Medicine Bow Nordic Ski Patrol
Patrol Log: Sun. December 13, 2009: N.
Mathsion / L. Finley-Blasi
N. Mathison
Just FYI for the folks in the back country.
While on the search this last weekend, and at the ski area on Friday, I
was feeling some definite “whumpfing” with the wind drifted snow.
Several spots were collapsing under my weight for about a 12 foot
circle. The snowmobile and my weight was more constant collapsing in
smaller areas. I assume this had something to do with the added weight
of the machine.
While I did not have the time to dig a pit I felt that there was a wind
crust with softer snow or sugar underneath on the north to northwest
facing slopes.
More level country with no wind crust was firm feeling snow while on
the snowmobile, but when getting off of the machine, I several times
sank to my waist right beside the snowmobile.
Hopefully warmer weather will stabilize the snow a bit but I would
suspect the snow of being unstable especially where there is evidence
of wind loading.
Has anyone dug a pit in the high country yet?
I see that Southern Montana has issued a high avalanche danger warning
and has had one avalanche fatality involving an ice climber near
Bozeman already this year.
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L. Finley-Blasi
I was in the backcountry on Sunday and felt/heard some of the same
things that Neil mentions in his note. I was with two other people and
we were attempting to find the old ski area off of Barber Lake Rd. Our
approach took us down the path of the poker run. Above the private
cabins we departed from the trail and crossed Libby Creek towards a
treed slope we thought looked promising. As we approached the
ridge-line we felt many "whumpfs" . One large whumpf on a shallow open
area (approx. 30x60 ft.) had small shooting cracks and shook all the
trees within it. The decision to dig a snow pit was unanimous.
Snow pit summary:
Location: Just down from the ridge on a northwest facing slope in the
trees.
Elevation: approximately 9600 ft.
Snow pack: approx. 3ft. total depth,
1) top 6-8 inches of very new snow that was light and fluffy
2) next 6-8 inches (going down) of consolidated fine-grained new-ish
snow
Note: contrast in consolidation created potential failure plane between
layer 1 and 2
3) next ~12 in. of large (0.5 cm. max.) "sugar" size grains, very
light, lots of air space, very loose
Note: It is highly likely that layer 3 is what is compacting and
creating the "whumpfing" sound.
4) bottom foot of consolidated sugar grains appearing to be fairly
stable
No uniform stress tests were performed.
We decided to spread out and descend within the trees. Upon descent it
was found that shooting cracks and whumpfing were ubiquitous features
of the snow pack. Slabs were prone to break, and slide a few inches.
Though nothing moved very much, it was decided to abandon the decent
and return to the ridge. A shallower (sorry, no quantitative values)
slope with about 4 inches of snow was chosen for descent. My skis have
the core shots to prove it.
Almost every open area we passed through that day made the whumpfing
sound. I heard more whumpfing that day than in all my previous
experiences combined. I'm tempted to call the sugar crystals depth
hoar, and site the -10 degree temperatures for their formation.
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