THIS SNOWPACK SUMMARY EXPIRED ON March 10, 2017 @ 3:00 pm
Snowpack Summary published on March 9, 2017 @ 10:00 pm
Issued by Ben Bernall - Kootenai National Forest
Avalanche Character 1: Storm Slab
Storm Slab avalanches release naturally during snow storms and can be triggered for a few days after a storm. They often release at or below the trigger point. They exist throughout the terrain. Avoid them by waiting for the storm snow to stabilize.

The National Weather Service is forecasting another wallop of snow to land in the mountains of Northwest Montana beginning tonight (Thursday) and continuing into Friday.  This additional rapid loading of snow will cause an increase in sensitivity of any existing weak layers within the snowpack.

BOTTOM LINE: Backcountry travellers should exercise caution by choosing low angle terrain until this new snow has had time to settle and strengthen.  There will be a high risk of natural and human triggered avalanches throughout the weekend due to this rapid loading of storm snow.

 

Avalanche Character 2: 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 snowpack of the Purcell Range (i.e. Buckhorn/Yaak/NW Peaks) is still harboring some persistant weaknesses that have gained strength since last weak but still remain an issue for backcountry travellers.

1. Februrary 16th rain crust: Due to the colder temperatures of the Purcell Range this rain crust layer is not likely to be found above 6,500' in elevation.  Below this altitude the snow that sits on this crust layer is very reactive and likely to propagate in steep open terrain.

2. The buried surface hoar layer can still be seen but is no longer proving to be reactive in stability tests.

BOTTOM LINE: The stability of the Purcell snowpack has improved considerably during the past week; however, with the incoming weather predicted I would still urge caution in terrain and travel selection until the new storm snow has had time to settle.

Snowpack Discussion

Check out the forecast for the Idaho Panhandle by clicking the link below!

http://www.idahopanhandleavalanche.org/selkirks-cabinets/advisory#null

recent observations

Today we travelled back into the Pete Creek drainage to check on the progress of the weak layers we found in last weeks stability tests.  The buried surface hoar layer was found in 2 of 3 pits but showed no reactivity in stability tests.  The new snow is still bonded poorly to the rain crusts and is quite reactive in pits  below 6,000'.   The base of the Purcell snowpack consists of facets and depth hoar that developed during the early part of the winter when the snow was thin and the temps were cold.  This weakness is likely to stick around through the winter and is unlikely to be a problem to riders unless a thin weak spot is found.  It would be prudent to avoid areas where the snow is thin and scoured as this would be the most likely place to trigger and avalanche on this weak layer.  Avoid areas where you see rock sticking out of the snow.  Remember, thin snow is weak snow!

weather

Backcountry Forecast from NWS Missoula issued:

220 PM MST Thu Mar 9 2017



DISCUSSION: Widespread light snow down to 1500 feet this morning

will slowly rise throughout today, particularly for all ranges

south of I-90.



Significant heavy snow remains in the forecast for northwest

Montana ranges, particularly in the vicinity of Glacier Park, this

evening through Friday. An arctic airmass will move west of the

Continental Divide today and stall along the Canada border today.

The stalled edge of the arctic air will be a focal point for

periods of intense snowfall. Up to 2 feet accumulation is expected

along the Continental Divide. The arctic air will slowly retreat

east late tonight, with snow levels and snow density increasing

over time (i.e., upside down snow pack). Wind will also be a

factor right along the Continental Divide, where east winds

gusting to 30 mph will transport fresh snow to west facing slopes.

Large fluctuations in temperatures will add to dangerously

variable backcountry travel conditions.




Conditions for backcountry travel improve this weekend with high

pressure overhead, however it is not necessarily a dry forecast.

Snow levels will increase substantially through the weekend,

reaching 5000 feet at times. Additional storm systems will bring

more snow, mostly above 5000 feet, through the next week
. Mountain

snow pack will continue to steadily increase.




Kootenai:



--------------------------- 5000-7000 FT ----------------------------

                                 Tonight          Fri                 Fri Night       Sat     

Cloud Cover              95%               90%              75%              80%     

Hi/Lo Temps             21 to 30        34 to 39        22 to 27        32 to 38

Winds(mph)              SW 11G31     SW 21G47     SW 13G33     S  7    

Precip Chc                100                100               30                 40      

Precip Type              snow              snow             snow            snow    

Liquid Amt               0.77                0.49              0.01              0.05        

Snow Amt(in)           9-11                5-8               0                   0-1     

Snow Level             1000                3000            4500             3000  

Disclaimer

This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires midnight on the posted day unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.