THIS AVALANCHE ADVISORY EXPIRED ON December 15, 2019 @ 9:36 am
Avalanche Advisory published on December 14, 2019 @ 9:36 am
Issued by Dave Bingaman - Payette Avalanche Center
bottom line

Early season, shallow snowpack is the name of the game across the West Central Mountains.  We have a fairly complex and problematic snowpack right now with multiple weak layers.  The old October and early November snow has turned into a sugary junk pile near the ground with 2 additional weak layers above including buried surface hoar.  Avoid steep slopes especially on protected northerly slopes.  Our next update will be out on Wednesday, Dec 18.

How to read the advisory

recent observations

 

PAC is open for the season and a HUGE thanks to everyone that has been submitting observations so far.   George and I were out getting to know our 2019/2020 season snowpack on Thursday and Friday and found a shallow early season snowpack with a lot of rocks, logs and stumps still uncovered, it's kind of a junk show.  Once you get above 7000 feet it looks a lot more like winter... with brush. Snow depths range right now from 33 inches around 7000 ft to 45 inches in the upper elevation Northerly terrain.  Sledding in anything but meadows is risky business right now with a lot of buried treasure to be found.  We found great coverage and confidence inspiring skiing yesterday in between some very large brush patches up high but overall it is still thin and risky on most slopes.

The snowpack structure is kind of a mess right now too.  Over the last 2 days we found a widespread buried surface hoar layer that demonstrated the ability to initiate and propagate in stability tests.   This layer is likely responsible for a sled triggered incident on Tuesday and several shallow, soft slab avalanches reported near Tamarack Resort on Wednesday.  This layer is still relatively shallow and can be found between 10 and 18 inches down in the snowpack.  In addition a layer of near surface faceted snow is hiding mid pack and produced failures in some of our pits on Thursday.  Most notable is the layer of old snow that was deposited in October and early November that has turned into a sugary mess near the bottom of the snowpack.  This is likely the layer that has been producing most of the whumphing and collapsing that has been widely reported over the last week.  This is a layer that is going to be a problem for awhile and should dictate your travel habits until we get alot of snow above it to compress it or trigger it and flush it out.  If you dig down, it will be easy to find around 85 cm in the snowpack.  It is below a decomposing crust and is showing both the ability to initiate and propagate.  If you trigger this layer, you are talking about an avalanche that will fail almost to the ground with 3-4 feet of snow above it.  This layer is going to be most prevalent on upper elevation slopes that kept snow from early October.  Northerly and upper elevation slopes are the prime suspects and if you find yourself in the upper elevation where the coverage is confidence inspiring...think about this layer and avoid the urge to poke into steep slopes.

These photos correspond to the Granite and Twin Lake pit profiles you can find on our observations page.  The video is from an ECT on a North Aspect with full block propagation at the buried surface hoar layer.   https://youtu.be/McWPKFa8Ejc  Our next update will be on Wednesday, December 18.

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: 26 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 11 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 Tuesday...Upper trough positioned over
e-central Oregon early this morning will track into eastern Idaho
by afternoon. The focus of precipitation will continue to be
across the Snake Plain and higher terrain to the south. Have added
southern Twin Falls county to the Winter Weather Advisory as an
additional 1-2 inches is expected through sunrise. Rain/snow will
taper off from west to east this morning with the passage of this
wave. Many lower elevation sites now reporting rain/snow mix or
all snow and this transition to snow will continue as snow levels
gradually fall to valley floors. Not expecting much if any snow
accumulation in the lower Snake Plain before precipitation ends.
Much of the region trends drier for Sunday and Monday under a
colder northwest flow. The w-central Idaho mountains will keep a
chance of showers through Monday with the northwest flow and
passage of a weak wave driving the precipitation development. An
upper ridge moves in from the coast on Tuesday, bringing in
warmer air aloft which will develop an inversion. Temperatures
are near normal through Tuesday.

.LONG TERM...Tuesday night through Friday...Conditions will remain
relatively dry on Wednesday, though an increase in cloud cover
is expected as a weak feature tracks through the Intermountain
West. Thursday into the weekend, an approaching longwave trough
with a significant subtropical moisture tap will initiate several
days of precipitation across the region. Breezy winds will also be
anticipated Thursday through Saturday as this system tracks
closer. The stronger wind pattern will likely lead to enhanced
mountain upslope flow, as well as the potential for shadowed
valleys. Warm air advected in the southwest flow will trend snow
levels higher to around 3500-5000 feet MSL. Models are still in a
bit of disagreement with the arrival of the colder air which will
determine the rain/snow line for the valleys/foothills.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Saturday Saturday Night Sunday
Weather: Scattered snow showers, mainly after noon. Partly sunny, with a high near 29. Light west wind. Chance of precipitation is 30%. Total daytime snow accumulation of less than a half inch possible A 20 percent chance of snow showers. Patchy fog after midnight. Otherwise, mostly cloudy, with a low around 16. Calm wind. A 20 percent chance of snow showers. Patchy fog before noon. Otherwise, partly sunny, with a high near 26. Light and variable wind.
Temperatures: 29 deg. F. 16 deg. F. 26 deg. F.
Wind direction: W Calm Variable
Wind speed: 29 Calm Light
Expected snowfall: Trace in. 0 in. 0 in.
Granite Mountain at 7700 feet.
  Saturday Saturday Night Sunday
Weather: Scattered snow showers, mainly after 11am. Partly sunny, with a high near 16. Light west northwest wind. Chance of precipitation is 40%. Total daytime snow accumulation of less than one inch possible. A 30 percent chance of snow showers. Mostly cloudy, with a low around 10. Light west wind. New snow accumulation of less than a half inch possible. A 30 percent chance of snow showers, mainly after 11am. Mostly cloudy, with a high near 15. Calm wind becoming west around 5 mph in the afternoon. New snow accumulation of less than one inch possible.
Temperatures: 16 deg. F. 10 deg. F. 15 deg. F.
Wind direction: W W W
Wind speed: Light Light 15
Expected snowfall: Trace in. Trace in. Trace 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.