Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 212132

Area Forecast Discussion
National Weather Service Great Falls MT
232 PM MST Sat Jan 21 2017


This afternoon through Monday...The large upper level low that was
sitting off the Pac NW coast over the last couple of days has begun
lifting north and weakening while yet another deep low dropping
south out of the Gulf of Alaska takes its place. This transition
will allow the jet stream to briefly shift north through Sunday,
before diving back south early next week. As the jet shifts south,
it will carve out a broad trough across the western US.

This afternoon, satellite imagery continues to show a small
circulation associated with a weak shortwave moving slowly north
through western MT. Light snow has continued to mainly be confined
west of the Divide thus far. However, radar suggests some of that
snow may now be edging just over the Divide. Given the s/w
appears to be weakening and with some downsloping effects, it
still appears that most of the snow with this feature will remain
along/west of the Divide through tonight. Across SW MT, the
generally light flow appears to be allowing snow shower activity
to underperform there, but I still expect at least scattered to
widespread snow showers there through this evening, mainly along
the MT/ID border.

Regarding snow on SUN/MON, a few things will be going on. Large-
scale ascent should gradually increase across SW MT as the above
mentioned jet pushes into the central Rockies. Through Monday, the
focus of greatest lift is expected across SW MT as the jet moves
through the central Rockies and into the Plains, and is where the
heaviest snowfall rates are expected. On the leading edge of the
jet, it appears that a low/mid level warm front will push north
through the entire CWA, providing an additional mechanism for lift,
albeit somewhat weaker compared to the lift with the jet, itself.
At the same time, high pressure will begin nosing south out of
Canada bringing colder low-level air into central Montana. This
should allow a broad area of overrunning light snow to develop
across much of the area late Sunday into Monday.

At this point, it appears lift/convergence associated with the
warm front will be maximized along the Hi- line where 1-3" of snow
appears possible. Across SW MT, the highest snowfall totals are
expected along the MT/ID border. Of note, the models suggest a
700mb low will develop over southern ID/Wyoming, but disagree on
the track. An area of enhanced lift and heavier snowfall rates
appears possible along/north of the low track and some models have
this area glancing far SW MT. If this were to pan out, snowfall
rates/amounts there could end up higher than currently forecast.
For now, though, thinking 4-8" along the border, with lower
amounts of an inch or two further north (ie. Dillon/Bozeman).
Given the uncertainty regarding the evolution of the SUN/MON
system, we`ll hold off on any winter weather products across SW MT
for now, but could certainly see us needing at least a Winter
Weather Advisory there as some travel impacts will likely occur
over passes. Despite a fresh coating of snow likely across
central Montana, it appears impacts there will be limited due to
lighter snowfall rates and lower snowfall amounts. At this point,
then, I do not see any need for any winter weather products. But,
this area will continue to be monitored in later forecasts as
well. Martin

Monday night through Saturday...A broad upper level trough with axis
from Central Canada SW into the Western US will bring cool and
unsettled conditions early next week, followed by dry northerly flow
aloft as the trough progresses east into the central US with a ridge
amplifying along the west coast and pushing inland by the end of
next week. Shortwave energy tracking east from southern ID into WY
Monday night will bring widespread light snow to southern portions
of the forecast area while a northerly flow with embedded energy
also spreads light snow showers across areas to the north Monday
night through Tuesday night. Amplifying ridge to the west will bring
dry conditions from late Wednesday into next weekend with
temperatures moderating from somewhat below seasonal averages early
next week to slightly above average by next weekend. Surface high
pressure will also strengthen over the interior western US late next
week and will likely lead to the redevelopment of temperature
inversions in the SW MT valleys, likely leading to lingering cold
temperatures and potential air quality issues in some SW MT valleys.


Updated 1730Z.

A moist southwest flow aloft will prevail over Montana through late
Sunday morning. Several weak disturbances in the flow aloft will
bring a chance for light snow to the western mountains and to much
of southwest Montana through the forecast period. Expect mountain
obscurations with occasional IFR/MVFR conditions in precipitation
impacting lower elevations, including the KBZN, KEKS and KHLN
terminals. Mostly VFR conditions will prevail across north central
Montana today but a weak surface cold front will bring a surge of
low level moisture for periods IFR/MVFR conditons this evening
through Sunday morning. Occasional fog will also continue to affect
KHLN and KHVR, mainly during the late night through early afternoon
hours. mpj


GTF  19  33  18  27 /   0   0  30  40
CTB  17  29  17  25 /  10  10  40  40
HLN   8  26  10  22 /  10  10  30  30
BZN   6  28   9  25 /  30  10  30  40
WEY   7  22  12  24 /  60  60  70  70
DLN  11  33  15  29 /  30  20  30  50
HVR  18  30  18  27 /  10  10  30  40
LWT  19  34  16  26 /   0  10  20  30



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