Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 010537
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1035 PM MST Wed Nov 30 2016

UPDATED AVIATION

.UPDATE...Westerly downslope winds have caused temperatures to
remain steady or increase a few degrees across western portions of
North Central Montana this evening. However, in my far northeast
zones lack of cloud cover has allowed Havre to cool down into the
teens. Have updated the forecast to account for this colder air in
the northeast and have also made some minor changes to PoPs, sky
and weather but have left the Winter Weather Advisory unchanged.
mpj

&&

.AVIATION...Updated 0535Z.

A couple weather disturbances will move generally east-southeastward
over the region through 06Z/Fri. Expect mountain obscuration along
the Continental Divide and in southwest MT due to widespread cloud
cover and periods of snow. Periods of light snow showers are
expected at KBZN and KEKS, mainly after 12Z/Thu. In north-central MT,
a swath of steadier snow may develop over the eastern half of the
area after 00Z/Fri. This includes KGTF, KHVR, and KLWT. Expect MVFR
to IFR CIGS and VSBY in light snow showers, while LIFR are possible
in any heavier snow. KCTB should remain precipitation-free throughout
the TAF period due to downslope flow.
Jaszka

&&

.PREV DISCUSSION...
/ISSUED 429 PM MST Wed Nov 30 2016/

Tonight through Friday...An upper level low pressure trough will
move through the forecast area during this period, maintaining
cool temperatures and bringing occasional snow snow showers,
mainly in the mountains. Only light accumulations, if any, are
expected over the plains, but an inch or two is likely in the
southwest valleys. Mountain passes will likely get 2 to 4 inches,
while areas above mountain passes could get 4 to 8 inches. As a
result, have issued a Winter Weather Advisory for the Rocky
Mountain Front, much of Southwest Montana, and the mountains of
Central Montana. The main impact from this snow will be snow
covered and icy roads, especially over the mountains passes. The
main periods for heavier snow will be associated with snow bands
tonight and Thursday night. Snowfall will decrease from the west
late Thursday night into Friday as the upper trough begins
exiting the area. Temperatures will remain near to slightly below
normal through the period, with highs mostly in the mid 20s to mid
30s and lows mostly in the teens to lower 20s.
Coulston

Friday night through Wednesday...The extended period starts with
a ridge over the Pacific NW, but this will quickly transition to
an active NW flow pattern. This will occur as a trough over the
Gulf of Alaska drops SE into the western US, amplifying a broad
trough already in place across much of the US. This pattern
transition will feature a period of stronger winds, the potential
for widespread snow, and much colder temperatures. Details on
each below...

WIND: As the ridge weakens, a lead s/w trough will move across
the northern Rockies late Friday into Saturday. As the s/w moves
through, a lee-side trough will develop east of the Rockies along
with a tightening SFC pressure gradient. At the same time, winds
aloft will gradually increase with 40-60+ kt winds possible in the
850-700mb layer. This should allow a period of stronger winds to
develop across central and parts of SW MT Friday night into
Saturday. At this point, it appears High Wind Watches or Warnings
may be needed for a portion of the CWA during this time.
Confidence is highest along the Rocky Mtn Front where gusts in
excess of 60-70 mph will be possible. Elsewhere, wind gusts of
30-50 mph look possible. Even some of the SW valleys may warm up
enough to allow decent mixing of the winds aloft.

SNOW: Initially, the pattern transition will feature isolated to
scattered rain/snow showers across much of the area on Saturday,
but accumulations should generally be confined to the mountains.
Attention then turns to Sunday and Monday. The models continue to
forecast a deepening SFC low to move through Montana during this
period. 12z guidance has shifted the track a bit further north
compared to previous runs which, if this verifies, would put the
emphasis on widespread snow across the Hi-Line counties where
several inches would be possible. It should be noted, though, that
the models may shift where the heaviest band of snow sets up and,
in fact, if the trend continues further north, then most of our
CWA may miss out on the heavier snow. Regardless, snow showers
will remain possible through early next week with a moist, NW flow
in place.

TEMPS: In the wake of the Sunday/Monday system, the models
continue to insist on a strong surge of arctic air dropping down
from Canada. Confidence in much colder temps is high, but
confidence remains lower on just how cold it will get. 12z
guidance is fairly well clustered on most locations seeing sub-
zero temps, but guidance has come in a bit warmer (ie. 0 to -10 as
opposed to -10 to -20). These temps are not record-breaking, but
noticeably colder than the past several weeks. How cold it gets
may ultimately depend on how much snowcover is put down over the
weekend and how much cloudcover is around. Stay tuned through the
weekend for continued updates on the evolving weather pattern.
Martin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  28  36  23  37 /  20  30  50  10
CTB  24  36  22  34 /  20  30  30  10
HLN  24  37  21  39 /  30  20  30  10
BZN  21  34  21  32 /  40  30  50  20
WEY  14  26   3  23 /  50  50  30  10
DLN  19  32  15  31 /  50  20  40  10
HVR  17  36  22  35 /  30  20  30  10
LWT  24  35  24  34 /  30  20  40  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 AM MST Friday Beaverhead...
Broadwater...Cascade...Central and Southern Lewis and Clark...
Gallatin...Jefferson...Judith Basin...Madison...Meagher...
Northern Rocky Mountain Front...Southern Rocky Mountain Front.

&&

$$

weather.gov/greatfalls


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