THIS AVALANCHE ADVISORY EXPIRED ON December 19, 2018 @ 5:25 am
Avalanche Advisory published on December 18, 2018 @ 5:25 am
Issued by Ben Bernall - Kootenai National Forest

Kootenai

bottom line

A subtropical storm will push through the forecast area on Tuesday and Wednesday bringing heavy mountain snow and strong southwesterly winds. This will create thick and reactive storms slabs to build above 4,500'. Expect avalanche danger to rise as this storm materializes.  Avoid riding on or below slopes steeper than 30° until this snow has had time to settle.

How to read the advisory

A subtropical storm will push through the forecast area on Tuesday and Wednesday bringing heavy mountain snow and strong southwesterly winds. This will create thick and reactive storms slabs to build above 4,500'. Expect avalanche danger to rise as this storm materializes.  Avoid riding on or below slopes steeper than 30° until this snow has had time to settle.

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: Storm Slab
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Mountain weather forecasts are calling for 1-2' of snow to fall along the Idaho/Montana border begining Tuesday through Wednesday morning. This snow will be dense, heavy and may come in with strong southwesterly winds. These conditions will be ripe for creating thick and cohesive storm slabs at elevations above 4,500'. Avoid riding on or below steep terrain (over 30°) until this new snow has had time to settle and strengthen.

Avalanche Problem 2: Persistent Slab
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We now have a couple of "persistent weak layers" in the snowpack at this time. There is a layer of buried surface hoar that is scattered across the area that was failing in pit tests with moderate force. In the thin snowpack of the Purcell range there is also a layer of facets (weak, sugary snow) that was also failing with moderate force. These weaknesses will become more likely to fail in the coming days as the snowpack takes on the added weight of heavy snow and rain. 

advisory discussion

There will be a Level 1 Avalanche Course for backcountry skiers and snowboarder on January 11th-13th. This course will be on the Kootenai and hosted by IPAC, to register visit idahopanhandleavalanche.org email Jeff Thompson at thompsonipac@gmail.com

 

recent observations

December 17th we travelled to Flatiron Mountain in the Purcell Range where we found a snowpack that was much thinner than the Western Cabinets. We found buried surface hoar on north-easterly aspects above 5,000' which was propagating in extended column tests with moderate force. There was also a layer of weak, sugary snow at the ground which was failing with moderate force. We did not observe either of these weaknesses last week in the Western Cabinets, I believe there are likely pockets of surface hoar buried out there; but, the thicker snowpack and moderate temps have kept the development of facets at bay in the Cabinets. In the past few days the mountains have been recieving multiple shots of snow that is coming in wet and with mild temperatures. This new snow has been bonding well to the existing snowpack; but, things may change with the storm on Tuesday and Wednesday. 

Check out the video below to see the snowpack on Flatiron Mountain!

<iframe width="560" height="315" src="https://www.youtube.com/embed/hixj3OSCrWs" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

 

Weather and CURRENT CONDITIONS
weather summary
Backcountry Forecast from NWS Missoula issued:
350 AM MST Tue Dec 18 2018

DISCUSSION: A relatively warm and weak storm system will move over
the Northern Rockies today, with 5000+ feet snow levels slowly 
decreasing down to 3500 feet through tonight. Low snow to liquid 
ratios will be found in the up to 6 inches of new snowfall. Some 
of the snow around 5000 feet will change between rain and snow at 
times. 

A more significant subtropical moisture surge will be present over
the region Tuesday through Wednesday. A strong jet stream overhead
will create a dynamic environment for 1 to 2 feet snow to fall
along the Montana-Idaho border and near to 1 foot new accumulation
for many other ranges across the region. Considering the origins
of the Tuesday moisture surge, snow levels will be pushed above
5000 feet for a time Tuesday evening. Again, relatively low snow 
to liquid ratios will be common throughout this snow event and the
rapid cooling during Wednesday could leave an icy layer on the top
of the snowpack in the 4000-5000 feet range.

Kootenai:
--------------------------- 5000-7000 FT ----------------------------
                      Today        Tonight      Wed      
Cloud Cover           95%          90%          75%      
Hi/Lo Temps           30 to 37     24 to 29     27 to 33 
Winds(mph)            SW 25G55     SW 28G55     SW 23G45 
Precip Chc            100          100          60       
Precip Type           sno/fzra     snow         snow     
Liquid Amt            0.97         0.34         0.09     
Snow Ratio(SLR)       11:1         12:1         15:1     
Snow Amt(in)          9-18         3-11         1-5      

Snow Level            4500         4000         3000     
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.