Avalanche Advisory published on January 27, 2018 @ 6:02 am
Issued by Dave Bingaman - Payette Avalanche Center
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The Avalanche Hazard is Moderate today above 6000 feet where new snow and gusty winds have created new windslabs near ridges and on exposed terrain.  In addition, local mountains  have received 10 and 15 inches of new snow over the last week, watch for shallow instabilities on steeper slopes in the layers created this week.   Pay attention to changing conditions with 6-12 inches forecasted in the upper elevations over the next 24 hours.

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

This week produced another round of snow with accumulations in the 10-15 inch range above 6500 feet.  Winds have been gusty throughout the week and mostly out of the S and SW.  North and South Valley areas have both seen the same weather this week with cornices slowly growing and wind loading occurring on mostly E, NE, N and NW  facing slopes.  We observed active wind loading and scouring on a SW aspect just south of Granite Mountain yesterday with loading occurring on the NE throughout the day.  You should expect to see and anticipate reactive wind slabs on upper elevation leeward terrain today.  These slabs will continue to build through the day today with and additional 3-7 inches of new snow, increasing temperatures and winds from the S around 17 mph.


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

The snow that fell throughout the week this week came in small increments of light density snow that did not have a major effect on stability.  Loose, unconsolidated snow has been the trend for over a week creating a soft and unconsolidated upper snowpack with a firm and strong slab below it.   In our pit tests yesterday we saw several of these individual storm layers that were only partially bonded to the layers below creating moderate failures in compression but lacking propagation or the energy to spread out over large areas.  It is possible that you could trigger a weakness in one of these layers on steep terrain resulting in a shallow slab in the 6 to 12 inch range today.  Worth noting and watching over the next 24 hours is a subtle crust created by a freezing mist event that occurred mid morning on Monday.  It is buried between 6 and 12 inches down in the snowpack.  This was one of the layers that is failing in compression and may become more reactive as we add more snow to it over the next 24 hours.   We found this layer to be fairly widespread yesterday and Thursday in the mid to upper elevations and had reports of it from the South Valley area as well.

See the photos below for a comparison of the difference in the snowpack on a SW facing slope at 7100 feet and a NNE slope at 7600 feet. Notice the layering in the SW pit and the depth of the freezing mist crust and new snow between the two pits.


recent observations

No new avalanche activitiy has been reported or observed.

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CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 14 deg. F.
Max. temperature in the last 24 hours: 15 deg. F.
Average wind direction during the last 24 hours: WSW
Average wind speed during the last 24 hours: 10 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

.SHORT TERM...Today through Sunday...Warm-frontal snow event is
about to begin. A significant moisture plume is moving onto the
coast as the warm front develops and heads our way. The NAM is
still an outlier with high QPF values, but as all other models
continue to trend down with QPF, have lowered snow amounts again
in this forecast. I am concerned that the NAM could end up
verifying best based on high PW values arriving later this
morning, but decided to go with the model consensus. Temps will be
tricky as always in these types of systems, with precip holding
temps down initially, and then the warm frontal influence trying
to increase them later. Have again gone below guidance for most
areas, with little change from previous forecast. Guidance agrees
that a weak impulse will follow the warm front this evening,
producing another, though much lighter, precip event. Snow levels
for this will range from 4000 feet in the north to 6500 feet in
the south, so this will be rain for most folks. Sunday will be
warmer, with highs near 10 degrees above normal. Precip will be
moving off to the northeast, with lingering showers mainly in the
mountains of the north.

No changes to locations/timing of the winter weather warnings and
advisories, but will issue an update to indicate slightly lower
snow accumulations.

.LONG TERM...Sunday night through Saturday...A pattern change
will bring cooling starting Wednesday as the upper level flow
shifts to northwest. An upper level high pressure ridge will keep
our area dry Sunday night and Monday. Moist westerly flow will
replace the ridge Monday night, spreading mountain snow and valley
rain into Baker County Oregon and the west central Idaho
mountains, reaching as far south as Burns and Boise on Tuesday.
Tuesday night and Wednesday a high pressure ridge building off the
coast will shift the upper level flow into the northwest over the
Intermountain Region. Precipitation will be mainly confined to
our northern mountains, and temperatures will be around 10 degrees
cooler at all locations. This pattern will persist through
Saturday, but precipitation coverage will become more uncertain by
the weekend as models diverge.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Saturday Saturday Night Sunday
Weather: Snow. High near 30. South wind around 8 mph. Chance of precipitation is 100%. Total daytime snow accumulation of 2 to 4 inches possible Snow. Low around 29. South southwest wind 3 to 7 mph. Chance of precipitation is 90%. New snow accumulation of 1 to 3 inches possible. A chance of snow before 11am, then a chance of rain and snow. Cloudy, with a high near 36. South southeast wind 3 to 5 mph. Chance of precipitation is 40%. New snow accumulation of less than a half inch possible.
Temperatures: 30 deg. F. 29 deg. F. 36 deg. F.
Wind direction: S SSW SSE
Wind speed: 8 3-7 3-5
Expected snowfall: 2-4 in. 1-3 in. Trace in.
Granite Mountain at 7700 feet.
  Saturday Saturday Night Sunday
Weather: Snow. High near 24. Wind chill values between -3 and 7. South wind 14 to 17 mph. Chance of precipitation is 100%. Total daytime snow accumulation of 3 to 7 inches possible. Snow. Low around 23. Southwest wind 9 to 13 mph. Chance of precipitation is 100%. New snow accumulation of 3 to 5 inches possible. Snow likely, mainly before 11am. Cloudy, with a high near 32. South southwest wind 8 to 10 mph. Chance of precipitation is 70%. New snow accumulation of less than one inch possible.
Temperatures: 24 deg. F. 23 deg. F. deg. F.
Wind direction: S SW
Wind speed: 14-17 9-13
Expected snowfall: 3-7 in. 3-5 in. Trace 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 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.