THIS AVALANCHE ADVISORY EXPIRED ON December 23, 2018 @ 5:57 am
Avalanche Advisory published on December 22, 2018 @ 5:57 am
Issued by Dave Bingaman - Payette Avalanche Center
bottom line

Our snowpack is widely variable right now and so are our avalanche problems.  Multiple weak layers exist within our snowpack including a well preserved layer of buried surface hoar and basal facets.  Wind slabs can be found on leeward facing terrain, additional snow and wind will add to the problem.  Give the snowpack a VERY healthy dose of respect right now until conditions change. If triggered, avalanches have the possibility of spreading out over large areas. 

How to read the advisory


  • Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Persistent Slab
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Variability is the name of the game right now in our local snowpack.  This variability runs the spectrum from solid and consolidated snow to weak and unconsolidated with extremely weak layers waiting for a trigger. 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 snowpack.  The layer of buried surface hoar that we have been watching is now 2-3 feet down in the snowpack 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 snowpack 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 ridgetops.
  • 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 snowpack 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 snowpack 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 snowload 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

Avalanche Problem 2: Wind Slab
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We have had quite a bit of wind over the last week that have created wind slabs on multiple aspects as winds have shifted from S to SW and W and last night to NW.  Wind slabs of varying thickness and strength can be found on upper elevation terrain on E, N and W terrain.  Some of these may be overlying buried weak layers.  Watch for visual clues such as ripples, pillows or changes in texture of the snowpack as you travel through the wind affected upper elevations.   In addition, the amount of wind transport is allowing a dense and surprisingly deep snowpack to build quickly in the upper elevations(See photo). 

As the next storm enters our area later today expect to see prefrontal winds increasing into the lower to mid 20mph range.  With a lot of light density snow available on the ground right now, additional wind loading should be expected on all leeward terrain.

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 week after our Tuesday storm was a surprise and just means our snowpack has been able to adjust to the weight of the new snow so far.   Observations from Tamarack Resort and the Payette Powder Guides in the Lick Creek Summit area show similar snowpack structure throughout the West Central Mountains right now. 

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: 7 deg. F.
Max. temperature in the last 24 hours: 19 deg. F.
Average wind direction during the last 24 hours: W
Average wind speed during the last 24 hours: 7 mph
Maximum wind gust in the last 24 hours: 10 mph
New snowfall in the last 24 hours: na inches
Total snow depth: na inches
weather

.SHORT TERM...Today through Monday...Mostly clear and cold this
morning (except for patchy valley fog) under dry northwesterly
flow aloft. Upper level ridge along the coast will pass through
our area this evening followed by a Pacific frontal system that
will bring light to moderate amounts of snow to all areas late
tonight through Sunday. Clouds will increase in Harney and Baker
Counties late today with snow beginning there this evening and
spreading east through the night. Lower valleys, including
the Treasure Valley, will receive up to an inch of snow Sunday
morning. Largest amounts, 3 to 6 inches, will occur in the
central Idaho mountains. A winter weather advisory has been
issued for the Upper Weiser River area late tonight through
Sunday for 1 to 3 inches of snow in that area. Warmer air will
change the snow to rain in southern areas Sunday afternoon. A
break in precipitation is expected Sunday night before another
Pacific storm brings more light rain and snow Monday.
Temperatures will rise slightly above normal Sunday and Monday,
with snow levels ranging from around 3000 feet MSL in Baker County
and the west central Idaho mountains, to 4500 feet MSL in Idaho
south of the Snake River, and in Oregon south of Burns to Vale.

.LONG TERM...Monday night through Saturday...Widespread rain and
snow will start the period under moist zonal flow aloft through
Monday night. Snow will be limited to elevations above 4000 feet
at first. A cooler northwest flow aloft sets up Tuesday evening
onward helping to drop snow levels down to valley floors.
Precipitation after Tuesday looks to stay confined to higher
elevations. However, timing and placement continue to be a
struggle among models leaving confidence in precipitation
forecast low for the end of the year.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Saturday Saturday Night Sunday
Weather: Patchy fog before 11am. Otherwise, increasing clouds, with a high near 25. Calm wind becoming south around 5 mph in the afternoon. Snow likely after 11pm. Mostly cloudy, with a low around 22. South wind 5 to 8 mph. Chance of precipitation is 60%. New snow accumulation of less than one inch possible. Snow. High near 30. South wind around 8 mph. Chance of precipitation is 80%. New snow accumulation of 1 to 2 inches possible.
Temperatures: 25 deg. F. 22 deg. F. 30 deg. F.
Wind direction: S S S
Wind speed: 5 5-8 8
Expected snowfall: 0 in. 1 in. 1-2 in.
Granite Mountain at 7700 feet.
  Saturday Saturday Night Sunday
Weather: Increasing clouds, with a high near 19. Light and variable wind becoming south 6 to 11 mph in the afternoon. Snow likely after 11pm. Mostly cloudy, with a low around 16. Breezy, with a south wind 11 to 16 mph increasing to 18 to 23 mph after midnight. Chance of precipitation is 70%. New snow accumulation of 1 to 3 inches possible. Snow. High near 21. Breezy, with a south wind 14 to 20 mph. Chance of precipitation is 80%. New snow accumulation of 2 to 4 inches possible.
Temperatures: 19 deg. F. 16 deg. F. 21 deg. F.
Wind direction: S S S
Wind speed: 6-11 18-23 14-20
Expected snowfall: 0 in. 1-3 in. 2-4 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.