THIS AVALANCHE ADVISORY EXPIRED ON January 1, 2020 @ 5:33 am
Avalanche Advisory published on December 31, 2019 @ 5:33 am
Issued by Ben Bernall - Kootenai National Forest

Kootenai

bottom line

Our current snowpack conditions are very stable but that will begin to change by Tuesday night. A winter storm will bring 1-2 feet of fresh snow and strong winds out of the southwest which will cause avalanche hazards to rise by Wednesday. Be on the watch for fresh storm slabs and wind slabs at tree-line and above throughout the week!

How to read the advisory

Our current snowpack conditions are very stable but that will begin to change by Tuesday night. A winter storm will bring 1-2 feet of fresh snow and strong winds out of the southwest which will cause avalanche hazards to rise by Wednesday. Be on the watch for fresh storm slabs and wind slabs at tree-line and above throughout the week!

1. Low

?

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

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.
    Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
Avalanche Problem 1: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

The weak layer at the base of our snowpack continues to show gains in strength and is unlikely to create a human triggered avalanche at this time. This problem has some potential in terrain above 6,000' and potential trigger points will be steep and rocky terrain with a thin snowpack.

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

We have been observing lots of wind and evidence of slab development in the higher terrain of the West and East Cabinets the last two weeks. Yesterday we found that they are unlikely to react and fairly small in size. This will begin to change by Wednesday as another storm with strong southwest winds enters the area. Expect windslabs to become a problem by Wednesday on north and east aspects at treeline and above.

advisory discussion

On December 30th I toured into the East Cabinets with the goal of assessing the wind slabs at upper elevations and to check on the strength of the weak crust/facet combo at the base of our snowpack. The deep weak layer we have been looking at since the begining of the season is continuing to improve and become more difficult to trigger. We observed lots of older evidence of avalanches on this weak layer that occurred during the warm and rainy weather that rolled in on December 20th. Much of the steep terrain below 6,500' had slid on this weak layer leaving behind large piles of debris and setting the snowpack back to almost nothing in these locations. Stability tests above 6,500' are showing very little reactivity on this layer but every so often it rears it's ugly head to show some potential, at this point the most likely trigger will be another rain on snow event. Outside of this scenario it is unlikely to fail. At upper elevations we spent alot of time checking out the wind slabs on all aspects and found very little to be concerned about, overall conditions were very stable on all aspects and elevations. BUT. My wife called me the "King of Caveats" the other day. As an avalanche guy I consider caveats an integral part of my life. Things will change, as they always do. 

The weather forecast is calling for multiple shots of snow throughout the week with strong southerly winds that are likely to cause a spike in avalanche hazard. The snow looks to be coming in with lighter snow densities on Tuesday then transitioning to heavier snow on Wednesday night. This will cause an upside down scenario in the upper snowpack and create a stormslab issue that will be with us throughout the week. Much of the current snow surface has a crust layer and we were seeing a fresh growth of surface hoar below treeline, these conditions are poor surfaces for the new snow to bond to. And, as mentioned earlier, more wind to add to the problem. 

So, don't let the LOW hazard rating on Tuesday cause you to drop your gaurd. Bring your A-Game to the mountains and be ready for changes, because you can always rely on changes!

 

Weather and CURRENT CONDITIONS
weather summary 
Backcountry Forecast from NWS Missoula issued:
340 AM MST Tue Dec 31 2019

DISCUSSION: An atmospheric river will bring heavy snowfall across 
mountain locations Tuesday into Wednesday. The heaviest 
accumulations will impact the Clearwater, Bitterroot, Swan, and 
Mission mountains where several feet of snow is expected. Snow
densities initially will be on the low side, with ratios of 
15-20:1 this morning, before increasing this afternoon into 
Wednesday morning with warmer air and strong winds up to 40 mph. A
few snow showers could produce graupel Wednesday afternoon as the
atmosphere becomes unstable and convective. Flow will turn out of
the northwest Wednesday night into Thursday. Widespread snow 
shower activity is expected for the Bitteroot, Clearwater, and 
Mission mountains, along with higher snow ratios (~20:1).


Kootenai:

--------------------------- 5000-7000 FT ----------------------------
                      Today        Tonight      Wed      
Cloud Cover           90%          95%          90%      
Hi/Lo Temps           27 to 33     24 to 30     29 to 35 
Winds(mph)            SW 20G48     SW 29G57     SW 24G53 
Precip Chc            100          100          80       
Precip Type           snow         snow         snow     
Liquid Amt            0.38         0.72         0.22     
Snow Ratio(SLR)       16:1         11:1         15:1     
Snow Amt(in)          5-12         6-15         2-8      
Snow Level            2000         3000         3500
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.