THIS AVALANCHE ADVISORY EXPIRED ON March 2, 2019 @ 4:49 am
Avalanche Advisory published on March 1, 2019 @ 4:49 am
Issued by Ben Bernall - Kootenai National Forest

Kootenai

bottom line

General snowpack stability is improving area wide on the Kootenai and the primary avalanche concern will continue to be windslabs this weekend. With southwest flow today followed by another incoming cold front expect to find new wind slabs developing on any aspect at upper elevations. The likely trigger spots will be below ridgelines, steep and cross-loaded chutes or steep convex rolls. 

How to read the advisory

General snowpack stability is improving area wide on the Kootenai and the primary avalanche concern will continue to be windslabs this weekend. With southwest flow today followed by another incoming cold front expect to find new wind slabs developing on any aspect at upper elevations. The likely trigger spots will be below ridgelines, steep and cross-loaded chutes or steep convex rolls. 

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

Earlier this week we had extensive northeast winds that developed firm slabs primarily on westerly aspects. These slabs are now hidden by 2-4" of fresh snow and have become stubborn and difficult to trigger. With light and cold snow resting on the surface and more moderate winds predicted from the southwest and northeast this weekend we will likely see some fresh windslabs develop above treeline on all aspects. With a cold front don't rule out the potential to find them below these elevations as well in open areas that are exposed to the effects of the wind. New slabs will likely be fairly thin and soft but may be quite reactive in areas where they will develop on top of firm surfaces (i.e.-old slab and crust layers).



Photos taken on 2/25 in Keeler Rattle area.

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

The "Persistent Slab Problem" will continue to persist with the cold temperatures that gripped the area this past week. The likely problem areas will be steep ribs or steep ridgeline features where the thinner snowpack is more susceptible to faceting that is driven by the cold temperatures. These thinner areas in the snowpack are where you are also likely to "trigger" a persistent weak layer. Use caution in the above mentioned features and be alert to this potential when riding in steep and rugged terrain of the Purcells (NW Peak/Rock Candy area). With pit results and observations this past  week are bumping the likelihood of triggering this layer down to unlikely.

advisory discussion

So, it's cold and windy. From and avalanche standpoint I would say that wind slabs in steep and open terrain continue to be your biggest concern. But, right now it is worth thinking about the fact that even a minor accident will have the potential to rapidly decline into a bad situation with the cold temps and wind chill factor. Bear this in mind when making your decisions on where to ride, who to ride with and any contingency items that you may want to have in your pack when Murphys' Law slaps you in the face.

As far as observations go: Our snowpack is getting thicker and generally we are under some pretty stable conditions area wide. Stability tests were performed in the Keeler area on Monday and in Northwest Peaks area on Thursday. In locations sheltered from the wind the general stability of the snowpack looks really good right now. I poked my nose out into some steeper and open terrain looking for some "worst case scenario" in the snowpack and found some weakness below the last stiff windslab layer. Two tests showed it failing with moderate force but it did not fully propagate. The shears are fairly clean but they also showed to be pretty resistant to moving. It is possible to trigger one of these older slab layers in isolated locations if playing in "extreme terrain" at upper elevations. The continuous winds last week created some very firm slabs and there are also some thin sun crusts on solar aspects that may provide a slippery bed surface. These surfaces will be the locations where the new wind-slabs will be most reactive and easily triggered. The Persistent weak layers are out there still but they will be stubborn and difficult to trigger. At this point I believe it is "unlikely" that you will trigger these deeper week layers. This can be bad thing as it often allows riders to get away with riding in terrain that may be suspect and lure them into a sense of false security. I would continue to approach steep terrain with caution in the Purcell Range, especially if it appears to have thin coverage. 

Have fun this weekend and don't get frostbite! 

Weather and CURRENT CONDITIONS
weather summary
Backcountry Forecast from NWS Missoula issued:
350 AM MST Fri Mar 1 2019

DISCUSSION: Light snow has already moved into northwest Montana
ahead of the arctic. This snow will be enhanced as the arctic
pushes into the region later this morning through the evening. A
secondary feature swings into the region from the west which will
cause scattered snow showers. These showers will dissipate after
sunset. The aforementioned arctic still appears to be quite deep
with easterly winds being experienced at elevations of 8000 to
9000 feet. Due to the high ratio snow with these strong winds
blowing and drift is likely for all locations. The east winds will
even make their way into Idaho by Friday evening.

Kootenai:
--------------------------- 5000-7000 FT ----------------------------
                      Today        Tonight      Sat      
Cloud Cover           90%          90%          65%      
Hi/Lo Temps           15 to 23     -10 to -1    4 to 13  
Winds(mph)            SW 10G29     NE 14G29     NE 13G25 
Precip Chc            60           60           0        
Precip Type           snow         snow         none     
Liquid Amt            0.11         0.05         0.00     
Snow Ratio(SLR)       21:1         20:1         0        
Snow Amt(in)          2-4          1            0        
Snow Level            0            0            0    
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.