Avalanche Advisory published on January 6, 2020 @ 7:04 am
Issued by George Halcom - Payette Avalanche Center
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Human triggered avalanches remain possible today. New snow and Southern winds will help increase the likely hood of triggering Wind Slabs near ridge tops. You may be able to trigger avalanches on a layer of buried surface hoar 18-24 inches down in the snow pack.  A deeper layer may be reactive to large triggers or triggers in shallow areas on a layer of facets near the November crust near the ground.  Avoid steep, wind loaded terrain, and steep, rocky terrain.

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 ?
    Very Likely
  • Size ?
    Very Large

The weather forecast looks good for us. We are lined up to get snow and wind over the next week along with some moderate to eventually strong winds. The wind and snow will be perfect for forming new wind slabs, and possibly overloading other slabs and weak layeyers possibly causing some small natural avalanches in wind loaded terrain.  Exposed ridges and fingers are likely spots to trigger a wind slab, as well as just below ridgelines on leeward slopes. Look out for sculpted or pillowed snow, or hollow sounds, and or cracking.


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

Our persistent weak layers are doing exactly what they are supposed to do. They are lingering and still preserved in the mid and ground level of our snow pack. We now have a new layer of buried surface hoar about 1-2 feet down that is propagating in our snowpit tests that was buried by the New Year's storm. This is going to be a layer to watch as we add more storm and wind deposited snow today and through the next few days.

The other persistent problem is lower in our snow pack and it could have serious consequences: facets sitting on a crust that are producing planar failures that show propagation potential.  While it is deep enough and relatively hard to initiate, triggering a slide in this layer would be fatal.  There now exists the possibility of step down avalanches where you could trigger an avalanche in the new or wind blown snow that could overload the weak layer near the old crust which is now buried close to 4 feet down. Click here for a recent video of a Propagation Test on this layer.  You will be more likely to trigger this early season problem where the snowpack is shallow and still weak above the buried layers, especially in rocky terrain where the ground is uneven across a slope.

It is easy to get complacent about a persistent avalanche problem but it should be in the back of your mind as you travel right now,  The presence of these layers is enough to keep most seasoned backcountry travelers on lower angle slopes and away from committing terrain.  Persistent weak layers are unpredictable based on variations in the terrain and snow pack, they can also fail in unpredictable ways across a slope sometimes weeks after being buried. 

Persistent weak layers in snowpack on Sargeant's Mt. near Brundage Resort:

recent observations

We ski toured Granite Mountain yesterday near our weather station and found about 41 inches or 105 CM of snow.  We dug a pit just below the weatherstation on an east aspect that showed stronger results than we had been seeing. Our tests lacked propigation in the surface hoar layer in the mid pack, and in the facets sitting on the crust about a foot off the ground. Above 8,000 feet nearing the summit of granite, the fog layer was thick and hollow drum like wind slab snow blanketed the shoulder. Good snow was plentiful in lower protected areas, depth is variable up high. Coverage is improving, but there are still a lot of rocks and logs to be hit.Youtube video of the ECTN

Tip of the week:  If you are venturing out of bounds at the local resorts, go with someone who knows more than you and make sure everyone in your group is packing a beacon, probe and shovel.  Based on observed behavior and travel decisions, lots of folks would benefit from the upcoming FPAC Intro to Avalanche Classes over the next few weeks.  #KnowBeforeYouGo, #GetTheGear(for the millennial crowd).

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 14 deg. F.
Max. temperature in the last 24 hours: 15 deg. F.
Average wind direction during the last 24 hours: W
Average wind speed during the last 24 hours: 8 mph
Maximum wind gust in the last 24 hours: 14 mph
New snowfall in the last 24 hours: NA inches
Total snow depth: 41 inches

Area Forecast Discussion
National Weather Service Boise ID
255 AM MST Mon Jan 6 2020

.SHORT TERM...Today through Thursday...Moist northwest flow aloft
will continue through early Tuesday, bringing snow mainly to the
higher terrain in the northern half of the CWA. An upper level
ridge axis will pass over the area Tuesday, but even then snow
showers will continue over the same area. Behind this ridge, we
move into a period of southwest flow aloft in advance of an upper
level trough that will arrive early Wednesday. This trough will
bring colder air and more precipitation. By midday Wed, snow
levels will fall to valley floors. Late Wed, we`re right back into
moist northwest flow as the trough axis moves through. An
additional shortwave trough will move rapidly through the
northwest flow on Thursday, bringing even more cold air and
snowfall. Temps will be above normal through Wed, then fall to
near normal Thu. The heaviest snowfall will occur Monday into
Monday evening, then again late Tue night into Wed morning. This
is a fairly long period of on-and-off snowfall, and snow totals by
Thursday afternoon will range from 2 to 6 inches in the Upper
Weiser River Basin, 6 to 10 inches in the Long Valley, and 12 to
24 inches above 7500 feet. However, there is no one intense period
in which a great deal of snow will fall, and therefore we do not
have an advisory or warning out.

.LONG TERM...Thursday night through Sunday...The active winter
time pattern will continue through the extended period as a series
of low pressure systems move through. Precipitation associated
with these systems begins on Friday night with a cold frontal
passage. On Saturday, a system with more promising moisture drops
over the region, spreading higher chances for snow especially in
the Central Mountains of Idaho. Ensembles and most operational
models have trended slightly warmer for the region with the large
scale upper level low over central Canada for early next week.
Temperatures through the extended will remain within a couple
degrees of normal.


Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Monday Monday Night Tuesday
Weather: Snow showers. High near 28. South wind 3 to 7 mph. Chance of precipitation is 90%. New snow accumulation of 1 to 3 inches possible. Snow showers. Low around 26. South wind 3 to 6 mph. Chance of precipitation is 80%. New snow accumulation of less than one inch possible. Snow likely. Mostly cloudy, with a high near 34. South wind 3 to 5 mph. Chance of precipitation is 70%. New snow accumulation of less than one inch possible.
Temperatures: 28 deg. F. 26 deg. F. 34 deg. F.
Wind direction: S South South
Wind speed: 3-7 mph 6 mph 3-5 mph
Expected snowfall: 1-3 in. Less than one in. Less than one in.
Granite Mountain at 7700 feet.
  Monday Monday Night Tuesday
Weather: Snow showers. High near 21. Wind chill values between -2 and 7. South southwest wind 11 to 15 mph. Chance of precipitation is 100%. Total daytime snow accumulation of 4 to 8 inches possible. Snow showers. Temperature rising to around 25 by 2am. South southwest wind 11 to 13 mph. Chance of precipitation is 90%. New snow accumulation of 2 to 4 inches possible. Snow. High near 25. South wind 11 to 13 mph. Chance of precipitation is 80%. New snow accumulation of 1 to 3 inches possible.
Temperatures: 21 deg. F. 25 deg. F. 25 deg. F.
Wind direction: SSW SSW S
Wind speed: 11-15 mph 11-13 mph 11-13
Expected snowfall: 4-8 in. 2-4 in. 1-3 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.