THIS AVALANCHE ADVISORY EXPIRED ON December 27, 2018 @ 6:56 am
Avalanche Advisory published on December 26, 2018 @ 6:56 am
Issued by George Halcom - Payette Avalanche Center
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The Avalanche danger is Moderate. Human triggered avalanches are still possible. Winds from around the Compass have continued to form slabs on leeward facing terrain.  Within our variable snow pack, multiple persistent weak layers exist including a well preserved layer of buried surface hoar and basal facets Be Patient and Give the snow pack a VERY healthy dose of respect right now until conditions change, which may be a couple of weeks or more

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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Conditions were prime for creating fresh wind slabs from Northern winds last night. The weather station on Granite last night showed gusts out of the NE upwards of 27 MPH. All of the wind over this past week has created wind slabs on multiple aspects. Wind slabs tend to heal with time, but the wind has not taken a day off for Christmas or anything, so expect slabs on all aspects in areas that are not protected from the wind.

Watch for visual clues such as ripples, pillows or changes in texture of the snow pack as you travel through the wind affected upper elevations.   In addition, the amount of wind transport is allowing a dense and surprisingly deep snow pack to build quickly in the upper elevations. 

As the next storm enters our area later today we will likely see more wind slab development on all aspects in the upper elevations as the weather forecast is calling for moderate winds and another few inches of snow...meaning we could see fresh slabs as thick as six inches or more on leeward terrain.

Avalanche Problem 2: Persistent Slab
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Variability is the name of the game right now in our local snow pack.  This variability runs the spectrum from solid and consolidated snow to weak and unconsolidated with extremely weak layers waiting for a trigger. Snow depth, elevation, aspect, and wind history are all influencing the variability. The problems are also varied creating a complex forecasting and decision making process.  In a nutshell:

  •  Buried surface hoar exists in a growing and strengthening snow pack.  The layer of buried surface hoar that we have been watching is now 2-3 feet down in the snow pack below a slab that is gaining density and depth but not getting stronger.  So far the weight of the new snow has not been enough to tip the scale on a widespread natural avalanche cycle.  This layer is also mostly confined to shady, wind protected areas.
  • Basal Facets-leftover snow from early season storms and the cold start that we had to December.  Our entire snow pack is resting on a pile of kitty litter in some places creating the whumps and collapses that are still being reported as this layer fails when weighted by a skier or snowmobile. Shallow rocky areas and north facing terrain are still harboring alarming pockets of loose faceted snow.
  • Wind slabs ranging in thickness and sensitivity near the ridge tops.
  • Incremental loading from a series of small storms that have not provided the weight to trigger the weak layers in a natural avalanche cycle or squash them down enough to take them out of the equation.

 Snow pros and savvy backcountry travelers have a healthy respect for this kind of a snow pack scenario.  A great analogy is that we are building a house of cards right now and it is getting progressively taller and more wobbly.  As we add either ourselves or additional snow weight from storms or wind we know the house of cards is going to fall.  Roulette might be an overstatement but this level of uncertainty in a growing snow pack is enough to keep experienced recreationist out of steep terrain right now. 

If triggered either of our current weak layers have the potential to spread out or propagate over large distances.  Even a shallow avalanche may have the ability to step down into the deeper layers below.  Your best and safest bet right now is to avoid protected slopes steep enough to produce avalanches.  This lingering problem won't go away until we have enough snow load to crush the weak layers or trigger a natural cycle.

 

Saw Propagation Test Video with impressive failure in Buried Surface Hoar Layer 12/20/18

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

The lack of natural avalanches this past week means our snow pack has been able to adjust to the weight of the new snow so far.  Observations from different ends of our advisory area have revealed a variable snow pack that is more consolidated in some places more than others. 

Saturday near Josephine Lake, we saw around 6 feet on this North aspect Just North of Josephine Lake. The snow pack is holding a couple of strong over weak layers, one being near the ground. Coverage is getting a lot better, but still not inspiring enough to cover up all the rocks and logs to want to get after it on a sled. 

Please take the time to report your observation on the PAC site.  Just click Submit Observations and fill in what you saw.  You don't have to fill in all of the fields, just add what you can.  Aspect, Elevation and photos are great and add valuable content to our site.

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 8 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: 7 mph
Maximum wind gust in the last 24 hours: 27 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: na inches
weather

 

 

 

Area Forecast Discussion
National Weather Service Boise ID
250 AM MST Wed Dec 26 2018

.SHORT TERM...Today through Friday...Patchy fog in northern
valleys this morning otherwise mostly clear until noon.
Clouds will increase from the northwest this afternoon as an
upper trough approaches from the BC coast. This trough will
move directly over our CWA tonight then exit into Utah
Thursday. It will bring light snow to all areas tonight with
QPF limited by the partly over-land track of the system.
Snowfall will be light, with mountain areas receiving 1-2
inches and valleys generally less than one inch. These
amounts are not enough for a winter weather advisory, but the
snow will stick to roads and likely affect travel. The Blue
mountains of western Baker County and the Owyhee mountains of
southwest Idaho may receive 3 inches, and Steens Mountain in
southern Harney County may get 3 to 6 inches. Models again
target the Magic Valley and southern Twin Falls County and
southeastern Owyhee County for relatively more snow than the
other areas, generally 1-2 inches in the Magic Valley and
2 to 4 inches in the southern Sawtooths. These amounts do
qualify for a winter weather advisory south of Twin Falls,
which is being issued with this forecast package. On Thursday
the snow will move off to the southeast, except in southern
Twin Falls County, and skies will clear from the northwest.
West/northwest winds will increase, reaching 15 to 25 mph
with gusts to 35 mph in the Snake Basin east of Boise.
Mostly clear and cold Thursday night with decreasing winds.
Temperatures will dip below zero in the Camas Prairie Friday
morning, with other mountain lows zero to 10 above zero, and
valleys 10 to 20 degrees. Saturday looks sunny and cold under
northerly winds aloft.

.LONG TERM...Friday night through Wednesday. Models have come
into much better alignment for the overall pattern during this
forecast periods. Our upper level ridge offshore begins to flatten
at the start of the weekend allowing for a system to push south
from off the British Columbia coastline. Snow levels remain at
valley floors so any and all precipitation will fall as snow.
Northern areas of Baker County as well as the central Idaho
mountains look to be the most promising locations for snowfall
on the weekend. As we head into the start of January, mostly dry
but cool conditions will dominate.

&&

.AVIATION...Mostly VFR. Patchy fog through the lower valleys around
KMYL and KTWF/KJER through late morning hours. Snow will push into
SE Oregon after 21z causing MVFR/IFR conditions and mountain
obscuration. Snow will reach SW Idaho after 00z. Main bulk of
snowfall will push out of the area by 10z leaving isolated snow for
higher elevations. Surface winds, west to northwest 5-10kts. Winds
aloft to 10k feet MSL, northwest 25-30kts.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Wednesday Wednesday Night Thursday
Weather: Patchy fog before 11am. Otherwise, partly sunny, with a high near 24. Calm wind. Snow, mainly before 11pm. Low around 17. Calm wind. Chance of precipitation is 90%. New snow accumulation of around an inch possible. Partly sunny, with a high near 26. Calm wind becoming northwest 5 to 8 mph in the afternoon.
Temperatures: 24 deg. F. 17 deg. F. 26 deg. F.
Wind direction: Calm Calm NW
Wind speed: Calm Calm 5-8
Expected snowfall: 0 in. 1 in. 0 in.
Granite Mountain at 7700 feet.
  Wednesday Wednesday Night Thursday
Weather: Partly sunny, with a high near 17. Northwest wind around 8 mph becoming southwest in the morning. Snow, mainly before 11pm. Low around 12. South southwest wind 8 to 11 mph becoming north after midnight. Chance of precipitation is 90%. New snow accumulation of 1 to 3 inches possible. A 20 percent chance of snow showers. Partly sunny, with a high near 15. North wind 11 to 13 mph.
Temperatures: 17 deg. F. 12 deg. F. 15 deg. F.
Wind direction: NW becoming SW SSW becoming N N
Wind speed: 8 5-9 11-13
Expected snowfall: 0 in. 1-3 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.