THIS AVALANCHE ADVISORY EXPIRED ON December 30, 2017 @ 7:08 am
Avalanche Advisory published on December 29, 2017 @ 7:08 am
Issued by Dave Bingaman - Payette Avalanche Center
bottom line

Small wind slabs on upper elevation leeward slopes will remain our primary hazard today.  A warm Pacific storm system is forecasted to impact the West Central Mountains later today through Saturday and the avalanche hazard will rise as we add storm and wind slabs to the buried crust and weak layers above it.  As the new snow piles up, watch for obvious signs of instabilities in the snowpack such as cracking, whumphing or newly formed wind slabs near ridgetops.

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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Winds have affected the West Central Mountains over the last week, as a result, Northerly slopes are starting to gain depth quicker than their Southerly conterpoints.  Areas of wind impacted and drifted snow are visible throughout the northerly terrain in the form of rounded wind pillows and drifts near the ridgetops.  For the most part, what is out there right now lacks the energy or slab strength to be a large avalanche concern but isolated areas of thicker denser slabs exist and are currently resting on a weak layer of rounding facets overlying our new best friend the Thanksgiving rain crust.  This crust and facet combo is shaping up to be a long term problem that will likely evolve into a persistent slab problem after the warm storm that will impact the area later today and through Saturday.

Avalanche Problem 2: Storm Slab
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Unfortunately, we have not added much in the form of storm slab since last weekend's storm cycle but light dustings in the valley have added a few additional inches to the upper elevation snowpack.  Temperatures have kept the previous storm snow light and unconsolidated for the most part which has kept our avalanche hazard fairly low.  Yesterday was one of the warmest days we have had and today's storm will add some higher temperature snow above the cold snow below.  The impact of this storm has already been downgraded as it slips slightly to the North of our advisory area but it will still add up to 2 inches of Snow Water Equivalent to the upper elevations by tomorrow afternoon.  Expect the avalanche hazard to rise as this new storm slab adds depth to the existing snowpack and stresses the weak layers below it.  

advisory discussion

The PAC will only be operating 3 days per week this year.  Your observations are more important now than ever before, please let us know what you are seeing while you are out riding or skiing in the local backcountry.  It's super easy to send us info and photos with date, location, pictures, general or specific snow observations, just click on the submit observations page on the PAC website and add what you saw or found in the snow.  You can also email the forecasters directly at:  forecast@payetteavalanche.org

recent observations

We toured the ridges near Squaw Point yesterday and found plenty of light, unconsolidated snow on all aspects.  The snowpack showed a wide array of conditions and variability as we toured up South facing slopes and then checked out the high elevation northerly and east facing slopes.  The depth of the snowpack varies quite a bit with between 30 inches on mid elevation southerly slopes and just shy of 5 feet on wind loaded northerly slopes around 8000 feet. Our pit tests showed very clean shear planes above the Thanksgiving crust layer which ranged from a few inches thick to almost a foot thick in some areas, but these layers and the lack of a strong overlying slab showed no propagation potential on the rounding facets above the crust or an upper layer of facets that evolved during our last high pressure.  Variability means it is difficult to forecast conditions based on a handful of observations and while we have a fairly solid snowpack right now, the presence of weak layers on multiple slopes, wind impacted snow near the ridgetops and the absence of a cohesive slab on most slopes should make you stop and consider your terrain choices before committing to bigger, steeper lines over the next few days.  Watch out for areas of stiffer, denser snow on the leeward slopes where small wind slabs have already formed and pay attention to changes in the snowpack as the next storm enters our area today.  

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 25 deg. F.
Max. temperature in the last 24 hours: 27 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 6 mph
Maximum wind gust in the last 24 hours: 12 mph
New snowfall in the last 24 hours: .5 inches
Total snow depth: 60 inches
weather

SHORT TERM Boise NWS forecast...Today through Saturday...Snow showers continue
across the east-central Oregon and west-central Idaho mountains
this morning. Models have continued to trend the moisture
northward with the bulk falling to the north of the forecast
region. A southerly flow aloft continues to pour over the region,
bringing a continued threat of freezing rain across valleys where
cold-pooling has stayed in place. This means that the lower
Treasure Valley near Ontario, the Weiser Basin and to the
northwest towards Baker City, look to remain under the treat of
freezing rain through the late morning when surface temperatures
warm (generally below 5000 feet). Have kept the Winter Weather
Advisory going for snow in the West Central and Boise Mountains,
and for the threat of freezing rain in the Weiser Basin and Baker
County. The Advisory will be assessed again later this morning.
Saturday remains a difficult forecast as a cold front passes
quickly through the region. The front will bring precipitation
across the region, mainly before noon MST, with precipitation type
being the challenge. Current thinking is that cold-air will
continue to pool in low-lying valleys such as the Weiser Basin and
the Lower Treasure Valley (including Ontario), bringing another
threat of freezing rain to these areas. Otherwise, valley
locations will see a mixture of rain and snow, with a few inches
of snow falling across the mountains. The tallest peaks could
receive up to 3 to 5 inches.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Friday Friday Night Saturday
Weather: Snow before 11 am today Rain and snow mix Snow before 11 then clearing
Temperatures: 33 deg. F. 19 deg. F. 29 deg. F.
Wind direction: South South W/SW
Wind speed: 7 9 5-7
Expected snowfall: Trace in. Trace in. Trace in.
Granite Mountain at 7700 feet.
  Friday Friday Night Saturday
Weather: 2-4 inches of snow, 100% chance 3-7 inches of snow, 100% chance 1-2 inches of snow, 70% chance. Temp falling in the afternoon.
Temperatures: 32 deg. F. 21 deg. F. 25 deg. F.
Wind direction: W/SW SW W/SW
Wind speed: 14-16 18 10-15
Expected snowfall: 2-4 in. 3-7 in. 1-2 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.