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INTRODUCTION:
This is Stan Bones with the backcountry avalanche advisory issued
Tuesday, March 18, 2008, at 5:00 AM by the U.S. Forest Service Glacier
Country Avalanche Center for the Glacier Park and Flathead and Kootenai
National Forest areas.
HAZARD ANALYSIS
Between 5,500' and 7,500' elevation the avalanche danger is rated
MODERATE, with a special caution for buried surface hoar on northerly
aspects. Below 5,500 ' we're rating the avalanche danger LOW.
These danger ratings expire at midnight Tuesday, March 18th. The
outlook is for the avalanche danger to remain unsettled through the
week as new snowfall, moderate winds, and cooling temperatures impact
the region.
Because of the general nature of this advisory message, each
backcountry party will always need to make their own time and site
specific avalanche hazard evaluations.
WEATHER ANALYSIS:
Weather over Northwestern Montana this past week was a bit of in and
out. Last Thursday generally dawned clear and cold across the
region. By mid day sunshine quickly pushed air temperatures to above
freezing, especially on S-facing slopes. Friday night and into
Saturday morning a NW flow of cooler and more moist Pacific air
produced 3-6" of new snowfall at most mtn locations. Another front
Sunday pushed temps down a bit further into the mid to low 20's and
added another 2-4" of new snow depth by Monday morning. At the mtn
peak and ridge top levels Sunday night and Monday, winds were
moderately strong blowing mostly from the SW with some local
topographic channeling.
SNOWPACK ANALYSIS:
Our backcountry observations Monday were on Banfield Mtn in the Purcell
Range on the Kootenai, N of Libby and in the Cilly Cr area in the Swan
Range, SE of the town of Swan Lake. We received two reports on
Saturday from the Cascadilla -Wahoo area above Hwy 2 in the Middle Fork
of the Flathead drainage and another report on Sunday from the Graves
Creek-Jewel Basin area in the Swan Range, NE of Bigfork. These latter
three reports all told of skier and cornice drop triggered avalanches
on northerly aspects.
In our snowpit investigations around the Flathead we continue to find
reason to wave a red flag, beat the drum, and cry, "Beware of the
buried surface hoar from mid February!" All the triggered slides over
the weekend in the Middle Fork and Jewel Basin areas were failing upon
this weak layer. In our stress tests, shear failures were occurring
regularly on it.
Although the buried surface hoar layer is showing signs of
strengthening as it becomes more deeply buried, warms, and ages, it
continues to posses elastic energy enough to propagate failures a
significant distance when stressed by forces equivalent to a skier,
snowboarder, or snow machine. The surface hoar is generally buried
16-48" deep and is most prevalent on steep, gladed, northerly facing
slopes. Victims caught in avalanches in these scattered tree
situations face a serious threat of trauma injury.
In the west on the Kootenai we are finding the most significant
instability is generally associated with the near surface, melt freeze
crust frequently found there.
AVALANCHE - INSTABILITY DESCRIPTION:
We're currently rating the avalanche danger between the elevations of
5,500 and 7,500' elevation as MODERATE. Unstable slab layers are
possible, on all steep terrain. Natural avalanches are unlikely, but
human triggered avalanches remain possible, especially on northern
aspects. Backcountry travelers should use caution, should always carry
and know how to use avalanche safety equipment, and expose no more than
a single person at a time to any potential danger. Watch, feel, and
listen for signs of collapsing snow. In the Flathead area travelers
should view all steep northerly aspects with suspicion, well worth
investigation prior to traveling upon them.
Below 5,500 ft. elevation we're rating the avalanche danger as LOW.
Generally stable snow exists there with only isolated areas of
instability. Natural avalanches are very unlikely, while human
triggered avalanches are unlikely. Backcountry travel is generally
safe, but normal caution is still advised on any steep slope.
WEATHER FORECAST & AVALANCHE OUTLOOK:
The weather forecast is for a series of disturbances to move over
Northwestern Montana Tuesday through Sunday. 2-3'' of new snowfall is
expected in the mountains Tuesday. Another front is forecasted
Wednesday and Thursday, but new snowfall amounts then should be
modest. W-SW winds blowing 10-15 MPH are forecasted Tuesday thru
Thursday. Daytime high temperatures are expected to peak in the upper
20's to low 30's F, with nighttime lows cooling into the upper teens to
low 20's.
Cooling temperatures, with some new snow fall, combined with moderate
winds should maintain the current level of avalanche danger thru the
week. Buried surface hoar is a persistent crystal form. Cold snow
temperatures promote its preservation and elasticity. New snow loading
from precipitation and wind add to the stress upon these buried
layers. Backcountry travelers need to exercise caution until these
buried weak layers gain significant strength.
The next regularly scheduled update of this advisory message will be
Friday, March 21st.