THIS AVALANCHE ADVISORY EXPIRED ON February 26, 2018 @ 5:00 am
Avalanche Advisory published on February 25, 2018 @ 5:00 am
Issued by George Halcom - Payette Avalanche Center
bottom line

Natural and Human triggered Loose snow avalanches/sluffs are likely on slopes over 35 degrees. Wind slab hazard will be increasing, and may be found on upper and middle elevation, wind exposed slopes. Be careful near ridgelines and cornices that may be tender and overhanging.  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


  • Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Loose Dry
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The new snow is light and dry, and wants to move with you on slopes that are steeper than 35 degrees. The loose snow sluffs are somewhat predictable, but could easily take you into treewells, rocks and other obstacles that might not be on your to do list.  We watched them failing naturally yesterday and had many running away from our skis and sleds

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Avalanche Problem 2: Wind Slab
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Wind slab avalanche hazard has been on our radar as most of the snow that we have received has come with ideal winds and temperature regimes to create slabs that have failed naturally and which have been triggered by people. The slabs built up on top of crusts that have been exposed to cold temperatures and a mixture of graupel and facets....as we put more snow and wind over the next 24 hours the wind slab avalanche hazard will increase. Signs on the snow surface have likely been erased by the calmer light snow over the last 24 hours. Likely tender cornices are overhanging quite a bit right now and will be a great indicator of where the snow has been blowing and shaping the monsters on the ridges above slopes that have been wind loaded.

recent observations

Saturday, we toured through the Louie Lake Twin Peaks, Rapid cr, South Fork of Lake Fork zone. The new snow was very light, deep, deeper throughout the day, and had a tenancy to run in front of your sled and or skis. We were able to kick loose sluffs that wanted to move within the new snow on steep slopes greater than 35 degrees, and observed natural sluffing that was widespread along steep ridgelines. Test pits columns were failing on isolation within the upper six inches of new snow.

weather

 

Area Forecast Discussion
National Weather Service Boise ID
357 AM MST Sun Feb 25 2018

.SHORT TERM...Today through Monday...A vigorous upper level trough
is moving south along the British Columbia coast this morning with
moisture streaming into the Pacific Northwest. Temperatures this
morning are colder than anticipated, thus have lowered high
temperatures for today slightly. Snow will develop across the
west central Idaho mountains and Baker County OR this morning and
become widespread across much of the region by this evening, with
the exception of precipitation shadowing directly east of the
Owyhee Mountains of SW Idaho and Steens Mountain in SE Oregon.
Westerly flow aloft will favor heavy snow across the West Central
Mountains and Boise Mountains ahead of the cold front. The cold
front will move through the region late Sunday night and into
Monday morning. Snow will taper off quickly behind the cold front
from NW to SE on Monday, with isolated snow showers continuing
across the central ID mountains and south along the NV ID/OR
border through Monday evening. Breezy northwest winds are expected
on Monday afternoon behind the cold front.

.LONG TERM...Monday night through Saturday...Snow showers end Monday
night as the trough axis swings through and the main upper low moves
far to the southwest. Northwest flow Tuesday will back to westerly
on Wed in advance of another system, and mountain snow showers are
possible through this time. This large strong system will spread
snow to all areas Wed night through Friday, with several periods of
snowfall for lower elevations and nearly continuous snow for the
mountains. Mountains could easily end up with over a foot of new
snow with this storm. A cold front will move through Friday,
continuing the chance for precipitation. Snow showers remain
possible through next weekend as we reside under a broad upper level
trough. Temperatures will remain well below normal through the
extended period.

&&

.AVIATION...VFR initially. Mountains deteriorating into IFR with
snow after 12Z. Lower elevations VFR/MVFR after 15Z in snow showers.
A cold front will move through tonight, bringing additional snow.
Surface winds: southeast to southwest 10-20 kts except 15-25 kts
over the higher terrain, transitioning to northwest after 26/00Z.
Winds aloft at 10k ft MSL: northwest 25-35 kts becoming west 30-40
kts after 21z.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Sunday Sunday Night Monday
Weather: Snow. High near 23. South wind 5 to 14 mph. Chance of precipitation is 100%. New snow accumulation of 4 to 8 inches possible. Snow showers. Low around 13. South southwest wind 8 to 13 mph becoming light and variable after midnight. Chance of precipitation is 100%. New snow accumulation of 3 to 7 inches possible. A 20 percent chance of snow showers. Mostly sunny, with a high near 25. Northwest wind 3 to 6 mph.
Temperatures: 23 deg. F. 13 deg. F. 25 deg. F.
Wind direction: S SW NW
Wind speed: 5-14 8-13 3-6
Expected snowfall: 4-8 in. 3-7 in. 0 in.
Granite Mountain at 7700 feet.
  Sunday Sunday Night Monday
Weather: Snow. Areas of blowing snow after 11am. High near 18. Wind chill values between -5 and -14. Breezy, with a south southwest wind 18 to 23 mph increasing to 24 to 29 mph in the afternoon. Chance of precipitation is 100%. New snow accumulation of 8 to 12 inches possible. Snow showers. Low around 8. Wind chill values between -5 and 1. Windy, with a south southwest wind 21 to 31 mph becoming west northwest 8 to 13 mph after midnight. Winds could gust as high as 40 mph. Chance of precipitation is 100%. New snow accumulation of 5 to 9 inches possible. A 20 percent chance of snow showers. Mostly sunny, with a high near 13. Wind chill values between -5 and 1. North northwest wind 7 to 9 mph.
Temperatures: 18 deg. F. 8 deg. F. 13 deg. F.
Wind direction: SW SSW NNW
Wind speed: 18-29 21-40 7-9
Expected snowfall: 8-12 in. 5-9 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.