THIS AVALANCHE ADVISORY EXPIRED ON December 18, 2018 @ 6:27 am
Avalanche Advisory published on December 17, 2018 @ 6:27 am
Issued by George Halcom - Payette Avalanche Center
bottom line

 Use caution in upper elevation areas today. Northerly slopes contain deeper snowpacks and multiple weak layers including buried surface hoar a foot from the surface and basal facets creating a VERY weak snowpack in some areas.   New windslabs formed in the last 48 hours on NW, N and NE slopes.  With multiple weak layers, widespread variability and newly created wind slabs, avalanche season has begun and may be on the rise this week given more snow load.  

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 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 since Friday 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 in the upper foot: buried surface hoar, which showed moderate potential for human triggering and propagation. It failed moderately in compression, and in an ECT... as we put more snow on this weak layer, we may see conditions get touchy, and even create a natural cycle mid week...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.  

 

Avalanche Problem 2: Wind Slab
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Friday afternoon a low pressure system entered the area producing high winds.  By early evening, ridgetop winds were out of the SE and SW gusting into the low 40 mph range.  With plenty of low density snow available for transport, wind slabs will be a problem on or near NW, NE and N facing slopes and possible on E and W facing cross loaded slopes. 

More snow and wind over the next 48 hours will create some new wind slab hazard in lee terrain...keep an eye out for wind sculpted snow surfaces that indicate where loading has occurred.

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!

recent observations

Friday saw multiple older crowns on steep Northerly terrain. These were leftovers from the storm last week.  most of them were relatively shallow but partially filled in with new snow, and yesterday all of the evidence had disappeared from all of the wind transport.  Hand pits and stability tests showed easy to moderate failures in the upper buried surface hoar layer that was visible to the naked eye about a foot down. The air temperature has been on the rise over the last week, and has changed the snow pack drastically: it's doing some packing slowly, and we hope that colder temperatures will help. 

We have seen some folks in the backcountry that have broken A arms on their sleds, and I can't believe where some people are riding with such little low density snow that allows you to go to the ground...give it a little more time, even skiing is a high stakes scary experience in most all places.

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 26 deg. F.
Max. temperature in the last 24 hours: 28 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: 26 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: na inches
weather

 

Area Forecast Discussion
National Weather Service Boise ID
312 AM MST Mon Dec 17 2018

.SHORT TERM...Today through Wednesday...Active conditions
continue. Mountain snow and valley rain observed across the area
this morning, which will continue to progress eastward today.
This initial system will move out of the area by this evening,
with the second system close behind. Snowfall will return to
eastern Oregon late tonight, with activity spreading east through
Tuesday. The best snowfall looks to fall across the West Central
Mountains of Idaho, though lighter amounts expected across the
remaining mountain ranges, with rain or a rain-snow mix over the
valleys. This second system departs late Tuesday with a ridge
anticipated for Wednesday. Lingering moisture may result in a few
mountain showers for Baker County Oregon and the West Central
Mountains in Idaho for Wednesday, with otherwise dry conditions.
Temperatures will generally remain 5 to 10 degrees above seasonal
normals through the period.

.LONG TERM...Wednesday night through Sunday...Conditions briefly dry
out Thursday under a mid-level ridge as temperatures remain roughly
5-8 degrees above normal. Winds pick up ahead of an incoming cold
front with a tight pressure gradient arriving on Friday. With snow
levels between 3000-5000 ft MSL, this system will usher in mixed
precip across the area with the highest chance in the West Central
Idaho Mountains. Strong winds will persist through this event and
will weaken as the system progresses out of the region on Saturday.
Temperatures and snow levels will lower sharply behind the front.
Into Sunday, unsettled flow continues as widespread snow showers
move into the region which will persist through the beginning of
next week.

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.AVIATION...Mostly VFR with MVFR/IFR within precipitation.
Widespread rain and snow showers will continue to move northeastward
across the region through the afternoon. Snow levels: 4500-5500 ft
MSL lowering to 3500-4000 ft after 18/06z. Surface winds: south-
southeast 10-15kts with gusts in and around precipitation of 25-
35kts. Winds aloft to 10k feet MSL: southwest 35-45kts through
17/18z, then 20-25kts.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Monday Monday Night Tuesday
Weather: Rain and snow, mainly before 11am. High near 35. South wind 3 to 6 mph. Chance of precipitation is 90%. Total daytime snow accumulation of less than a half inch possible. A slight chance of rain and snow before 11pm, then snow likely. Cloudy, with a low around 28. South wind 5 to 9 mph. Chance of precipitation is 70%. New snow accumulation of less than one inch possible. Snow before 11am, then rain and snow. High near 34. South wind around 11 mph. Chance of precipitation is 100%. New snow accumulation of 2 to 4 inches possible.
Temperatures: 35 deg. F. 28 deg. F. 34 deg. F.
Wind direction: S S S
Wind speed: 3-6 5-9 11
Expected snowfall: less than one half in. Less than one in. 2-4 in.
Granite Mountain at 7700 feet.
  Monday Monday Night Tuesday
Weather: Snow. High near 27. South southwest wind 9 to 16 mph. Chance of precipitation is 90%. Total daytime snow accumulation of 1 to 3 inches possible. Snow, mainly after 11pm. Low around 22. South wind 9 to 18 mph. Chance of precipitation is 80%. New snow accumulation of 1 to 3 inches possible. Snow. High near 28. Breezy, with a south wind 20 to 22 mph. Chance of precipitation is 100%. New snow accumulation of 3 to 7 inches possible.
Temperatures: 27 deg. F. 22 deg. F. 28 deg. F.
Wind direction: SSW S S
Wind speed: 9-16 9-18 20-22
Expected snowfall: 1-3 in. 1-3 in. 3-7 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.