THIS AVALANCHE ADVISORY EXPIRED ON December 20, 2018 @ 6:29 am
Avalanche Advisory published on December 19, 2018 @ 6:29 am
Issued by George Halcom - Payette Avalanche Center
bottom line

The Avalanche danger is Considerable. Use caution and good travel protocols near slopes above 30 degrees! The new snow has created a sensitive storm slab on all aspects. There are still multiple weak layers including buried surface hoar, and basal facets creating a VERY weak snow pack in some areas.  Strong South winds created Wind slabs on NW, N and NE slopes.  

How to read the advisory

  • Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Storm Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Very Likely
  • Size ?
    Very Large

We like it when it snows more than forecasted, but the snow pack doesn't feel the same. Around a foot of new snow that was fairly warm and denser than the snow below it  came in with some winds that reached 50 MPH at the top of Tamarack yesterday creating wind slabs on lee slopes, and a storm slab on all aspects and elevations. If you trigger a storm slab you may be dealing with a 2 foot deep slab, or it could entrain enough snow and step down to the ground creating a very deep avalanche. 

Skiers in the Lick Cr Canyon observed a storm slab that ran naturally on a West aspect yesterday, and patrollers at Tamarack were able to get snow to move near the ride tops. The weight of the new snow is significant, and may be too much for the snow pack along with the weight of a skier or rider? Time will likely make the storm slab problem go away as the new snow will eventually bond with the old snow.  Use good travel protocols, and be very cautious near slopes above 30 degrees!

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

Our snow pack contains multiple persistent weak layers right now.  Buried surface hoar is fairly widespread throughout the advisory area on many aspects where the terrain protected it from being destroyed by the wind, and has been reactive in our stability tests.  There are some density changes within the snow pack and a thick layer of basal facets that have created the opportunity for shallow or deep avalanches to be possible in some areas. Travel cautiously not only because of the weak layers, but because of the early season shallow snow pack that is hiding things for sledders and skiers to run into. Select your routes with a very critical mindset especially on shaded, north facing aspects that have been accumulating snow since mid November. Tests have shown some improvement of developing a stronger snow pack near the ground eventually, but it's still weak, and we have a weak layer of great concern about two feet down now: buried surface hoar, which has shown potential for human triggering and propagation. Caution is the name of the game right now, the deepest (and best skiing and riding) areas are exactly where the hazard is highest right now.  Lower angle, East and West aspects are the place to be right now, the snow pack is more dense and will allow you to travel higher in the snow pack even though there is less snow.  


advisory discussion

Please submit your observations to the PAC.  It's easy, just click on the observations tab, and you don't need to worry about filling in every blank.  Fill out what you know and add a picture if you have them.  Watch your bases and A arms our there!

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 22 deg. F.
Max. temperature in the last 24 hours: 25 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 20 mph
Maximum wind gust in the last 24 hours: 24 mph
New snowfall in the last 24 hours: 12 inches
Total snow depth: na inches



Area Forecast Discussion
National Weather Service Boise ID
258 AM MST Wed Dec 19 2018

.SHORT TERM...Today through Friday...Radar continues to show a few
showers over the Western Magic Valley and over the West Central
and Boise Mountains this morning as the most recent system
continues to head east. Lingering moisture will keep a few
mountain showers across the area into the afternoon hours, though
any snowfall accumulation should be insignificant. A developing
ridge will produce generally dry conditions by this evening, which
will continue into Thursday afternoon. Models continue to slow
down the arrival of the next trough, with showers now looking to
move into eastern Oregon on Thursday evening. This trough will
move through the area Thursday night and into Friday, with much of
the area seeing the potential for some shower activity. Snow
levels will keep most activity as rain across the valleys, though
a rain-snow mix will be possible Friday morning as the system
departs. Temperatures will cool through the forecast period, but
will remain at least a few degrees above normal.

.LONG TERM...Friday night through Tuesday...A Pacific system
clears out of the region Friday night, ushering in colder
temperatures under drier northwest flow aloft. Another north
Pacific storm approaches on Sunday, bringing in widespread snow
showers and windy conditions. Unsettled flow continues as a
stronger and colder wave moves through the region on Monday,
potentially bringing in significant snow. PoPs during both waves
were raised to reflect the NBM, GFS, and ECMWF, which all show
high confidence of precip Sunday and Monday. A chance of snow
showers lingers across the region into Wednesday as a mid-level
trough deepens.


.AVIATION...VFR. Surface winds: light and variable across most of
the region, with west 10-15 kts/gusts up to 25 kts in the Magic
Valley through 19/18z. Winds aloft to 10k feet MSL: west-southwest
30-40 kts.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Wednesday Wednesday Night Thursday
Weather: A 20 percent chance of snow. Cloudy, with a high near 34. South southwest wind around 5 mph. Mostly cloudy, with a low around 22. Light southeast wind. Partly sunny, with a high near 36. South wind 3 to 7 mph.
Temperatures: 34 deg. F. 22 deg. F. 36 deg. F.
Wind direction: SSW SE S
Wind speed: 5 Light 3-7
Expected snowfall: 0 in. 0 in. 0 in.
Granite Mountain at 7700 feet.
  Wednesday Wednesday Night Thursday
Weather: A 40 percent chance of snow. Cloudy, with a high near 25. South wind around 9 mph. Total daytime snow accumulation of less than one inch possible. Mostly cloudy, with a low around 21. South wind around 10 mph. Partly sunny, with a high near 31. South wind 11 to 17 mph.
Temperatures: 25 deg. F. 21 deg. F. 31 deg. F.
Wind direction: S S S
Wind speed: 9 10 11-17
Expected snowfall: Less than one in. 0 in. 0 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.