until the winds calm back down.  While our snowpack continues to slowly grow in depth and is generally getting more supportive, there is still a LOT of buried treasure to be found off trail whether you are riding sleds or sliding on boards.  Travel lightly and look for more supportive snow on East and West aspects.

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 7 deg. F.
Max. temperature in the last 24 hours: 15 deg. F.
Average wind direction during the last 24 hours: N
Average wind speed during the last 24 hours: 8 mph
Maximum wind gust in the last 24 hours: 19 mph
New snowfall in the last 24 hours: 3 inches
Total snow depth: na inches

SHORT TERM...Today through Sunday...A drier northerly flow aloft
will continue across the area today. Patchy fog is possible in
portions of southwest Idaho this morning - east of Mountain Home
and in the mountain valleys of Central Idaho. Otherwise, expect
dry conditions and lighter winds today. Pacific moisture riding
over top of an upper ridge off the coast will result in snow
across mainly northern portions of our forecast area tonight and
Saturday. Total snow accumulations of less than an inch in the
northern valleys and 1-2 inches in the mountains are expected
through the period. An upper trough moving across the northern
Rockies Saturday night will drag a strong cold front across our
area on Sunday. Precipitation associated with the trough and front
will occur mainly over southwest Idaho, with the greatest amounts
expected in the Boise/West Central Mountains. Snow levels will
rise ahead of the front with a rain/snow mix expected in the lower
valleys. It will become windy on Sunday with the frontal passage,
especially in the Western Magic Valley and Southern Twin Falls
County where it will be very windy in the afternoon.

.LONG TERM...Sunday night through Friday...Models all in good
agreement indicating the upper ridge remaining amplified off the
west coast through early Tuesday. The next system pushes into the
Pac Nw and somewhat flattens the ridge as it moves further east
but also weakens considerably Tuesday afternoon. Any threat of new
snow should be minimal and confined to Idaho`s west central
mountains. Confidence by Thursday and Friday begin to diminish as
another system threatens the Pac Nw Thursday morning. Models
differ on the timing and the strength, with the ECM about 12 hrs
faster than the GFS and the ECM also weakens it quicker than the
GFS. One thing the ECM/GFS/Canadian all agree on is that the flow
into the Pac Nw remains active and off the Pac vs coming out of
Canada and the Gulf of Ak. This therefore should keep temperatures
somewhat near normal after Wednesday through the weekend.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Friday Friday Night Saturday
Weather: Increasing clouds, with a high near 22. Wind chill values between -1 and 9. North northeast wind around 5 mph becoming calm in the morning. A 50 percent chance of snow, mainly after 11pm. Mostly cloudy, with a low around 14. Calm wind. New snow accumulation of less than a half inch possible. Snow likely, mainly before 11am. Cloudy, with a high near 24. Light south wind. Chance of precipitation is 60%. New snow accumulation of less than one inch possible.
Temperatures: 22 deg. F. 14 deg. F. 24 deg. F.
Wind direction: NNE Calm S
Wind speed: 5 Calm Light
Expected snowfall: 0 in. Trace in. 1 in.
Granite Mountain at 7700 feet.
  Friday Friday Night Saturday
Weather: Increasing clouds, with a high near 18. Wind chill values between -3 and 7. North northeast wind 3 to 8 mph. Snow likely, mainly after 11pm. Mostly cloudy, with a low around 13. Light and variable wind becoming south southwest around 6 mph in the evening. Chance of precipitation is 70%. New snow accumulation of 1 to 2 inches possible. Snow, mainly before 11am. High near 22. South southwest wind 6 to 8 mph. Chance of precipitation is 90%. New snow accumulation of 1 to 2 inches possible.
Temperatures: 18 deg. F. 13 deg. F. 22 deg. F.
Wind direction: NNE SSW SSW
Wind speed: 3-8( currently gusting to 19) 6 6-8
Expected snowfall: 0 in. 1-2 in. 1-2 in.

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.