THIS AVALANCHE ADVISORY EXPIRED ON January 29, 2020 @ 5:29 am
Avalanche Advisory published on January 28, 2020 @ 5:29 am
Issued by Ben Bernall - Kootenai National Forest

Kootenai

bottom line

Expect to trigger windslabs on steep and open terrain on all aspects. Wind slabs will be most dangerous in cross-loaded gullies, chutes and leeward terrain below ridgelines where the surface snow has been deposited into a deep and cohesive slab. This problem can be avoided by staying in the sheltered timber locations and avoiding the steep and open areas through the week. This will also give the new storm snow time to settle out and bond with the existing snowpack.

How to read the advisory

Expect to trigger windslabs on steep and open terrain on all aspects. Wind slabs will be most dangerous in cross-loaded gullies, chutes and leeward terrain below ridgelines where the surface snow has been deposited into a deep and cohesive slab. This problem can be avoided by staying in the sheltered timber locations and avoiding the steep and open areas through the week. This will also give the new storm snow time to settle out and bond with the existing snowpack.

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: Wind Slab
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Strong and steady south winds were developing wind slabs on all aspects in open terrain above 5,500' on Monday and will continue to build on Tuesday as more snow and wind makes its way through the mountains. We observed thin and reactive slabs on aspects directly exposed to the wind (south & west) and deeper pockets of dense and reactive snow in leeward terrain (north & east). Areas of particular concern were "cross-loaded" features, gullies, chutes and shallow depressions where the snow had filled in on steep and open slopes. 

Avalanche Problem 2: Storm Slab
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Up to a foot of new snow is forecast to fall on the mountains by the end of the day on Tuesday. In many mountain locations this new snow will be falling on a very consolidated snowpack that was covered with about 4 inches of light and loose snow. So, we have good firm bed surface, a nice layer of light fluffy powder and a storm blowing in. The wildcard here is how dense this new snow will fall on the existing snowpack. Pay attention today to how the new snow is interacting with the existing snowpack. The instabilites should be easy to see and close to the surface. If you are seeing a dense and cohesive layer that shears easily off the top then you will want to avoid steep terrain.

advisory discussion

I will start with the good news. The existing snowpack that we have been observing lately is very cohesive and well consolidated due to the warm temperatures and consistent moisture that has graced the area during the past week. The avalanche instabilities we are seeing are all close to the surface and a result of wind and fresh snow. This makes them much easier to spot without a whole lot of digging. Wind slabs will cut loose easily on steep terrain below cornices and steep rolls. If storm slabs are developing at your location you will be able to see them in the top of the snowpack as you travel, take note of how the snow reacts when you cut a high bank on your sled or make a switchback on skis.

Here's what we observed yesterday in the East Cabinets. Travelling in from the drainage bottom we were immediatley blasted by strong southerly winds and steady snowfall that persisted through the day. As we climbed above 5,500' we transitioned from a very consolidated snowpack with a light dusting of snow to a consolidated snowpack with reactive windslabs. The windslabs were observed on all aspects and released under skis on any steep terrain that had a convex surface or wind lip. With a firm bed surface on south and west aspects they were thinner but very weak and reactive. Wrapping around into depressions, and northeasterly locations the drifting was much deeper and resulted in large shooting cracks that were surprisingly deep in spots. 

 



Video of Nate demonstrating the reactivity of developing wind slabs on steep rolls, South east aspect at 5,500'.

For me the big question mark out there today will be the storm snow. Yesterday we observed a pretty firm snowpack that was dusted with a fresh layer of light new snow. How the incoming snow falls through today will determine the storm slab hazard. The weather forecast is showing a low snow-to-liquid ratio (SLR) and mild temperatures. This will likely result in snowfall that falls on the dense and heavy side; thus, potentially creating a slab on the surface. But, I'm inside looking at a computer right now and I left my crystal ball at the house. So, just pay attention out there as you travel and do some quick shear tests in the new snow to check the interaction with the old snow. 

Weather and CURRENT CONDITIONS
weather summary 
Backcountry Forecast from NWS Missoula issued:
400 AM MST Tue Jan 28 2020

DISCUSSION: 

Northwest Montana: Light to moderate snow is currently falling
along the Idaho/ Montana border, snow totals by Wednesday morning
will be up to a foot along the Idaho border and for the Swan and 
Mission Ranges. This active weather pattern will persist all week,
with multiple systems affecting the area. After Wednesday westerly
flow will cause snow levels to rise to 6000 feet by Saturday. 
Sunday will see a change with a very strong cold front, dropping 
temperatures and snow levels.

Kootenai:
--------------------------- 5000-7000 FT ----------------------------
                      Today        Tonight      Wed      
Cloud Cover           95%          85%          85%      
Hi/Lo Temps           28 to 34     21 to 28     28 to 34 
Winds(mph)            S 15G25      W 10         SW 14G32 
Precip Chc            90           60           50       
Precip Type           snow         snow         snow     
Liquid Amt            0.41         0.09         0.06     
Snow Ratio(SLR)       15:1         16:1         15:1     
Snow Amt(in)          5-12         1-5          0-4      
Snow Level            3500         3500         2500
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.