THIS AVALANCHE ADVISORY EXPIRED ON February 19, 2018 @ 6:58 am
Avalanche Advisory published on February 18, 2018 @ 6:58 am
Issued by George Halcom - Payette Avalanche Center
bottom line

Strong winds and a combination of denser snow on top of light density snow has created Considerable wind slab avalanche hazard in lee terrain, and terrain exposed to the swirling mountain winds.  Moderate to high W/SW winds hammered the upper elevations over the last 2 days forming sensitive wind slabs on leeward slopes.  Slopes protected from the wind will be prone to storm slab avalanches.  Steep, especially wind protected terrain may also be prone to loose sluffing.

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  slabs are going to be the major concern today in the upper elevations. 

West and Southwest winds hit the West Central Mountains over the last 2 days, and had plenty of light density snow to transport. Yesterday the wind slabs North of 8302 were sensitive to the weight of a skier, and were getting more consolidated and will likely be brittle and able to transmit energy given the warm cold temperature regime that they have seen over the last 24 hours. We were able to trigger a slab about a foot plus deep on an East facing ridge around 7500 feet. We also had cracking developing in the upper 15cm of snow that was denser.

 Friday on the high ridges near Diamond Rock we observed cornices growing and becoming very sensitive to the weight of a skier.  We also observed wind slabs growing in thickness and density through the day with gusts in the 25-30 mph range. 

This problem should be mostly confined to North and East facing slopes BUT given smaller terrain features and undulations, you should expect wind slabs on any terrain with a North or East facing tilt, including gullies, small bowls and other natural catcher mitt features.  These slabs are resting on a slick crust in some areas and will propagate and move quickly if triggered. As winds pick up later in the day, expect the wind slab hazard to rise as well.

Avalanche Problem 2: Loose Dry
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On slopes over 35 degrees, that were protected from the last 2 days of winds, Loose/Dry avalanches or Sluffs are possible.  Sluffs are an indicator of great ski and riding conditions but can also push you around especially in treed, or confined terrain.  Below all of the new snow is a stout and in some areas slick crust which the new snow is not bonding to. Be extra careful if your line involves terrain traps such as benches or gullies where a little bit of moving snow can pile up quickly. 

Avoid traveling above your companions in steep terrain and keep your group corralled today; good travel practices, group and slope management as well as keeping eyes on all members of your group in steep terrain should be priorities.

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Avalanche Problem 3: Storm Slab
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The surface of the snow yesterday had grown denser than the snow below: a foot of light density, cold snow already on the crust and old snow layers below, the addition over the next 24 hours of another 8-16 inches of warmer higher density snow will help form a storm slab in the new snow.  Rising temperatures created a layer of more dense snow  and will be resting on the low density snow below which will make storm slab avalanches of between 1 and 2 feet more possible today.

advisory discussion

Don't forget about the FPAC fundraiser at the McCall Golf Course next Friday 2/23 at 7pm.  Admission includes raffle ticket with over $1000 worth of cool stuff, music by local bluegrass band Jughandle Parade and a short state of the snowpack address by the PAC staff.

Our website just went through a routine update and our email server is currently offline.  No advisory emails will be sent out today or tomorrow.  We are sorry for the inconvienience.  The problem should be resolved by early next week.

recent observations

Yesterday we toured 8302 up the Lick Creek drainage and the wind slabs North of 8302 were sensitive to the weight of a skier, and were getting more consolidated and will likely be brittle and able to transmit energy given the warm cold temperature regime that they have seen over the last 24 hours. We were able to trigger a slab about a foot plus deep on an East facing ridge around 7500 feet. We also had cracking developing in the upper 15cm of snow that was denser.

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 17 deg. F.
Max. temperature in the last 24 hours: 17 deg. F.
Average wind direction during the last 24 hours: W
Average wind speed during the last 24 hours: 10 mph
Maximum wind gust in the last 24 hours: 19 mph
New snowfall in the last 24 hours: NA inches
Total snow depth: 64 inches
weather

 

Area Forecast Discussion
National Weather Service Boise ID
243 AM MST Sun Feb 18 2018

.SHORT TERM...Today through Monday...Showers and wind gusts to 40
mph are accompanying a cold front, which is oriented west-east
just to the south of Boise. The front will continue to slowly
shift southward today. Showers will remain focused near the front
with snow levels falling to valley floors. Snow accumulations
near the ID/NV border warrant a Winter Weather Advisory for today.
Elsewhere, snow showers will accompany an upper trough as it
settles into the region. Bulk of the snow showers and accumulating
snows will be in the mountains. A Winter Weather Advisory remains
in effect for the West Central Mountains this morning. Another
cold front will cross the region from the north tonight and bring
modified arctic air to the region on Monday. Snow showers will
continue tonight through Monday, although additional snow
accumulations will be light and mainly in the mountains. Northwest
winds of 15 to 25 mph on Monday along with high temperatures only
in the 20s to lower 30s will create wind chills in the single
digits and teens.

.LONG TERM...Monday night through Saturday...Modified arctic air
will be with us Monday night through Tuesday night under strong
northerly flow aloft. Gusty northwest surface winds will produce
significant wind chill Monday night. Gradual moderation and less
windy thereafter but still colder than normal through Saturday.
Troughs in the northerly flow will bring a chance of snow to our
zones, especially Thursday and Saturday.

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.AVIATION...Cold front in south-central Idaho moving southeast
this morning. Areas of MVFR and mountain obscuration in showers
near the front and in the central Idaho mountains. Generally VFR
elsewhere. Snow level lowering to valley floors by 18Z with all
pcpn becoming snow. Surface winds west/northwest 15-25 kts, except
20-30 kts in southeast Oregon. Winds aloft at 10k ft MSL westerly
30-40 kts.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Sunday Sunday Night Monday
Weather: Snow showers likely. Mostly cloudy, with a high near 28. West southwest wind around 7 mph. Chance of precipitation is 70%. Total daytime snow accumulation of less than one inch possible. Snow showers likely, mainly before 11pm. Mostly cloudy, with a low around 9. Calm wind becoming north northwest around 5 mph after midnight. Chance of precipitation is 60%. New snow accumulation of less than a half inch possible. A 40 percent chance of snow showers. Cloudy, then gradually becoming mostly sunny, with a high near 20. Wind chill values between -2 and 8. North northwest wind 6 to 11 mph. New snow accumulation of less than a half inch possible.
Temperatures: 28 deg. F. 9 deg. F. 20 deg. F.
Wind direction: WSW NNW NNW
Wind speed: 7 mph 5 mph 6-11 mph
Expected snowfall: less than one in. less than one half in. less than one half in.
Granite Mountain at 7700 feet.
  Sunday Sunday Night Monday
Weather: Snow showers. High near 16. Southwest wind 10 to 14 mph. Chance of precipitation is 90%. Total daytime snow accumulation of 2 to 4 inches possible. Snow showers likely. Mostly cloudy, with a low around 3. Wind chill values between -1 and -11. Calm wind becoming north 5 to 9 mph after midnight. Chance of precipitation is 70%. New snow accumulation of 1 to 3 inches possible. A 50 percent chance of snow showers. Mostly cloudy and cold, with a high near 6. Wind chill values between -11 and -17. North wind 11 to 15 mph. New snow accumulation of less than a half inch possible.
Temperatures: 16 deg. F. 3 deg. F. 6 deg. F.
Wind direction: SW N N
Wind speed: 10-14 mph 5-9 mph 11-15 mph
Expected snowfall: 2-4 in. 1-3 in. less than one half 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.