THIS AVALANCHE ADVISORY EXPIRED ON December 31, 2019 @ 7:15 am
Avalanche Advisory published on December 30, 2019 @ 7:15 am
Issued by George Halcom - Payette Avalanche Center
bottom line

Human triggered avalanches remain possible on buried Surface Hoar 1-2 feet deep, and where the snow is shallow and rocky basal facets may be reactive. Shallow wind slabs 2-6 inches are not bonded, and are failing under the weight of a person. You are most likely to trigger avalanches in steep, wind loaded, or wind protected slopes where surface hoar is still intact. Your decision making should reflect the conditions.

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
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
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    Very Large
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Our persistent weak layers are doing exactly what they say. They are persisting, or still preserved in the mid and ground level of our snowpack. We have a layer of buried surface hoar about 1-2 feet down that is our primary concern as it is still propigating in our snowpit tests Sunday. 

The other persistent problem is our basal facets, or sugar snow near the ground that can be anywhere from 6-12 inches sitting on rocks. This is a poor base and is faiing in shallow rocky areas with a stronger layer above it consisting of crusts and denser snow.

The development of a layer of faceted snow sitting above an old crust about 30cm/12in above the ground is worth noting,  it is becoming more reactive and showing a potential to fail where the crust is still intact and the overall depth of the snowpack is shallow.

It is easy to get complacent about a persistent avalanche problem but it should be in the back of your mind as you travel right now,  The presence of these layers is enough to keep most seasoned backcountry travelers on lower angle slopes and away from committing terrain.  Persistent weak layers are unpredictable based on variations in the terrain and snowpack, they can also fail in unpredictable ways across a slope sometimes weeks after being buried.   

 

 

Avalanche Problem 2: Wind Slab
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  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
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  • Size ?
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The 2-6 inch thick wind slabs that we saw Sunday were cracking under our skis while climbing an East face of Bruin. A quick pit revealed very poor bonding with facets in between. Most of the wind slab problem is gone, but steep wind effected areas could fail if you were to jump on the right spot.

advisory discussion

FPAC is hosting a bunch of avalanche awareness classes in January including a women's only class.  Check out our Education page here.

It is easy to get complacent about a persistent avalanche problem but it should be in the back of your mind as you travel right now,  The presence of these layers is enough to keep most seasoned backcountry travelers on lower angle slopes and away from committing terrain.  Persistent weak layers are unpredictable based on variations in the terrain and snowpack, they can also fail in unpredictable ways across a slope sometimes weeks after being buried.   

recent observations

Yesterday, out ski touring Bruin Mtn, North of McCall, near Fisher Creek Saddle, we had a layer of buried surface hoar about 1-2 feet down that is our primary concern as it is still propagating in our snowpit tests. We also had some shallow wind slabs cracking and woomphing under our skis while climbing an East trending ENE face. The bond between the stiff, shallow wind slabs were poor, and failed on isolation in our tests.

Extended column test faied on buried surface hoar on 21 taps propigating across the block.

Yesterday, we got a report with no details of an Avalanche Above Cascade near Rock Lake. It would be nice if someone could submit an observation.

Saturday, in the Sawtooths, people were triggering a buried surface hoar layer remotely, or from a distance.

 

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 18 deg. F.
Max. temperature in the last 24 hours: 24 deg. F.
Average wind direction during the last 24 hours: E
Average wind speed during the last 24 hours: 4 mph
Maximum wind gust in the last 24 hours: 18 mph
New snowfall in the last 24 hours: NA inches
Total snow depth: NA inches
weather

Area Forecast Discussion
National Weather Service Boise ID
343 AM MST Mon Dec 30 2019

.SHORT TERM...Today through Thursday...A backdoor cold front from
the east will keep snow flurries going across most of the area
through this afternoon along with mostly cloudy skies and patchy
fog. Snow flurries should taper off this afternoon. Upper level
ridging builds into the region on Monday and Tuesday with dry
conditions expected. A more significant system will arrive on
Tuesday night, bringing moderate snow accumulations to the
mountains through Wednesday. Expect 8-12" of snow across the
mountains above 5000ft during a 36 hour period from Tuesday night
through Thursday morning. Snow levels will start at the valley
floors and rise to around 4000-5500ft in Idaho and Baker County,
OR and up to 6500ft in southeast Oregon on Wednesday. Temperatures
in the Snake Basin and Harney/Malheur Counties will reach the mid
40s ahead of the cold front on Wednesday. The cold front will
move through the region on Wednesday evening lowering snow levels
down to 2500-3500ft by Thursday morning. Snow showers will mainly
be focused on the higher terrain during this time, so not
expecting any significant accumulations in the valleys. Snow
showers will generally taper off late Thursday afternoon.
Temperatures will remain above normal through the period, with the
exception of today, where temperatures are near normal under the
low clouds and fog.

.LONG TERM...Thursday night through Monday...Northwest flow aloft
will bring the potential for precipitation through the bulk of the
period, favoring the higher elevations in the north. Snow levels
will be around 4000 feet on Friday, lowering to valley floors by
Sunday. Temperatures will be above normal through the weekend,
cooling to near normal on Monday.

&&

.AVIATION...Mountains obscured. Areas of MVFR and local IFR
conditions in snow flurries and patchy fog, mainly through 18Z.
Surface winds variable less than 10 kts. Winds aloft at 10K ft MSL
northwest 15-30 kt

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Monday Monday Night Tuesday
Weather: Patchy fog. Otherwise, partly sunny, with a high near 29. Calm wind. Patchy fog. Otherwise, mostly cloudy, with a low around 20. Light and variable wind. A 50 percent chance of snow showers after noon. Cloudy, with a high near 28. Light south southeast wind. New snow accumulation of less than a half inch possible.
Temperatures: 29 deg. F. 20 deg. F. 28 deg. F.
Wind direction: Calm Light Light
Wind speed: Calm Varible SSE
Expected snowfall: 0 in. 0 in. Less than one half in.
Granite Mountain at 7700 feet.
  Monday Monday Night Tuesday
Weather: Patchy fog. Otherwise, partly sunny, with a high near 23. North wind 5 to 8 mph becoming calm in the morning. Patchy fog. Otherwise, mostly cloudy, with a low around 18. Calm wind becoming south southeast around 5 mph. Snow showers likely, mainly after 11am. Cloudy, with a high near 22. South southwest wind 7 to 9 mph. Chance of precipitation is 60%. New snow accumulation of 1 to 2 inches possible.
Temperatures: 23 deg. F. 18 deg. F. 22 deg. F.
Wind direction: N SSE SSW
Wind speed: 5-8 5 7-9
Expected snowfall: 0 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.