THIS AVALANCHE ADVISORY EXPIRED ON December 22, 2018 @ 6:59 am
Avalanche Advisory published on December 21, 2018 @ 6:59 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 have also been building on leeward facing terrain over the last 3 days. 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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Our current avalanche hazard is a bit of a head scratcher.  Buried surface hoar and basal facets exist in a growing and strengthening snowpack.  Tuesday's storm added another 12-15 inches of snow on top of a house of cards.  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. 

Forecasters and savvy backcountry travelers know this problem as the sleeping dragon.  We know it is a serious concern and it is very reactive in snow tests, right now it hasn't produced widespread natural avalanches.  What we don't know  and what makes it tricky to forecast is when it is going to wake up into a serious problem or what the trigger will be.   If triggered this layer has the potential to spread out or propagate over large distances. 

The layer of basal facets is still present throughout our snowpack as well, it is buried deeply enough in most places that a skier or snowmobiler is going to have a hard time triggering it.  It is also starting to warm up slightly and become less sugary BUT in the right area, especially shallow rocky areas you could still trigger an avalanche that would break near the ground.  There is also a possibility of triggering the more shallow buried surface hoar and having it step down into the basal facets resulting in a large and dense and very un-survivable avalanche.

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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3 days of moderate and strong winds have produced wind slabs on multiple aspects as winds have shifted from S to SW and W.  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.

recent observations

We were able to trigger the buried surface hoar easily in our test pits yesterday.  See the video above in the persistent slab discussion.  We also had reports and saw the remnants of a natural avalanche that occurred earlier in the week.  This natural cycle was not widespread so that indicates that the buried surface hoar layer is still waiting for a trigger in many areas.

We also observed widespread wind transport throughout the leeward terrain.   The photo below shows the buried surface hoar layer marked by the snow saw at 80cm/31 inches down that was very reactive in our tests.

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 16 deg. F.
Max. temperature in the last 24 hours: 26 deg. F.
Average wind direction during the last 24 hours: SSW
Average wind speed during the last 24 hours: 16 mph
Maximum wind gust in the last 24 hours: 26 mph
New snowfall in the last 24 hours: na inches
Total snow depth: na inches
weather

SHORT TERM...Today through Sunday...The quick moving cold front
continues to progress through Idaho this morning. Snowfall and
rain showers were left in its wake, along with periods of gusty
wind. Patchy valley fog will be possible through the morning
hours. The front should generally be east of the area by this
afternoon. Periods of precipitation will be possible near the
ID/NV border through this evening as a follow-up disturbance and
its associated moisture move through the area. A dry northwest
flow will be found Saturday morning, with dry conditions expected
through Saturday evening. A fairly potent Pacific system will then
begin to move into eastern Oregon Saturday evening, and continue
into Idaho overnight. Precipitation will be possible across the
entirety of the forecast area on Sunday, with the mountains
remaining the most favorable location to see potentially
significant snowfall. Stay tuned for updates regarding this
upcoming system, especially those traveling for the holidays.

.LONG TERM...Sunday night through Thursday...A wet zonal flow begins
our extended forecast period bringing widespread rain and snow. Snow
levels hover around 3000 to 4500 ft Monday and Tuesday meaning lower
valleys will see all rain. Tuesday morning a northwest flow sets up
which helps lower snow levels down to valley floors. The remainder
of the period stays active, however confidence in timing and
placement is still less than desirable. Precipitation that does fall
Wednesday onward will be in the form of snow for all locations.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Friday Friday Night Saturday
Weather: A 50 percent chance of snow before 11am. Patchy fog before 11am. Otherwise, partly sunny, with a high near 27. Southwest wind 5 to 8 mph. Total daytime snow accumulation of less than a half inch possible. Mostly cloudy, with a low around 12. Calm wind. Patchy fog before 11am. Otherwise, mostly sunny, with a high near 25. Light south wind
Temperatures: 27 deg. F. 12 deg. F. 25 deg. F.
Wind direction: SW Calm S
Wind speed: 5-8 Calm Light
Expected snowfall: Trace in. 0 in. 0 in.
Granite Mountain at 7700 feet.
  Friday Friday Night Saturday
Weather: A 50 percent chance of snow, mainly before 11am. Partly sunny, with a high near 18. Southwest wind 6 to 11 mph. Total daytime snow accumulation of around an inch possible. Mostly cloudy, with a low around 11. South southwest wind 3 to 5 mph. Mostly sunny, with a high near 19. South southwest wind 3 to 8 mph.
Temperatures: 18 deg. F. 11 deg. F. 19 deg. F.
Wind direction: SW S S
Wind speed: 6-11 3-5 3-8
Expected snowfall: Trace in. 0 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.