THIS AVALANCHE ADVISORY EXPIRED ON February 25, 2018 @ 7:14 am
Avalanche Advisory published on February 24, 2018 @ 7:14 am
Issued by Dave Bingaman - Payette Avalanche Center
bottom line

Human triggered avalanches are possible today where isolated wind slabs may be found on upper and middle elevation, wind exposed slopes.  The new snow we received this week may be hiding older wind slabs left from  strong winds earlier this week.  Loose snow avalanches/sluffs are likely on slopes over 35 degrees.  Numerous natural and human caused avalanches occurred in the last 6 days, most of these failed in wind loaded areas where faceted snow was resting on a crust below.

How to read the advisory

  • Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Very Likely
  • Size ?
    Very Large

Strong winds last week and steady winds in the mid to upper teens over the last few days combined with 20-24 inches of light density (blower!) snow in the last 7 days have created a recipe for wind slab avalanches throughout the West Central Mountains.  Winds have swirled around from the S to the NE during this time creating wind slabs on multiple aspects.  The newest crop of wind slabs is in the 4-14 inch range with older wind slabs as deep as 2.5 feet possible.  Some of these wind slabs from last week are now camouflaged by the addition of new snow above them.  A layer of faceted snow developed in some areas prior to the last 8 days of storms and is resting on the stout crust deposited in late January allowing avalanches in these areas to propagate over larger areas.   These slabs while widespread are not reactive in all areas, your best bet for triggering a shallow or deeper slab is going to be found on steep, upper elevation terrain especially with recently deposited drifts or wind pillows. Watch for visual clues such as sculpted or scalloped snow surfaces, wind spines or drifts or rounded pillow formations all of these features are signs of wind slabs.  Throughout the day today we are going to be adding even more new snow above our existing wind slab problem and these visual clues are going to be harder to pick out.

Winds are currently out of the SW around 10 mph but will be increasing throughout the day and overnight tonight. Pay attention to changing conditions, fluctuations in the temperature over the next few days and the addition of over a foot of new snow in the next 24-36 hours.  The avalanche hazard will rise later in the day today and through the night tonight.

Watch for signs of instability today and through the weekend including:  recent avalanche activity, cracking, collapsing and whumphing in the snowpack.  If you encounter these or signs of denser snow resting on the light snow below, stick to lower angle, less avalanche prone terrain.

Photos are cracking from Thursday in wind drifts on East aspect near 7400 ft, wind sculpted snow on S ridge from Friday close to 8000 feet and NW ridgetop at 8400 ft.


Avalanche Problem 2: Loose Dry
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Very Likely
  • Size ?
    Very Large

Loose, dry avalanches or sluffs are widespread right now on steep terrain.  Our mountains have received between 17 and 21 inches of new, low density snow over the last 7 days with an additional 3-5 overnight. Widespread natural sluffing and point releases in the light density snow are visible on multiple aspects and on steep terrain at all elevations.  While these small avalanches don't pack a lot of punch, they do have the ability to pile up in terrain traps or steer a skier where they don't want to go.  Use good travel etiquette and keep your eyes on your partners.  Avoid steep, confined and consequential terrain where a small sluff could create larger problems.  Additional storm and wind deposited snow will continue to add to this problem today and tomorrow.

advisory discussion

A big thanks to everyone that came out to the FPAC fundraiser last night!  Thanks for supporting PAC and the Friends!  Thank you also to all of our donors and the McCall Club House/Banyans Restaurant for hosting us!

recent observations

We experienced the full spectrum of snow conditions on our tour of the West face of Granite Mt yesterday.  Ridgetops were sculpted and drifted with occasional patches of leftover January crust exposed in between soft snow and firm drifts.  We also experienced widespread collapsing, cracking and several large whumps where the new firm windslabs were overlying the crust and facet layer around between 12 and 14 inches down.  We were also able to observe wind sculpted and scoured snow on the NW aspect of Granite as well as many small natural point release sluffs in steep terrain on all aspects.   We were able to easily trigger multiple small and very thin but extremely sensitive wind slabs on the 30-35 degree West facing terrain with our skis.  Middle and lower elevation protected slopes proved to have the best skiing and riding with plenty of light fluffy snow and no wind effect.

Given the sensitivity of these small slabs, today would be a good day to stick to lower angle, protected terrain and leave the upper elevation, wind affected terrain alone.  3-5 inches of new snow fell overnight with another 6-10 inches possible through the day today.  Old wind slabs are going to be camoflaged by the new snow which will add to the depth of the problem.  Expect the avalanche hazard to rise today as the snow increases and plan your day and routes accordingly.

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 5 deg. F.
Max. temperature in the last 24 hours: 14 deg. F.
Average wind direction during the last 24 hours: WSW
Average wind speed during the last 24 hours: 6 mph
Maximum wind gust in the last 24 hours: 17 mph
New snowfall in the last 24 hours: NA inches
Total snow depth: 72 inches


SHORT TERM...Today through Sunday...The next winter storm is upon
us, and we made some small changes to the warning and advisories
that were previously posted. The latest timing brings the storm
in a little slower, and we now expect temperatures to allow for
accumulating snows in some of the lower elevations into the early
afternoon. This necessitated some changes in the timing of some
of the advisories. We have also added blowing snow to the forecast
in areas where wind gusts are forecast to be 25 mph or greater
and temperatures less than 28 degrees. Blowing snow will be common
in the mountains and in the Southwest Highlands. Snow is expected
to begin in eastern Oregon before 12Z, then moving into southwest
Idaho. There will be a sharp north-south gradient in snowfall
across the Treasure Valley due to downsloping in the lee of the
Owyhees, with less than an inch expected in the southern part of
the valley, then 1 to 3 inches as you move north toward I-84, and
then 3 to 5 inches north of I-84 (generally speaking). Instability
will be sufficient for very vigorous showers this afternoon, and
even a chance for thunderstorms. Graupel, often confused for hail,
will be present with many of these showers and any thunderstorms
that form. Generally, the morning will bring steady snows, while
the afternoon becomes showery/stormy. This is not true for the
western Magic Valley, where steady snows will not begin until
afternoon, with showers/storms coming late in the afternoon and
into the evening. Tonight will bring a drying trend, and yet
another system begins to move in Sunday. Snows will move into
Baker County and the mountains of southwest Idaho during the
afternoon, and expand south and east overnight. Additional
advisories and/or warnings will most likely be needed for this
system, once today`s system clears the area. Temps will be a few
degrees warmer Sunday than today.

.LONG TERM...Sunday night through Friday...Unseasonably cool and
unsettled weather will continue in the extended period, with below
normal temperatures and above normal precipitation. A vigorous cold
front will move through the region on Sunday night into Monday
morning. Westerly flow will favor moderate to heavy snow
accumulations across the central Idaho mountains; with snow amounts
approaching warning criteria. The system will quickly move south on
Monday afternoon, with snow showers lingering across the higher
terrain through Monday evening. Tuesday will be generally dry and
cool in between systems. A system will move into the Pacific NW late
Wednesday into Thursday, bringing more precipitation into the region
through the weekend, although forecast details and timing are
unclear at this point.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Saturday Saturday Night Sunday
Weather: Snow before 11am, then snow showers after 11am. High near 24. South wind 7 to 9 mph becoming west in the afternoon. Chance of precipitation is 100%. Total daytime snow accumulation of 3 to 5 inches possible. A 40 percent chance of snow showers. Mostly cloudy, with a low around 9. West wind 5 to 7 mph becoming light and variable. New snow accumulation of 1 to 2 inches possible. Snow. High near 24. South wind 7 to 15 mph. Chance of precipitation is 100%. New snow accumulation of 3 to 7 inches possible.
Temperatures: 24 deg. F. 9 deg. F. 24 deg. F.
Wind direction: S W S
Wind speed: 7-9 5-7 7-15
Expected snowfall: 3-5 in. 1-2 in. 3-7 in.
Granite Mountain at 7700 feet.
  Saturday Saturday Night Sunday
Weather: Snow before 11am, then snow showers after 11am. High near 14. Wind chill values between -2 and -10. South southwest wind 11 to 14 mph becoming west in the afternoon. Chance of precipitation is 100%. Total daytime snow accumulation of 6 to 10 inches possible. A 50 percent chance of snow showers. Mostly cloudy, with a low around 6. Wind chill values between -4 and -9. Northwest wind 8 to 11 mph becoming southwest after midnight. New snow accumulation of 1 to 3 inches possible. Snow. Areas of blowing snow after 11am. High near 16. Wind chill values between -5 and -10. Breezy, with a south southwest wind 14 to 23 mph, with gusts as high as 33 mph. Chance of precipitation is 100%. New snow accumulation of 7 to 11 inches possible.
Temperatures: 14 deg. F. 6 deg. F. 16 deg. F.
Wind direction: SSW NW SW
Wind speed: 11-14 8-11 14-23, gusting to 33
Expected snowfall: 6-10 in. 1-3 in. 7-11 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.