THIS AVALANCHE ADVISORY EXPIRED ON December 21, 2019 @ 6:34 am
Avalanche Advisory published on December 20, 2019 @ 6:34 am
Issued by Dave Bingaman - Payette Avalanche Center
bottom line

Human triggered avalanches are likely in wind affected areas. Triggering a shallow wind slab could result in a deeper avalanche that is able to step down into the layer of buried surface hoar or to the rotten facets near the ground. Avoid wind affected terrain or slopes steep enough to slide.   Lower elevations still have a shallow, early season snowpack with a different set of hazards.

How to read the advisory


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

Winds picked up during the last 24 hours with gusts hitting 26 mph.  Wind slabs and wind loaded slopes should be anticipated and avoided today.  Look for new windslabs near ridgetops on on micro terrain features like gullies or other areas that can collect wind blown snow.  The possibility of a shallow wind slab stepping down into the layer of buried surface hoar mid pack is also likely, if you trigger a shallow wind slab it could turn into a much bigger problem very quickly.  The prime northerly slopes that are harboring the persistent weak layers are also the slopes that are most likely to have freshly deposited wind slabs today.  Obvious signs of windslabs are recent drifting, wind deposited pillows or a sudden change in the consistency of the snowpack from soft to firm.  Shooting cracks produced by your skis or sled are also a sure sign you are on a windslab. 

Avalanche Problem 2: Persistent Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

This week's storm just added up to an inch of Snow Water Equivalent or SWE in 24 hours in some areas throughout the West Central.  Temperatures went up during the storm as well with warmer temperatures forecasted today.  Those factors added to the effects of the wind just added a significant load and strain for a shallow, early season snowpack, especially one harboring multiple persistent weak layers.  Buried surface hoar from early December is widely distributed throughout the area and can be found 12-18 inches down in the snowpack.  A layer of sugary depth hoar is also lingering near the ground on any slope that held early season snow.  Both of these layers are going to be closer to the tipping point with the added weight of new and wind blown snow and are more likely to produce human triggered avalanches over the next few days than they were prior to the storm.  Surface hoar problems are also much more likely to propagate over larger distances when they are widely distributed.  This means that you could remotely trigger a slide or that a slide you trigger could spread out quickly.  

 

 

advisory discussion

Thanks to the Friends of the Payette Avalanche Center for a great season opener last night.  Big thanks also to Broken Horn Brewery for hosting us and providing great food and drinks.  If you haven't checked it out already, see our FB page or the education page on the FPAC site for a list of all of the upcoming classes.  Get some knowledge and Know Before You Go.

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: 26 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 8 mph
Maximum wind gust in the last 24 hours: 23 mph
New snowfall in the last 24 hours: NA inches
Total snow depth: NA inches
weather

SHORT TERM...Today through Monday...Gusty winds noted across the
forecast area this morning, with a few locations in eastern Oregon
also reporting fog. Generally dry conditions expected this morning
through Sunday afternoon with mild temperatures. An approaching
trough on Sunday will spread showers across eastern Oregon on
Sunday night, with activity spreading into southwestern Idaho by
Monday morning. Most activity is currently anticipated to be
confined to the higher elevations, but conditions will continue to
be monitored as this upcoming system approaches.

.LONG TERM...Monday night through Thursday...Models are all fairly
similar in showing some sort of weak split trough over the region
through a majority of the week. Most of the energy that moves
towards the west coast remains well south of the forecast area. This
pattern would give the region a slight threat of snow showers each
day with little in the way of snow accumulations. Temperatures for
the period should be near or slightly above normal.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Friday Friday Night Saturday
Weather: A chance of rain and snow before 11am. Areas of fog. Otherwise, mostly cloudy, with a high near 37. South southeast wind 5 to 7 mph. Chance of precipitation is 30%. Little or no snow accumulation expected. Patchy fog after 11pm. Otherwise, mostly cloudy, with a low around 29. South southeast wind around 6 mph. Patchy fog before 11am. Otherwise, cloudy, with a high near 39. South southeast wind 5 to 8 mph.
Temperatures: 37 deg. F. 29 deg. F. 39 deg. F.
Wind direction: SSE SSE SSE
Wind speed: 5-7 6 5-8
Expected snowfall: Trace in. 0 in. 0 in.
Granite Mountain at 7700 feet.
  Friday Friday Night Saturday
Weather: A 40 percent chance of snow before 11am. Cloudy, with a high near 31. Breezy, with a south wind 22 to 24 mph. Total daytime snow accumulation of less than a half inch possible. Mostly cloudy, with a low around 26. Breezy, with a south wind 20 to 22 mph. Mostly cloudy, with a high near 33. Breezy, with a south wind 21 to 25 mph.
Temperatures: 31 deg. F. 26 deg. F. 33 deg. F.
Wind direction: S S S
Wind speed: 22-24 20-22 21-25
Expected snowfall: Trace 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.