THIS AVALANCHE ADVISORY EXPIRED ON December 26, 2019 @ 7:46 am
Avalanche Advisory published on December 25, 2019 @ 7:46 am
Issued by George Halcom - Payette Avalanche Center
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Human triggered avalanches remain possible in wind loaded terrain, and wind protected terrain.  This week, Skiers have been able to trigger avalanches on multiple aspects, and we are finding persistent weak layers on all aspects.  The snow pack is shallow and rocky still, and complex...Make conservative choices in your route finding and avoid steep wind loaded terrain and protected terrain.

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 major concern right now is the persistence of a layer of buried surface hoar in our local snow pack. This is a dangerous weak layer that is hard to predict, and is responsible for the majority of avalanche accidents...Over the last few days, the snow pack has begun to consolidate and heal this layer somewhat, but both the surface hoar and the deeper basal facet/depth hoar layers are still intact and are still reactive.

Without new snow in the future and colder temps, the snow pack is going to struggle to gain strength anytime soon.  In addition, the surface hoar layer has been further stressed with the wind loading that occurred late last week during the Wednesday/Thursday storm.  In some wind loaded areas, the scales tipped and the surface hoar failed during the storm or as it was further loaded Friday.   In areas that did not fail naturally (like areas protected from the wind) , this layer remains sensitive enough that a skier or snowmobiler could trigger an avalanche.   

I would consider the snow pack a WEAK moderate on the danger scale right now.  Stability is generally improving but structure is still variable and there are areas where you could trigger an avalanche in a wind slab or wind loaded area but the real problem is the ability of that avalanche to initiate and  propagate in the surface hoar and create a larger avalanche.  

Avalanche Problem 2: Wind Slab
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The wind slab avalanche problem should be healing, but keep in mind that the wind has created deeper slabs on top of persistent weak layers. Scoured and wind sculpted windward slopes are the norm with some stiff, challenging snow to be found right now while the leeward slopes are still preserving good snow.  Close to the ridge tops, cornices have started to grow but stiff wind pillows grew quite a bit in some areas. 

We have had multiple reports of human and naturally caused avalanches in these areas over the last 3 days and the problem is compounded by the persistence of the weak layers below the recently deposited snow.  

recent observations

 

Yesterday, we went out North of Brundage Mtn on the ridge to Sgts in order to look at a skier triggered slide that was reported on 12/23. The slide was big, 300 feet by 300 feet (R3,D2-3). The kind that could have killed or injured the skier, but they triggered the slide in the shallowest spot near the flank which only buried them up to their knees. If they had dropped in a few feet further south, they may have been buried deeper and or bounced through trees and rocks.  The steep rocky start zones are variable in depth (overall shallow and on ROCKS) and are harboring some of the harder to predict or forecast layers that are common in fatalities, even in our latest fatality at Twin lakes a couple of years ago. 

It is time to be careful in the mountains. The weak layers are going to need some time and different weather events to heal, which may take more time. Stick to lower angle terrain, not steep rocky terrain where the snow pack is the weakest.

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 11 deg. F.
Max. temperature in the last 24 hours: 14 deg. F.
Average wind direction during the last 24 hours: SE
Average wind speed during the last 24 hours: 3 mph
Maximum wind gust in the last 24 hours: 11 mph
New snowfall in the last 24 hours: NA inches
Total snow depth: NA inches
weather

Area Forecast Discussion
National Weather Service Boise ID
236 AM MST Wed Dec 25 2019

.SHORT TERM...Today through Saturday...Fog and low clouds have
formed from Baker County through northeastern Malheur County,
and into the Treasure Valley near the Snake River. A separate
area has formed in the Magic Valley, and patchy fog has also
been noted in the Long Valley. These conditions will likely
continue through the morning hours and repeat tonight and
early Thursday morning. Isolated snow showers are also
forecast today and tonight near the Nevada border, closest
to the main upper low well to our south in California. As
the low tracks east across Arizona Thursday a short wave
trough will approach our area from the Gulf of Alaska. This
trough will bring in colder air Friday and Saturday as flow
aloft becomes more northerly and increases. The trough will
also bring a chance of snow showers to northern mountains,
but the rest of our area will stay dry. Air stagnation is
not yet a concern but it may become one when the colder air
becomes established in the valleys Friday night and Saturday.

.LONG TERM...Saturday night through Wednesday...Another upper-
level trough approaches the area late Saturday Night. The
deterministic runs differ quite a bit with the GFS splitting the
system off farther west, and heading to California, while the
ECMWF is faster and less split. The ensembles appear to favor
the GFS solution, so have leaned in that direction with timing
of precip chances for Sunday afternoon and then tapering off on
Monday. Temperatures would still be cold enough to support snow
down to the lowest valleys on Sunday, so this storm bears
watching - but with it splitting and heading south it does not
currently look like a major event. After that, there is weak
ridging later Monday, but the west or northwesterly flow looks
like it will bring another storm system early next week. The
ensembles have a wide range here - some having precip starting
Tuesday, others holding off till Wednesday - so having chance
PoPs throughout Tuesday/Wednesday looks like a reasonable
compromise for now. Again, this would likely start out as snow,
but ensembles are trending slightly warmer by Wednesday and
could change to rain in lowest valleys.

&&

.AVIATION...Developing fog/stratus layer with MVFR/IFR in Snake
Plain and most other mountain valleys likely to persist thru much
of the day with light winds and high clouds from system farther
south limiting sunshine. But some sites - especially KBNO, KBOI,
KTWF - should be VFR at times as local drainage winds push
stratus around. Surface winds variable less than 10 kts. Winds
aloft at 10k ft MSL generally west 5-10 kts.

 

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Wednesday Wednesday Night Thursday
Weather: Areas of fog before 11am. Otherwise, mostly sunny, with a high near 27. Calm wind becoming south southwest around 5 mph in the afternoon. Areas of fog after 11pm. Otherwise, partly cloudy, with a low around 12. Light and variable wind. Areas of fog before 11am. Otherwise, mostly sunny, with a high near 25. Light and variable wind.
Temperatures: 27 deg. F. 12 deg. F. 25 deg. F.
Wind direction: SSW Light Light
Wind speed: 5 Variable Variable
Expected snowfall: 0 in. 0 in. 0 in.
Granite Mountain at 7700 feet.
  Wednesday Wednesday Night Thursday
Weather: Areas of fog before 11am. Otherwise, mostly sunny, with a high near 16. South southeast wind 3 to 5 mph. Areas of fog after 11pm. Otherwise, partly cloudy, with a low around 6. North northeast wind around 5 mph becoming calm after midnight. Areas of fog before 11am. Otherwise, mostly sunny, with a high near 15. Calm wind becoming south around 6 mph in the afternoon.
Temperatures: 16 deg. F. 6 deg. F. 15 deg. F.
Wind direction: SSW NNE SSW
Wind speed: 3-5 5-calm calm-6
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.