THIS AVALANCHE ADVISORY EXPIRED ON March 17, 2018 @ 5:45 am
Avalanche Advisory published on March 16, 2018 @ 5:45 am
Issued by Ben Bernall - Kootenai National Forest

Kootenai

bottom line

There is potential to trigger deep weak layers on north aspects above 6,000', this potential is more prevalant in the Purcell Range. There are also large and unpredictable cornices looming above northeast aspects that should be avoided as the recent warm temperatures have caused them to weaken over the past weak.  Continue to pay attention to the weather as any rapid warming or sunshine will increase the likelihood of avalanches.

How to read the advisory

There is potential to trigger deep weak layers on north aspects above 6,000', this potential is more prevalant in the Purcell Range. There are also large and unpredictable cornices looming above northeast aspects that should be avoided as the recent warm temperatures have caused them to weaken over the past weak.  Continue to pay attention to the weather as any rapid warming or sunshine will increase the likelihood of avalanches.

2. Moderate

?

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

1. Low

?

Near Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

1. Low

?

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: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

There is still some potential to trigger the weak layer resting on the February 4th rain crust on northerly aspects above 6,000'. This problem is primarily a concern in the thinner snowpack of the Purcell Range where it can be found 32" or deeper below the surface. Triggering this layer would take alot of force but it will produce large avalanches if triggered, such a trigger would include a breaking cornice (see Erics write-up for the Panhandle) or a snowmobile hitting a thin spot in the snowpack. Continue to use caution on steep northerly aspects above this elevation. There is also a layer of buried surface hoar lurking 8" below the surface in sheltered locations on these same aspects. Stability tests are showing that it is unlikely to propagate but it is worth paying attention to as it may be reactive in isolated locations.



Pit on Flatiron Mt. 6,000' Sheltered North aspect. Surface hoar 8" deep. February 4th crust 32" deep. 

Avalanche Problem 2: Cornice
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Large cornices are precariously perched on the ridgelines above treeline on northeasterly aspects. The recent warm temperatures are causing these features to slowly pull away and break. There is no crystal ball to tell us when they will fail; but, they are fairly easy to avoid. Avoid being underneath them on warm days and pay attention to where you are when travelling on ridgeline features. A cornice break right now has the potential to trigger the deeper weak layers on north aspects and create a large slide as was observed on Jeru Peak in the Panhandle.



Cornice pulling away from ridgeline at 6,500' in East Cabinets.

 

advisory discussion

On March 15th I went to Flatiron Mountain in the Purcell Range to see how things were progressing in the thinner snowpack of the Yaak country. Stability tests revealed 3 weak layers in the snowpack on north aspects, the primary concern for me is still the deep weak layer of graupel and facets that is resting on the February rain crust.  We have been talking about it for awhile now as a low likelihood-high consequence problem.  It has been a few weeks since this layer has shown any reactivity in the thick snow of the Cabinet Ranges where it is buried 5-6' deep, the Purcells however are a different animal. It is much closer to the surface and the weak grains are slow to heal. Below treeline the cycle of warm days and cold nights have done an excellent job of stabilizing the snowpack. Most of the loose snow that we recieved last Friday has been reduced to a layer of crustiness and slop that makes for safe (and not very fun) skiing conditions.

On Tuesday March 12th we travelled into the Eastern Cabinets. On the way in we had a good look at multiple aspects in the high terrain and I was quite surprised by the lack of "loose-wet" slide evidence despite the recent warm temperatures. It seems as though the high winds on Friday had stripped most of the new snow off of the southwesterly aspects down to the firmer layers below. For this reason most of the "loose-wet" slide activity was observed on easterly slopes where the new snow had been deposited, these slides where mostly originating below steep and rocky terrain. The recent storm snow and wind slab is showing a low likelihood of triggering in stability tests. We did observe some thin crowns in very steep, leeward terrain that appeared to be a couple of days old.

Climbing into the high terrain above treeline we are seeing an abundance of "glide cracks" opening up in areas where the snowpack is resting on rock slabs or smooth bed surfaces of bear grass. There is also some fairly significant cornice development hanging over the northeast aspects. The cold nights will help keep these issues locked together; but, as I mentioned earlier, expect them to start moving during the warm afternoon hours. If we get into a cycle of continuous warm weather or mountain rains the probability of these problems coming to life will increase. Again, the best way to deal with this problem is to avoid being underneath of it.

 

Glide cracks on easterly aspect at 6,500"                                                                        Cornices hanging off of ridge and glide cracks on face.                                                           

Weather and CURRENT CONDITIONS
weather summary
Backcountry Forecast from NWS Missoula issued:
500 AM MDT Fri Mar 16 2018

DISCUSSION: A subtle disturbance is causing some light snow along
the Sapphire Range up into the Seeley/Swan region this morning. 
Accumulations associate with this feature is anticipated to be 
less than an inch. A second feature will be making its way into 
area this afternoon/evening. Isolated to scattered snow showers 
will be generated by this disturbance, but most of the activity 
will be south of the I-90 corridor.

The next biggest impact will be from a mid level circulation that
will be moving through the region late tonight through Saturday.
This feature will cause widespread snow with some mountain ranges
potentially getting up to a foot of new snow. The last model runs
have shifted the track of this feature farther to the west. Now
the crest of the Bitterroot Range appears to receive the heaviest
snow amounts. Confidence is moderate on this current path. 

Kootenai:
--------------------------- 5000-7000 FT ----------------------------
                      Today        Tonight      Sat      
Cloud Cover           65%          45%          45%      
Hi/Lo Temps           36 to 41     23 to 26     36 to 42 
Winds(mph)            SE  4        E  5         NE  7    
Precip Chc            0            0            0        
Precip Type           none         none         sno/shr  
Liquid Amt            0.00         0.00         0.00     
Snow Ratio(SLR)       0            0            15:1     
Snow Amt(in)          0            0            0        
Snow Level            2500         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.