THIS AVALANCHE ADVISORY EXPIRED ON December 12, 2018 @ 11:54 pm
Avalanche Advisory published on December 12, 2018 @ 6:54 am
Issued by Blase Reardon - Flathead Avalanche Center

AN AVALANCHE WATCH IS IN EFFECT until Wednesday at noon.  

Dangerous avalanche conditions exist in the Swan Range. Heavy snowfall and gusty winds have overloaded a fragile snowpack. Human-triggered avalanches remain likely and natural avalanches are possible. Fresh avalanches, whumpfing sounds, and shooting cracks are sure signs of danger. Avoid traveling on or below steep slopes at mid and upper elevations.

3. Considerable

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Above 6500 ft.
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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5000-6500 ft.
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

2. Moderate

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3500-5000 ft.
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
  • 1. Low
  • 2. Moderate
  • 3. Considerable
  • 4. High
  • 5. Extreme
Avalanche Problem 1: Storm Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Natural and human-triggered slabs breaking in the upper snowpack are likely today. These can break 8-20 inches deep, at the base of the new snow or on buried weak layers that exist on both sunny and shady slopes. The size of avalanches will be greatest on slopes where wind-drifted snow is thickening slabs, and these conditions are most widespread near upper-elevation ridgelines.  Wind-sheltered terrain is harboring fragile layers below the dense new snow which will be equally sensitive to human triggers.  Slides breaking in the new snow could gouge or step down into older, weaker snow, creating larger avalanches than expected.  Avoid crossing on or below slopes steeper than about 30 degrees.

advisory discussion

A powerful storm overnight has brought heavy snowfall and gusty winds to the Swan Range. Storm water totals (since Monday) look to be 0.4-1.0 inches of snow water equivalent. Wind stations in nearby ranges show that gusty southerly and westerly winds accompanied the snowfall. On Tuesday, these winds were drifting snow and forming slabs that were sensitive to a person’s weight.

The combination of new and drifted snow has created slabs that are overloading weak snow that was at the surface of the snowpack prior to the storm. With snowfall totals and rates peaking this morning, a natural avalanche cycle may be underway. The likelihood of natural avalanches will diminish as snowfall tapers off. Nonetheless, it would be wise to avoid crossing under steep start zones near ridges where winds are continuing to drift snow.

Steep slopes will remain reactive to a person’s weight today, especially those where wind-drifted snow is thickening and stiffening slabs. Some avalanches may break below the new snow, in weak layers that developed during recent cold, dry weather. Multiple failure planes exist in the upper half of the snowpack. These include facets at the surface and facets near crusts. We have not had reports of buried surface hoar in the Swan Range, though it exists in nearby ranges. Slides that break into older snow will be more dangerous, even on small slopes, because they’ll involve more snow. Heed clear signs of danger – fresh avalanches, whumpfing collapses, and shooting cracks.

The changing conditions make your observations even more valuable. If you get into the mountains, let us know what you see. 

Our next Avalanche Awareness talk is Thursday, December 20th at Penco Power Products at 6:30 p.m.!

 

 

 

Weather and CURRENT CONDITIONS
weather summary

A cold front trailing behind the storm will bring burst of snow and gusty westerly winds this morning. Snowfall and winds taper off after the front passes. Thursday will see snow showers and warmer temperatures.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Missoula NWS
For 5000 ft. to 7000 ft.
Wednesday Wednesday Night Thursday
Cloud Cover: Mostly Cloudy Mostly Cloudy Mostly cloudy
Temperatures: 245-28 deg. F. 18-21 deg. F. 27-29 deg. F.
Wind Direction: West Southwest Southwest
Wind Speed: 10-15 mph, gusting to 30 10-15 mph, gusting 25 to 30 12 mph, gusting to 25
Snowfall: 2-4" in. T-1" in. 1-2" in.
Freezing Level: 2500 feet 1500 feet 2000 feet
Disclaimer

This forecast applies only to backcountry areas outside established ski area boundaries. The forecast describes general avalanche conditions and local variations always occur. This forecast expires at midnight on the posted day unless otherwise noted. The information in this forecast is provided by the USDA Forest Service who is solely responsible for its content.

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