THIS AVALANCHE ADVISORY EXPIRED ON December 28, 2017 @ 6:11 am
Avalanche Advisory published on December 27, 2017 @ 6:11 am
Issued by Ben Bernall - Kootenai National Forest

Kootenai

bottom line

The snowpack and its weak layers are gaining strength but still have the potential to fail and create large avalanches due to the thicknesses of this layer and the weak, sugary snow below it. 

How to read the advisory

The snowpack and its weak layers are gaining strength but still have the potential to fail and create large avalanches due to the thicknesses of this layer and the weak, sugary snow below it. 

3. Considerable

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

3. Considerable

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Near Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

2. Moderate

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Below Treeline
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.
Avalanche Problem 1: Persistent Slab
  • Type ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

The abundant snow that fell on December 18th-19th has created a cohesive slab 2-3' thick across the area.  This slab overlies multiple buried weak layersStability tests show that both this slab layer and buried weaknesses are gaining strength; however, if triggered they will create very large avalanches with the potential to pull in snow from adjacent, lower angle slopes if triggered.

advisory discussion

Remember that most avalanche accidents occur when the rating is considerable.  This large slab has gained strength but can still be triggered and will create a large avalanche when it does move!  Weather forecast says more snow is on the way Thursday and Friday so expect stability to decline as this new snow buries the layer of surface hoar that grew on our snowpack.  Small slides created by this new snowfall may also have the potential to create enough force to trigger our current weak layer buried deep in the snowpack as well.  The next update will be posted on 12/29 at 7A.M.

recent observations

Today we travelled into the Eastern Cabinets where we again heard multiple collapses on weak layers below 5,000'.   These sounds are a sure sign of instability and we noticed that they were most prevalant in terrain with a thinner snowpack and anchored terrain.  They also proved to be surprisingly stubborn in stability tests.  Conclusion-some very weak snow exists below 5,000'; however, it is unlikely to pose a threat in most areas due to vegetative anchoring.  Strong caution is advise at or below this elevation if you are in areas that are naturally open and void of vegetation.  As we travelled into higher terrain the snowpack thickened and stability tests showed that the snow that fell last weak has gained significant strength.  We had consistent results with failures occurring 32" below the surface, again failing on weak facets that are resting on the Thanksgiving rain crust.



32" thick slab resting on Thanksgiving rain crust with a layer of greasy facets between!

Weather and CURRENT CONDITIONS
weather summary

Backcountry Forecast from NWS Missoula issued:

350 AM MST Wed Dec 27 2017

DISCUSSION: Today begins a prolonged period of heavy snowfall for

mountain ranges across the Northern Rockies, though admittedly

– today's snowfall will be the lightest amounts out of the next

three days worth of snow.

Things we know and feel confident about: 1) the focus of the

heaviest snow continues to be over the Clearwaters, Bitterroots,

Rattlesnake, and southern Swan/Missions. 2) In the aforementioned

regions, we still expect slowly rising snow levels resulting in

heavy, dense snow with accumulations nearing 3 feet by Friday.

Things we suspect, but feel less confident about: 1) The Kootenai,

Whitefish/Glacier, Flathead, northern Swans will see also see

persistent, heavy snow Thurs-Fri, but with higher snow-liquid

ratios, on the order of 20:1 to 25:1, since snow levels won't

vary as much (you'll be locked in the cold air the whole storm).

2) Snow totals in these locations could ultimately be affected by

an extremely tight pressure gradient which may end up affecting

dendrite formation, thus cutting snow amounts down. This will need

to be monitored as we go forward.

In any case, all mountains will pick up additional feet of snow in

the next three days – but who gets utterly whomped on, versus

just your average foot or two of snow, will be determined in the

next 24-48 hours.

--Allegretto



Kootenai:

--------------------------- 5000-7000 FT ----------------------------

                                    Today       Tonight       Thu     

Cloud Cover               95%          85%             90%     

Hi/Lo Temps              14 to 23    5 to 14         15 to 25

Winds(mph)               W  9G21  W  9G20      SW  6   

Precip Chc                  60             40                90      

Precip Type                snow         snow           snow    

Liquid Amt                 0.05          0.03             0.15    

Snow Ratio(SLR)       20:1         24:1              22:1    

Snow Amt(in)             1-2           0-1               2-4     

Snow Level                 0              0                  500    

Disclaimer

Avalanche conditions change for better or worse continually. Backcountry travelers should be prepared to assess current conditions for themselves, plan their routes of travel accordingly, and never travel alone. Backcountry travelers can reduce their exposure to avalanche hazards by utilizing timbered trails and ridge routes and by avoiding open and exposed terrain with slope angles of 30 degrees or more. Backcountry travelers should carry the necessary avalanche rescue equipment such as a shovel, avalanche probe or probe ski poles, a rescue beacon and a well-equipped first aid kit.  For a recorded version of the Avalanche Advisory call (208)765-7323.