THIS AVALANCHE ADVISORY EXPIRED ON January 18, 2020 @ 5:15 am
Avalanche Advisory published on January 17, 2020 @ 5:15 am
Issued by Ben Bernall - Kootenai National Forest

Kootenai

bottom line

Be on the lookout for the potential to trigger wind slabs in steep terrain below ridgelines and cornices. There is extensive evidence of wind loading in the mountains and these wind slabs are becoming less reactive with time. Recent Storm snow is quickly settling and gaining strength but still has potential to fail at upper elevations. 

How to read the advisory

Be on the lookout for the potential to trigger wind slabs in steep terrain below ridgelines and cornices. There is extensive evidence of wind loading in the mountains and these wind slabs are becoming less reactive with time. Recent Storm snow is quickly settling and gaining strength but still has potential to fail at upper elevations. 

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
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The last storm snow came in with consistent south and west winds. Slopes facing north and east have been loaded in and will likely harbor wind slabs just below ridgelines and in steep gullies where the wind has "cross-loaded" the slope. Be alert for cracking and density changes in the snow surface as these will be likely clues to the presence of wind slab. Avoid riding on steep slopes that are directly under cornices or steep rolls as these will be potential trigger points.

 

 

Avalanche Problem 2: Storm Slab
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 This weeks stability tests and observations showed very little reactivity in the new storm snow.   On Thursday the storm snow was light and cold at lower elevations. Above 5,200' the temps were warmer and the affects of light winds had caused the new snow to form a bit of a slab on the surface. These slabs were thin and showing low likelihood of propagating into an avalanche.

advisory discussion

This past week I have visited the Purcells and the East Cabinets. The consistent problem that jumps out at me is the extensive wind loading and cornice development that has transpired in the past week. Yesterday the wind slabs felt much less reactive than they were on Monday and I observed numerous spots where many of them had released naturally under their own weight. All of the natural cornice fall and small windslab slides appeared to be multiple days old. I think this problem has the most potential in very steep terrain that has been exposed to the wind and cross-loaded. Steep chutes above treeline come to mind!



Photo depicts wind loading and cornice development overhanging north and east facing terrain.

The last couple shots of storm snow have bonded well and settled out quickly based on my observations this week. There were a few failures in compression tests but nothing that showed the potential to propagate and cause a major concern. There are a couple layers of graupel that fell last week I have been watching but they are now over 3 feet deep and showed no failures in pit tests this week. One of the more interesting things I observed yesterday was a major temperature inversion in the East Cabinets. Valley bottom temperatures where in the high teens which kept the storm snow light and fluffy. As we broke out above the valleys the temperature increased up to the freezing mark and could be easily felt in the snowpack underfoot. The snow had a "slabby" feel in the top 4-6 inches at upper elevation but it really lacks all the necessary ingredients underneath to cause an avalanche in most locations. There are spots that this new snow may have fallen on a denser surface which will up it's reactivity and potential, south and west aspects in open areas above treeline are areas where you want to give the new snow a closer look before jumping in.

As far as "Persistent Weak Layers" go, there are still some out there but I am not getting concerning results out of them. They are pretty deep in the snowpack and well bridged at most locations. Crust layers and deep faceted snow is still evident in the Purcell Range and there are multiple crust layers, graupel and old buried surface hoar across the area. All of which are unlikely to cause a problem at this point unless we have another big rain on snow event at upper elevations. I am always cautious of steep rocky terrain in the Purcells that may be thin enough to trigger a deeper weakness; again, it's a low-likelihood scenario but worth paying attention to these weaknesses moving forward.

Join us in Libby at the Forest Supervisors office Friday night at 5 PM for a 4 hour presentation on avalanche safety. January is a great time for a refresher!  See you there.

Weather and CURRENT CONDITIONS
weather summary 
Backcountry Forecast from NWS Missoula issued:
340 AM MST Fri Jan 17 2020

DISCUSSION:  
NORTHWEST MONTANA: Widespread light to moderate snow will quickly
pass through this morning, followed by afternoon showers on the
higher terrain. Another round of light snow is likely on Saturday. Snow
ratios will be roughly 20:1. Westerly winds will also be notably
stronger today compared to yesterday. One thing to note will be 
the reintroduction of arctic air just east of the Divide. It looks
like it will creep along the Rocky Mountain Front but not make 
its way west of the Divide, although JFS Canyon and GNP mountains 
could feel a little of its presence (in the form of winds and 
dropping temps) today into Saturday. Beyond Sunday, a ridge of 
high pressure will build in, but will have high thin clouds 
littering it by Tuesday.
Kootenai:
--------------------------- 5000-7000 FT ----------------------------
                      Today        Tonight      Sat      
Cloud Cover           60%          80%          100%     
Hi/Lo Temps           18 to 25     7 to 15      21 to 28 
Winds(mph)            SW 17G32     SW 12G24     SE 11G25 
Precip Chc            0            20           90       
Precip Type           snow         snow         snow     
Liquid Amt            0.00         0.03         0.12     
Snow Ratio(SLR)       21:1         21:1         21:1     
Snow Amt(in)          0            0-3          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.