THIS AVALANCHE ADVISORY EXPIRED ON December 22, 2019 @ 7:10 am
Avalanche Advisory published on December 21, 2019 @ 7:10 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
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Wind gusts remained in the mid 20's yesterday and are increasing again this morning. Wind transport was taking place throughout the day yesterday and the snow surface got progressively stiffer as we climbed up to 8000 ft near Secesh Summit.  Leeward slopes were getting pretty punchy and producing cracking and shallow windslabs up to 1ft deep.  With continued loading,  these slabs will be thicker today.   Wind slabs and wind loaded slopes should be anticipated and avoided today.  We also got a report of a natural or remote trigger avalanche in Wong's Bowl on a north facing slope that left behind a 1.5-2 ft crown.   Look for new windslabs near ridgetops on on the leeward side of 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.  Rising temperatures will also increase the potential for freshly loaded slopes to fail naturally. 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
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Wednesday's storm just added up to an inch of Snow Water Equivalent or SWE in 24 hours in some areas throughout the West Central Mountains. Temperatures went up during the storm as well with warmer temperatures forecasted again 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 and naturally 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. This was possibly the case with the Wong's slide yesterday as it appears to have happened when a group of snowmobilers approached the bowl.  

recent observations

Early season conditions still remain in the lower elevations.  A shallow snowpack and the lack of holdover early season snow has limited the amount of depth hoar that has grown and the warmup yesterday will have a good chance of destroying the buried surface hoar in the lower elevations.  Yesterday's warmup also created a lot of point releases and small wet slide action on steep areas along road cuts in the lower elevation.  Upper elevation conditions were still user friendly with slightly stiff powder in the protected areas and a fairly stout wind board forming on exposed areas.  The snowpack was slightly upside down with this stiff layer on top of some soft snow below but still provided good skiing and great powder turns on sleds.  Overall, wind transport seemed less than expected given the warmer snow and the wind's inability to blow it around too much.  There were 2 easily identifiable wind layers on leeward terrain that produced cracking and failed in our stability tests.  Expect steep wind loaded terrain to be even more sensitive today especially with another bump in temperatures today.

Check out this weekend update and overview that was published by our friends to the east at the Sawtooth Avalanche Center, great overview with similar snowpack problems...

https://www.youtube.com/watch?v=bBBayY55cHY

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 30 deg. F.
Max. temperature in the last 24 hours: 30 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 10 mph
Maximum wind gust in the last 24 hours: 25 mph
New snowfall in the last 24 hours: inches
Total snow depth: NA inches
weather

 

.SHORT TERM...Today through Tuesday...An approaching trough will
keep a tightened pressure gradient across the area today, allowing
gusty wind to continue. Dry conditions continue into Sunday as the
trough progresses slowly eastward. By late Sunday afternoon,
moisture begins to move into eastern Oregon, and spreads into the
central mountains of Idaho quickly. The trough splits as it moves
over the area on Monday, with the northern piece of the trough
keeping showers over the higher terrain through Monday afternoon.
The southern half of the flow starts to come more into play on
Tuesday as the system begins to lift across the forecast area.
Models currently showing a good amount of moisture across the area
by Tuesday afternoon. Snow levels will be at valley floors,
allowing some snowfall to be possible across the valleys when
temperatures are low enough. Continued warming temperatures
expected today, with a cooling trend then starting on Sunday under
the influence of the approaching trough.

.LONG TERM...Tuesday night through Saturday...Models remain in good
agreement in showing a split trough over the region with energy
going north and south of the forecast area. This leaves a threat of
light snow showers across the region through Thursday with little in
the way of accumulation expected. Models showing the flow aloft now
consolidates Friday and Saturday bringing much better threat of
snow, especially for the mountain areas, as a warm front enters the
PacNW. Temperatures are expected to remain near or slightly above
normal through the period.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Saturday Saturday Night Sunday
Weather: Patchy fog before 11am. Otherwise, mostly cloudy, with a high near 39. Light south southeast wind becoming south 5 to 9 mph in the morning. Mostly cloudy, with a low around 29. South southeast wind 3 to 7 mph. Partly sunny, with a high near 39. Light south wind.
Temperatures: 39 deg. F. 29 deg. F. 39 deg. F.
Wind direction: S SE S
Wind speed: Light 3-7 Light
Expected snowfall: 0 in. 0 in. 0 in.
Granite Mountain at 7700 feet.
  Saturday Saturday Night Sunday
Weather: Partly sunny, with a high near 34. Breezy, with a south wind 17 to 23 mph. Mostly cloudy, with a steady temperature around 31. Breezy, with a south wind 16 to 21 mph. Partly sunny, with a high near 32. South wind 11 to 14 mph.
Temperatures: 34 deg. F. 31 deg. F. 32 deg. F.
Wind direction: S S S
Wind speed: 17-23 16-21 11-14
Expected snowfall: 0 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.