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INTRODUCTION:
This is Stan Bones with the backcountry avalanche advisory issued by
the U.S. Forest Service Glacier Country Avalanche Center on Tuesday,
January 22, 2008, for the Glacier Park and Flathead and Kootenai
National Forest areas.
HAZARD ANALYSIS
Between 5,000' and 7,500' elevation the avalanche danger is currently
rated MODERATE. Below 5,000' we're rating the avalanche danger LOW.
These danger ratings expire at midnight Tuesday, January 22nd. The
outlook is for the avalanche danger to remain near current level at
least until Thursday.
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:
Over the weekend Northwestern Montana plunged solidly into winter as a
wind blast of frigid Canadian air pushed into the region from the N
and NE. On Saturday many of our mountain locations were experiencing
snowy conditions and air temperatures that peaked by early afternoon
near the mid 20's F. Within 12 hours these temp's, chilled by strong
and gusty NE'erly winds, had plummeted to near zero. Sunday night
under a clear sky and radiant heat loss the free fall in temperatures
continued into the sub zero realm. It was only under calm and sunny
conditions the next morning, that we saw a reversal in the temperature
fall. Readings Monday climbed back above zero to around 10 degree
positive in many mountain locations. Low level clouds began forming
in some locals late Monday and here we saw temp's moderate and
stabilize for awhile in the mid to low single digits.
Saturday night's arctic blast produced locally heavy snowfall in some
locations. The cold air belly bumped and bullied the moist Pacific
air west of the Continental Divide and quickly shoved it out of the
region.
SNOWPACK ANALYSIS:
Our backcountry observations Monday were on Turner Mtn. in the Purcell
Range north of Libby and in Noisy Basin in the N Swan Range NE of
Bigfork. On Saturday we received an report from the Middle Fork
region along Highway 2.
In your face Sunday and Monday was the cold temperature and it's
effect, along with the recent winds, upon the snow surface. Easterly
aspects have generally been sculpted by the wind into dense, contorted
drifts of wind packed snow called sustrugi. Westerly facing slopes
that are customarily the wind scoured aspects, in these latest blows
have become the recipients of cold, wind deposited snow. On all
aspects we are finding a significant temperature gradient deep into
the snow cover. This gradient is a weakening agent of the surface
snow and a preserver of existing weaknesses found deeper in pack.
In our snowpits we are finding shear weakness associated with weakly
bonded snow captured beneath the cold elastic wind densified
sustrugi. We are also finding on wind loaded westerly aspects near
surface weakness near the base of the more recently wind deposited
snow. The prolonged cold is also preserving more deeply buried
weaknesses that were preexisting prior to the arctic blast.
AVALANCHE - INSTABILITY DESCRIPTION:
We're currently rating the avalanche danger between the elevations of
5,000 and 7,500' as MODERATE on steep open slopes. Unstable wind slab
layers are possible on all aspects. Natural avalanches are unlikely,
while human triggered avalanche are possible. Backcountry travelers
should use caution in all avalanche terrain.
Below 5,000 ft. elevation we're rating the avalanche danger as LOW. A
more shallow snow there is often anchored by vegetation and terrain
features and generally stable 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, open slope.
WEATHER FORECAST & AVALANCHE OUTLOOK:
The weather forecast is for the dry, cold northerly flow to remain
over Northwestern Montana through Wednesday night. Temperatures will
remain below normal with no precipitation expected. The upper level
flow will become more W'erly/SW'erly by Thursday. This should produce
increased cloudiness and warming, but not much precipitation is
anticipated. Backcountry travelers will need to watch as conditions
begin to change this week. Our current cold conditions are weakening
the snow pack. How we transition Thursday and into the weekend will
determine much. A rapid warming or loading of new snowfall upon the
current snow surface could produce significant avalanche activity.
The next regularly scheduled update of this advisory message will be
Friday, January 25th.