Avalanche Advisory published on December 17, 2017 @ 6:58 am
This Avalanche Advisory expires in 4 hours, 34 minutes
This advisory is valid for 12 hours
Issued by Mark Dundas - Flathead National Forest

Whitefish Range
Swan Range
Flathead Range and Glacier National Park

How to read the advisory

Wind speeds increased overnight and will remain elevated through the day forming thin wind slabs at upper elevations. The avalanche danger is MODERATE above 6000' where wind loaded slabs exist in leeward and crossloaded terrain and a loose dry avalanche problem exists in sheltered terrain. Human triggered avalanches are possible and may entrain substantial snow and travel long distances. Note cracking in the surface snow to identify slabs and look for areas with substantial loose snow above a crust. 

2. Moderate

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Above 6000 ft.
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

1. Low

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5000-6000 ft.
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

1. Low

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3500-5000 ft.
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
  • 1. Low
  • 2. Moderate
  • 3. Considerable
  • 4. High
  • 5. Extreme
Avalanche Problem 1: Wind Slab
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Southwest winds increased overnight and will remain elevated through the day forming thin wind loaded slabs on leeward and crossloaded slopes. 1-3" of snow deposited Friday, combined with an additional 1-3" today, will be redistributed by these winds. These slabs now rest on crusts and may not bond to this underlying layer. Triggering even a small wind slab today could have significant consequences due to the thin snowpack and numerous exposed hazards. These slabs are fresh and easy to detect by the cracking in the snow surface beneath your skis or machine. Look for pillows of snow below ridgelines and in gullies. 

Avalanche Problem 2: Loose Dry
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Today's 1-3" of new snow will be deposited on a 6-12" layer of low density surface snow in sheltered locations. The weak sugary surface layer sits on top of the slippery Thanksgiving rain crust in many middle and upper elevation shaded locations. Observations from the past week have confirmed that this surface snow is not bonding well to the underlying crust and slides within this layer have the ability to travel long distances and entrain substantial snow (see observation). Be cognizant of this hazard especially while traveling above terrain traps, cliff bands and other features where losing your footing or machine could result in a nasty ride.

advisory discussion

Fridays weak storm deposited 1-3" of snow accompanied by light west winds over most of our advisory area except in the Swan Range which picked up 9" at Noisy Basin. Due to its favored status the loose dry avalanche problem in the Swan increased more than the rest of the advisory area. The system moving through our area today will deposit an additional 1-3" but will be accompanied by moderate southwest winds which will form thin wind loaded slabs onto a surface crust. A warm moist weather pattern arrives tonight depositing heavy snow at upper elevations accompanied by strong winds. This weather will increase the avalanche danger and create a widespread avalanche cycle. This avalanche cycle will be the largest of the season to date and we recommend avoiding avalanche terrain and runout zones as this storm intensifies early week. 

Thanks to everyone for submitting observations as this helps us to produce a better product.

Weather and CURRENT CONDITIONS
weather summary

Over the past 24 hours our area experienced scattered light snow flurries, light west winds and maximum temperatures in the mid 20's. Today a weak system will arrive with 1-3" of snow accompanied by moderate southwest winds with temperatures in the low to mid 20's. The big news is the arrival of a warm moist system tonight into Monday depositing heavy snow at upper elevations and a mix of precipitation in the valleys. Unsettled weather will continue through mid week.

Two-Day Mountain Weather Forecast Produced in partnership with the Missoula NWS
For 3000 ft. to 5000 ft.
Today Tonight Monday
Weather: Cloudy with light snow and moderate winds. Heavy snow and strong winds. Heavy snow and strong winds.
Temperatures: 22-27 deg. F. 20-25 deg. F. 29-34 deg. F.
Wind Direction: Southwest Southwest West
Wind Speed: 10-15 mph with gusts to 30 15-20 mph with gusts to 50 15-20 mph5 with gusts to 50 mph
Expected snowfall: 1-2 in. 3-6 in. 4-8 in.
For 5000 ft. to 7000 ft.
Today Tonight Monday
Weather: Cloudy with light snow and moderate winds. Heavy snow and strong winds. Heavy snow and strong winds.
Temperatures: 20-25 deg. F. 20-25 deg. F. 25-30 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 15-20 mph with gusts to 30 25-30 mph with gusts to 60 25-30 mph with gusts to 60
Expected snowfall: 1-3 in. 4-7 in. 5-8 in.
Disclaimer

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.

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