THIS SNOWPACK SUMMARY EXPIRED ON December 24, 2017 @ 5:29 am
Snowpack Summary published on December 22, 2017 @ 5:29 am
Issued by Ben Bernall - Kootenai National Forest


bottom line:

CONSIDERABLE  avalanche danger means natural avalanches are possible and Human triggered avalanches are likely.

A thick slab layer overlying multiple persistant weak layers is still proving to be highly reactive despite having some time to strengthen.  Avoid travelling on or below steep terrain.  The thickness of this slab on the widespread weak layers will have the potential to create surprisingly large slides if triggered.

Avalanche Character 1: Persistent Slab
Persistent Slab avalanches can be triggered days to weeks after the last storm. They often propagate across and beyond terrain features that would otherwise confine Wind and Storm Slab avalanches. In some cases they can be triggered remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty.

The storm that rolled in earlier this week left us with a slab of snow overlying a multitude of weak layers.  This slab ranges in thickness from 1-3' across the forest.  There are a variety of weakness such as melt/freeze crusts, facets and buried surface hoar.  Although the weaknesses vary in type, they are consistently present and reactive on all aspects and elevations above 4,500'.  These weaknesses will take substantial time to heal.  Avoid traveling on slopes over 30° and remember that with a persistent slab problem it is very possible that you may trigger a slide from lower angle terrain that is below or adjacent to steeper slopes.

Avalanche Character 2: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

Some evidence of wind loading on south and westerly aspects has been observed at upper elevations and the approaching cold front will bring more wind out a northerly direction.  Look for pillowy appearance in the snow surface below ridgeline features and gullies.

recent observations

On December 21st we ventured into the Flatiron area of the Purcell Range.  As is often the case we found a thinner snowpack and abundant amounts of weak sugary snow on top of the Thanksgiving rain crust.  The most notable observations were the persistant and loud settling sounds from the snowpack on the way in.  Widespread whoomping and collapsing was present as the 2' thick slab settled on a thick layer of facetsStability tests revealed  a weak layer in the top 10-12" that was not fully propagating and should heal quickly.  The main layer of concern was the failure 2' below the surface where we have a thin melt freeze crust overlying about 6-7" of facets.  Below these facets sits the stout Thanksgiving rain crust.  ECT scores on this layer ranged from ECTP15-17.   The slab has strengthened and become less reactive; but, the take-home message here is that you can expect things to move in a big way on steep terrain when it does break

The red lines represent the thin melt freeze crust resting on a layer of weak facets (blue) resting on a thick rain crust.  You could call it an avalanche sandwich.

 Turner mountian ski patrol reported that there were some small soft slabs (8-10") that released and made small runs on steeper isolated slopes as well.

weather summary
Backcountry Forecast from NWS Missoula issued:
410 AM MST Fri Dec 22 2017

DISCUSSION: Snow is spreading into the region and the main change
was to linger snow through tonight and Saturday morning in north-
central Idaho and west-central Montana. Colder arctic air is 
still expected to bring increased northeasterly winds and colder 
temperatures. Single digits to below zero temperatures are 
expected by Sunday morning. Model guidance are converging on the 
timing of a Sunday afternoon through Monday weather system. This 
will again be a low density snow event with north-central Idaho 
and west-central Montana likely to see most of the snow, although, 
snow should again be widespread.


--------------------------- 5000-7000 FT ----------------------------
                      Today        Tonight      Sat      
Cloud Cover           95%          60%          10%      
Hi/Lo Temps           12 to 21     1 to 6       8 to 17  
Winds(mph)            NE 13G25     N 10G21      NW  6    
Precip Chc            70           0            0        
Precip Type           snow         snow         none     
Liquid Amt            0.08         0.00         0.00     
Snow Ratio(SLR)       20:1         21:1         0        
Snow Amt(in)          1-3          0            0        

Snow Level            0            0            0        


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.