THIS SNOWPACK SUMMARY EXPIRED ON December 16, 2016 @ 11:38 pm
Snowpack Summary published on December 16, 2016 @ 6:38 am
Issued by Ben Bernall - Kootenai National Forest
Avalanche Character 1: 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.

This problem is likely to increase over the weekend due to an increase in predicted winds at upper elevations.  Backcountry travelers should exercise caution on steep exposed slopes below any ridgeline feature.  Pay close attention to changes in snow consistency and look for obvious signs of instability. Also, the upper 12 to 16 inches of the snowpack consists of very light and unconsolidated snow overlying a thin layer of buried surface hoar.   We found this layer to be a very isolated concern yesterday, but a thin wind slab was beginning to form on top of this layer at upper elevations.



Wind slab formation on Bear Mountain in the West Cabinets. 6,000'

Snowpack Discussion

On Thursday, December 15, my partner and I traveled to Bear Mountain in the Keeler/Rattle drainage.  We dug 3 separate pits on multiple aspects where we gathered very consistent results from the snowpack.  All stability tests failed at a density change in the snowpack that is seperated by a thin layer of buried surface hoar.  The upper 12" to 16" failed with hard force and did not propagate in extended column testing due to the loose and unconsolidated nature of this upper layer.  As we traveled across  the ridgeline we observed a thin windslab (4" thick) begining to form.  This wind slab was not widespread across the landscape and was not reactive to ski cutting.  As mentioned in the "Problems" section, I would expect this layer to increase in sensitivity as north east winds further consolidate the upper snowpack going into the weekend.

For the winter of 2016/2017 snowpack observations will be performed on Mondays and Thursdays, the "Snowpack Summary" will be posted the following morning.

recent observations

We greatly value observations from the public. They help provide a more accurate snowpack summary. If you are out and about, we'd love to know what you are seeing. You can click on the links to the left to submit your observations, or simply email fac.admin@flatheadavalanche.org. Remember, observations need not be complex. They can be as simple as "Hey, we snowmobiled over 60 miles in Area A and didn't see any avalanches anywhere, and it was a really clear day." Thanks!

Below are recent observations from the Flathead Avalanche Center advisory area (Whitefish Range, Flathead Range, northern and central Swan Range, and southern Glacier National Park). 

weather

Previous 48 hours:

SNOTEL sites on the Kootenai N.F. have recorded low temperatures at 2-3 degrees Fahrenheit and max daytime temperatures of 11 to 14 degrees.  Recorded snowfall at these sites ranges from 1-3 inches, yesterdays winds were out of the north east at 1-3 with gust to 8 mph.

Backcountry Forecast from NWS Missoula issued:

350 AM MST Fri Dec 16 2016




DISCUSSION:

An arctic airmass will continue to move into the Northern Rockies

today causing easterly to northerly winds. These winds will

causing blowing and drifting through early afternoon. Lemhi county

into southwest Montana will experience light to moderate snowfall

from wrap around precipitation. Temperatures drop dramatically

tonight. An active weather pattern continues through next week.

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.