THIS AVALANCHE ADVISORY EXPIRED ON December 14, 2019 @ 5:20 am
Avalanche Advisory published on December 13, 2019 @ 5:20 am
Issued by Ben Bernall - Kootenai National Forest

Kootenai

bottom line

Thirteen inches of new storm snow in the Cabinets and a deep weak layer in the middle of the snowpack. Avoiding steep slopes (over 30°) through the weekend will be your best bet to avoid triggering an avalanche. Above 5,000' is where you will find avalanche concerns. Sticking to lower angled terrain and ridgelines is where you will find the safest riding.

How to read the advisory

Thirteen inches of new storm snow in the Cabinets and a deep weak layer in the middle of the snowpack. Avoiding steep slopes (over 30°) through the weekend will be your best bet to avoid triggering an avalanche. Above 5,000' is where you will find avalanche concerns. Sticking to lower angled terrain and ridgelines is where you will find the safest riding.

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.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Storm Slab
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Bear Mt. and Chicago Peak Snotel sites have recorded over 13" of new snow and 1.4" of snow water equivalent. This snow will take a couple of days to settle and bond to the existing snowpack. Avoid riding on steep slopes above 5,000 in elevation through the weekend to minimize your chances of triggering an avalanche. This problem will be less of a concern in the Purcell Mountains where storm totals have topped out around 3-4" this morning.

Avalanche Problem 2: Persistent Slab
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We have a thin ice crust layer that is sandwiched between weak sugary snow approximately 20-30" below the surface. This layer is found widespread across the Kootenai area above 5,000' and has been failing with moderate force in stability tests. With the additional loading of storm snow this layer is likely to be more sensitive to human triggers through the weekend. Triggering and avalanche on this layer will result in a deep and destructive avalanche.



Layer of concern! This ice crust and the weak snow adjacent to it will be more sensitive with new snow loads. This will be the layer to watch in the coming weeks as it will be sticking around for a while!

Avalanche Problem 3: Wind Slab
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The new storm snow came in with fairly strong winds out of the southwest. Expect to trigger pockets of windslab on north and east aspects in steep terrain below ridgelines.

advisory discussion

On December 12th I toured into the NW Peak area of the Purcell Mountains. The snowpack story looked very similiar to what I saw in the East Cabinets on Monday with the same buried ice crust in the middle of the snowpack. This layer is sandwiched by weak facets that look to be slowly gaining strength and are resistant to propagation. I do expect that the weight of the new snow will definitely test this layer through the weekend and there is a good chance that triggering a slide in the new storm snow will cause a "step-down" to this deeper weak layer. Snotel sites across the region show that most of the new storm snow fell on the Cabinet Mountians and mostly missed the Purcell Range to the north. This will make riding in the Purcells a little safer this weekend from an avalanche standpoint; but, with our current snow situation there are still abundant rocks and stumps out there that can wreck your day just as well. 

Weather and CURRENT CONDITIONS
weather summary
Backcountry Forecast from NWS Missoula issued:
340 AM MST Fri Dec 13 2019
DISCUSSION: Light to moderate snow will continue for northwest
Montana through mid morning, especially for the Seeley/Swan
Region. Ridge top winds will be gusty through the day today as 
well. Most regions will see snow tapering off and becoming more 
showery in nature this afternoon. However, the Seeley/Swan Region 
may keep the steady light snow through the evening hours. A weak 
shortwave will migrate through a moist northwesterly flow over the
weekend, keeping mountain snow showers in the forecast, with 
further accumulations.

Kootenai:
--------------------------- 5000-7000 FT ----------------------------
                      Today        Tonight      Sat      
Cloud Cover           85%          65%          90%      
Hi/Lo Temps           25 to 31     19 to 24     22 to 28 
Winds(mph)            SW 14G36     W 10G23      W  8G20  
Precip Chc            50           40           30       
Precip Type           snow         snow         snow     
Liquid Amt            0.09         0.03         0.02     
Snow Ratio(SLR)       17:1         18:1         19:1     
Snow Amt(in)          1-4          0-3          0-2      

Snow Level            2500         2000         1000     
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.