THIS AVALANCHE ADVISORY EXPIRED ON December 22, 2018 @ 5:11 am
Avalanche Advisory published on December 21, 2018 @ 5:11 am
Issued by Ben Bernall - Kootenai National Forest

Kootenai

bottom line

Strong southwesterly winds and a dose of fresh snow have created sensitive wind slabs on leeward terrain above treeline. Avoid steep slopes (over 30°) on north-east aspects below ridgelines and cross loaded gullies. Pay attention to changes in surface density and watch for "pillows" and "lens shaped" features in the upper snowpack. These windslabs will likely be sensitive through the weekend.

How to read the advisory

Strong southwesterly winds and a dose of fresh snow have created sensitive wind slabs on leeward terrain above treeline. Avoid steep slopes (over 30°) on north-east aspects below ridgelines and cross loaded gullies. Pay attention to changes in surface density and watch for "pillows" and "lens shaped" features in the upper snowpack. These windslabs will likely be sensitive through the weekend.

3. Considerable

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

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.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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On Thursday (Dec. 20th) strong winds entered the area and steadily increased through the night. With fresh snow available for transport it is likely that you will trigger a windslab in steep, leeward terrain over the next few days. Use caution on the northeasterly side of any ridgeline that is exposed to the wind, it would be prudent to avoid these locations all together through the weekend.

Check out this 2 minute tutorial below on wind slabs and how to avoid them!

<iframe width="560" height="315" src="https://www.youtube.com/embed/pHipKLqcO54" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

 

Avalanche Problem 2: Storm Slab
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Tuesday through Wednesday brought 10-12" of snow to the mountains above 5,000' and we picked up an additional 4-5" Thursday night across the forecast area. The snow-fall came in warm and moist and proved to be bonding well in yesterdays observations; however, failure is still a possibility in steep terrain.

Avalanche Problem 3: Persistent Slab
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There are two persistent weak layers in the snowpack and they are most likely to be found in the Purcell Mountains (Flatiron/Buckhorn/NW Peaks area). The first of note is the buried surface hoar layer and the second would be weak sugary snow at the base of the snowpack. The recent warm, wet weather followed by cooling temperatures has done a good job of stabilizing these weaknesses. There is a low likelihood of triggering these layers right now; but, it is possible, particularly at upper elevations and thin spots within the snowpack.

recent observations

December 20th we travelled into the East Cabinets. The snowpack was pretty firm with a layer of dense powder over a supportive rain crust up to 5,000' in elevation. Above this elevation the crust dissipated into fairly dense powder.  The warm nature of the mid-week storm followed by dropping temperatures allowed the snow to bond fairly well.  Stability tests confirmed the snowpack was gaining strenth and we observed no natural avalanches in the steep terrain of the Eastern Cabinets. Again, the important take home message is the wind. Readings taken from my Kestrel at 2PM were steady at 15-20 mph with loose snow being transported into leeward terrain. It continued to increase through the night as more moisture began to fall with these strong winds.

Weather and CURRENT CONDITIONS
weather summary
Backcountry Forecast from NWS Missoula issued:
340 AM MST Fri Dec 21 2018

DISCUSSION: Observations this morning for elevations roughly
around 4000 feet and higher are showing varying gusts in the 30 to
50 mph range, with the expectation of winds continuing in this
manner through much of the day today. Winds will stay the number 
one backcountry concern, particularly as they combine with new 
snowfall amounts. The Rattlesnake, Bitterroots, and Clearwater 
mountains all stand the best chance at receiving the heaviest 
precip through the morning and early afternoon hours. More 
convective, showery type of snow will then persist through 
tonight. A pesky, light upslope component over the Whitefish and 
Swan/Mission region of NW MT may keep lingering snowfall in these 
regions going longer than our forecast may indicate. Watch out for
this...it can sneak up on you.

A break in the action will occur Saturday before the next round of
weather settles in Sunday-Monday. This will be the pattern coming
up: quick-moving shortwaves moving through the flow pattern next
week, dumping light to moderate snow amounts as they go. 
Confidence is high, particularly in a colder air mass 
accompanying the moisture during this time.


Kootenai:
--------------------------- 5000-7000 FT ----------------------------
                      Today        Tonight      Sat      
Cloud Cover           75%          65%          50%      
Hi/Lo Temps           22 to 29     14 to 20     20 to 26 
Winds(mph)            SW 28G56     SW 15G35     SW 12G28 
Precip Chc            60           0            0        
Precip Type           sno/shr      sno/shr      sno/shr  
Liquid Amt            0.07         0.00         0.00     
Snow Ratio(SLR)       18:1         18:1         19:1     
Snow Amt(in)          1-3          0            0        
Snow Level            1000         500          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.