Avalanche Advisory published on December 31, 2018 @ 6:35 am
Issued by George Halcom - Payette Avalanche Center
bottom line

 Moderate Avalanche hazard exists throughout the West Central Mountains.  The snowpack is widely variable right now.  Wind Slabs may be found in wind exposed, upper elevation terrain.  Human triggered avalanches are also possible in wind protected 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.

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  over the last few days 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.   Expect to see increasing wind loading and slab development beginning tonight and increasing Sunday as winds increase with another storm system entering our area.

Wind slabs are relatively easy to spot and recognize, look for cornices and drifted pillows of snow, or areas where the snowpack is more firm and supportive to the weight of a skier or snowmobiler.  

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

Our snowpack continues to show us reasons not to venture into steep terrain right now.  Despite the fact that we have not seen any new avalanches throughout the local mountains in almost 2 weeks, we still have a VERY unusual and atypical snowpack for the West Central Mountains.  Avalanche Professionals to our East and West are echoing the same themes in both the Sawtooth area and in the Wallowas.  Cautious travel is advised, the weak layers that continue to get deeper in our snowpack are refusing to go away quickly this year.  While the upper snowpack continues to gain strength and depth, the weakest layer of Basal Facets(near the ground) continues to show the ability to both initiate and propagate avalanches that would run to the ground.  A weak layer in the middle of the snowpack continues to produce failures in our pit tests too.  This is a layer of Surface Hoar and Near Surface Facets that are getting stronger in most cases but remain well preserved in others. 

In many areas, the snowpack is still relatively thin and the lack of snowpack is allowing skiers and snowmobilers to affect this layer resulting in collapsing or whumphing as this layer fails or is triggered by our weight.  We experienced several of these yesterday in wind protected areas including one that displaced the snow in the flats, reverberated away from us for several hundred feet and shook snow out of nearby trees.

The skiing and riding is getting better by the day with the addition of light, dry snow but unfortunately, until we have a large snowfall event coupled with warmer temperatures our buried weak layers are going to be a problem.  Your best and safest bet is to avoid slopes steep enough to produce avalanches, especially where the snowpack is still thin and the weak layers are more likely to be influenced by your weight.

Variability is the name of the game right now, check out the pictures and pit profiles from our day out yesterday in the observations section of the advisory or on the observations page.

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.

Your observations are very helpful to the PAC staff and help create a better picture of the complex terrain in our advisory area.  You can click on the add observations link and add as little or as much detail as you have.  It is easy to navigate and will also upload pictures easily.  Please contribute to your local forecast by sharing what you see or experience even if it is just good snow. or a trip report.

recent observations

The skiing and riding continues to get better, you  really couldn't ask for better quality snow and the almost daily refresher of an inch or two of new snow has done a great job keeping it soft out there.   There is still a lot of buried treasure, especially for sleds. Watch your a arms and your bases but enjoy it while it is here.  

These two pits were on the same ridgeline about 300 vertical feet apart in elevation.  The upper pit showed a much denser and slightly deeper snowpack where the weak layers were much less reactive in our tests.  The lower pit showed hard to initiate failures but easy propagation on both of the buried weak layers in a more shallow snowpack.  The profile shows the hardness of the snowpack from the top down in the shaded blocks, they both show the problem we have been discussing:  more firm snow over weaker snow below. 

We also were able to trigger several localized collapses and one very widespread collapse in the protected terrain near the toe of this slope.   In addition, both pits were dug on slopes less than 30 degrees. 






CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 9 deg. F.
Max. temperature in the last 24 hours: 11 deg. F.
Average wind direction during the last 24 hours: NE
Average wind speed during the last 24 hours: 8 mph
Maximum wind gust in the last 24 hours: 28 mph
New snowfall in the last 24 hours: NA inches
Total snow depth: NA inches


Area Forecast Discussion
National Weather Service Boise ID
259 AM MST Mon Dec 31 2018

.SHORT TERM...Today through Wednesday...A drier northerly flow
aloft will replace an exiting trough today. Residual moisture
near the edge of the trough will maintain scattered light snow
showers near the Nevada border this morning, which will taper off
this afternoon as the trough continues to push further south. An
upper ridge off the coast will build into the area Tuesday and
Wednesday for dry but cold conditions to start the new year.
Temperatures will average below normal, with the coldest period
expected to be Tuesday morning.

.LONG TERM...Wednesday night through Monday...Models are all in
good agreement in showing a broad upper ridge over the western
United States through the end of the week. This keeps the
jetstream into Washington and the Idaho panhandle. Any threat of
precipitation could be confined to the Salmon River region
northward. High temperatures across the region are expected to be
at or above normal through the extended, but morning lows are
still expected to be fairly cold as a strong sfc high remains
parked over eastern Idaho.

For the weekend, models diverge in their solutions on how the
next trough affects the region. The GFS shows the most run to run
variability whereas the ECM and Canadian have slightly better
consistency. For Saturday and into Sunday, the next trough
amplifies a ridge out ahead of it bringing continued warm and
potentially breezy conditions to the forecast area. ECM and
Canadian push the trough into the region Monday bringing the next
threat of precipitation. The 00Z run of the GFS shows a strong
cutoff low pushing into southern California and thus keeping
precip south of the area.


.AVIATION...Mostly VFR. Low clouds and fog thru sunrise across
KJER/KTWF along with KMYL lowering ceilings to IFR/MVFR at times.
Mtns obscd. Surface winds: W to NW 5 to 15 kts becoming N to NW
less than 10 kts aft 01/00z. Winds aloft near 10 KFT MSL: N to NE
10 to 20 kts.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Monday Monday Night Tuesday
Weather: Sunny, with a high near 21. Wind chill values between zero and 10. North wind around 8 mph. Mostly clear, with a low around -1. North northwest wind around 6 mph becoming calm in the evening. Partly sunny, with a high near 19. Calm wind.
Temperatures: 21 deg. F. -1 deg. F. 19 deg. F.
Wind direction: N NNW calm
Wind speed: 8 6 calm
Expected snowfall: 0 in. 0 in. 0 in.
Granite Mountain at 7700 feet.
  Monday Monday Night Tuesday
Weather: Mostly sunny, with a high near 12. Wind chill values between -3 and -9. North wind around 11 mph. Mostly clear, with a steady temperature around 5. Wind chill values between -4 and -9. Northeast wind 7 to 9 mph. Partly sunny, with a high near 15. Wind chill values between -6 and 4. North northeast wind around 6 mph becoming light and variable in the morning.
Temperatures: 12 deg. F. 5 deg. F. 15 deg. F.
Wind direction: N NE NNE
Wind speed: 11 7-9 6
Expected snowfall: 0 in. 0 in. 0 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.  This advisory applies only to backcountry areas outside established ski area boundaries. and describes general avalanche conditions.  Local variations always occur. and should be expected. This advisory expires at midnight on the posted day unless otherwise noted