Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 172201

Area Forecast Discussion
National Weather Service Great Falls MT
301 PM MST Sun Dec 17 2017


An active weather pattern is developing over the region with
temperatures turning colder throughout the upcoming week. Strong
winds are likely tonight along the Rocky Mountain Front and
adjacent areas as well as over portions of central Montana late
tonight through Monday morning. Snow develops over the Rocky
Mountain Front tonight with rain and snow developing further east
across north central Montana Monday afternoon and continuing into



Tonight through Monday...WNW flow aloft increases through tonight
as a flat upper level ridge over the Pacific NW and Northern
Rockies is suppressed by energy moving into western Canada and an
incoming upper level jet. Main impact to weather tonight through
Monday morning will be strong winds, which are already occurring
along the Rocky Mtn Front and adjacent areas including Cut Bank.
Strong winds are likely to expand east later tonight through
Monday morning ahead of of frontal boundary that sets up along the
Hi-line by mid-day Monday and have included areas from Lewis and
Clark county east through Judith Basin county in a High wind
warning that begins tonight with strong winds continuing until a
wind shift to the NW occurs as the previously mentioned frontal
boundary drops southward. Snow will develop over the mountains of
the Rocky Mtn front tonight as PAcific moisture increases with a
long duration snow event anticipated to continue into the middle
of the upcoming week. Precipitation will also develop along and
north of the frontal boundary developing over northern portions of
the forecast area by Monday afternoon. Hoenisch

Monday evening through Wednesday night...Confidence continues to
increase regarding a long-duration and significant snowfall along
the Rocky Mountain Front from late tonight through WED. Elsewhere,
winter weather impacts are likely as well, although snowfall amounts
do not look to be nearly as high as along the RMF.

As for the setup, a 140+ kt jet will allow a stream of moisture to
be directed at the north-south oriented Continental Divide and will
combine with favorable jet dynamics, a surface frontal boundary, and
good orographic/upslope flow to produce a period of 1-2"/hr snowfall
rates for several hours on Tuesday (with light to moderate snowfall
rates before and after that). It is expected that this will lead to
1-2 feet of snow right along the Rocky Mtn Front, including Marias
Pass, St Mary, GNP, and possibly even as far east as Browning. This
is the area where confidence in significant impacts is highest and
is the reason for going ahead and issuing a Winter Storm Warning.

Over the plains, Monday`s front will stall, then lift back north as
a warm front, before stalling again near the US/Canadian border.
This should allow a period of light to moderate overrunning snow
from Monday evening through Tuesday. Of note, a surface low will
track along this boundary on Tuesday which may allow some drier air
to work north into southern sections of Central MT (ie Great Falls
to Lewistown), limiting precip rates/amounts there during the day.
It is this area where confidence in snowfall amounts is the lowest
as the surface low track, temps, and amount of moisture are more in
question. Along the Hi-line, confidence in snowfall amounts is
higher as all model guidance keeps the surface low south of that
area. Given some of the uncertainties over the plains, we have
decided to hold off on expanding the Winter Storm Warning. However,
confidence is high enough to go ahead and issue a Winter Storm Watch
for all of the plains and specific impacts will be ironed out in
later forecasts. Gusty winds over the plains will likely lead to
areas of blowing snow at times, especially Wednesday as the surface
low pushes through the area. Across SW MT, most of the precip should
be focused along the front as it surges back south on WED, with
generally light accumulations/travel impacts in the valleys. Also,
given a layer of warm air aloft on TUE, there is some potential for
patchy freezing rain, but confidence on this is low at this time.

For Thursday through Sunday, models are in pretty good agreement
that a northern system will develop into a deep Hudson Bay low
that expands southward across all of the central USA leaving
Montana beneath a cold northerly flow aloft. Reinforcing cold high
pressure will expand into the region Thursday night into Friday
leaving the forecast area with below average temperatures and
above normal precipitation through the weekend with potential for
an even colder airmass to move south into the region by Christmas.


Updated 1730Z.

Mid-high level cloud-cover increases today under an increasing WNW
flow aloft with VFR conditions prevailing through tonight. Gusty
winds persist across the N-central MT plains and increase further
late tonight through Monday morning. Frontal boundary will set up
from around KCTB to KHVR by mid-day Tuesday with lowering ceilings,
precipitation and winds decreasing/shifting to the northwest.


GTF  37  45  26  34 /  10  10  60  60
CTB  36  40  21  27 /  10  80  70  80
HLN  31  41  28  39 /  10  10  40  40
BZN  23  36  26  39 /  10  10  30  30
WEY  12  24  18  27 /  30  40  50  40
DLN  23  36  25  39 /   0  10  20  20
HVR  34  40  22  31 /  20  90  30  70
LWT  33  43  24  35 /  10  30  70  60


Winter Storm Watch from late Monday night through Wednesday
morning Eastern Glacier...Eastern Pondera...Liberty...Southern
Rocky Mountain Front...Toole.

Winter Storm Watch from Tuesday morning through Wednesday
morning Blaine...Cascade...Chouteau...Eastern Teton...Fergus...
Hill...Judith Basin...Meagher.

High Wind Warning from midnight tonight to 11 AM MST Monday
Cascade...Central and Southern Lewis and Clark...Eastern Teton...
Judith Basin.

High Wind Warning until 11 AM MST Monday Eastern Glacier...
Northern Rocky Mountain Front...Southern Rocky Mountain Front.

Winter Storm Warning until 11 AM MST Wednesday Northern Rocky
Mountain Front.


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