THIS AVALANCHE ADVISORY EXPIRED ON February 13, 2018 @ 10:47 pm
Avalanche Advisory published on February 13, 2018 @ 5:47 am
Issued by Zach Guy - Flathead Avalanche Center

Whitefish Range
Swan Range
Flathead Range and Glacier National Park

How to read the advisory

Recent wind drifting and deeply buried weak layers require cautious travel at higher elevations, especially on wind loaded slopes or in rocky, shallow areas.  Lower elevations have a safer snowpack.  

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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  • Aspect/Elevation ?
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    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Shifting winds on Sunday and last night formed stiff wind slabs on various aspects at high elevations and some mid elevation slopes.  Recent natural wind slabs observed in the Flathead Range (here) and Southern GNP (here) exemplify the problem.  These slabs formed over low density snow or slick rain crusts, which prevent good bonding. They are hard enough that they could break above you without giving good feedback.  Avoidance is the easy solution.  Wind slabs are most likely to be found below ridgelines, rollovers, and in gullies in wind loaded or crossloaded terrain.  Carefully observe recent wind drifting patterns and look for thicker or denser lenses of snow to identify the problem.  

Avalanche Problem 2: Deep Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

The most active buried weak layer in our snowpack is the Thanksgiving rain crust/facet near the ground.  Our last storm triggered another round of hard slab avalanches that produced very destructive and long-running debris piles (Observation 1, observation 2, observation 3).  Deep slabs are most likely to be triggered on alpine terrain near rock outcrops or shallow areas, especially from cornice falls.  Choose well-supported slopes with deep and uniform snow coverage if you are traveling in steep, high elevation terrain.  Don't linger below cornices.  

advisory discussion

Observations from the last few days suggest a mixed bag of surface conditions at higher elevations.  A skier yesterday near Felix Peak noted some shallow cracks up to 6 feet long, evidence that the wind slab problem has not gone away.  Varying snow amounts from Thursday's cold front (up to 14" in the Flathead/Glacier) have been redistributed by strong winds, first from the west switching to northeast on Sunday, and today from the southwest.  The drifted snow is shallow and benign in some places, but in favored locations, you could trigger a slab large enough to drag you into a terrain trap or bury you.  Wind slabs may exist on atypical slopes from recent northeast winds, so be attentive on all aspects today. Reading the surface texture is your best weapon.  The natural avalanche reported yesterday in John F Stevens Canyon highlights where the problem is most concerning: areas that recieved more low density snow on Thursday and more wind transport on Sunday.  

Deep slab avalanches tend to be less active during prolonged periods of dry and cold weather.  Yet, their behavior is notoriously tricky.   The odds are low today, but the consequences remain severe.  There were numerous deep slab avalanches that ran last week, from D3 to D4 in size, during our warm and wet storm cycle.  To find out more about our deep slab problem please visit our forecaster's corner blog post: http://www.flatheadavalanche.org/forecast-corner

Weather and CURRENT CONDITIONS
weather summary

Wind chill is topping the charts this morning with mountain temperatures in the single digits and blustery southwest winds gusting into the 30s.  Temperatures will thankfully warm through the day ahead of our next Pacific system which is onboarding the BC Coastline this morning.  Take advantage of our last day of sunny weather before we enter into an active weather pattern through the weekend.  A welcomed surface refresh is on the way; about a 6" to 12" is on tap by Thursday morning.

Two-Day Mountain Weather Forecast Produced in partnership with the Missoula NWS
For 3000 ft. to 5000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly sunny and warming temps Increasing clouds Moderate snowfall
Temperatures: 22 to 27 deg. F. 15 to 20 deg. F. 25 to 30 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 10 to 20, gusting to 30 10 to 20, gusting to 30 5 to 15, gusting to 25
Expected snowfall: 0 in. 0 to 2 in. 3 to 5 in.
For 5000 ft. to 7000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly sunny and warming temps Increasing clouds Moderate snowfall
Temperatures: 20 to 25 deg. F. 15 to 20 deg. F. 19 to 24 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 15 to 25, gusting to 40 15 to 25, gusting to 40 10 to 20, gusting to 30
Expected snowfall: 0 in. 0 to 2 in. 4 to 6 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.