Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 210502
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1000 PM MST Tue Feb 20 2018

...Aviation Section Updated...

.UPDATE...

Most of tonight`s forecast was left unchanged. However, added
patchy freezing fog through Wednesday morning to portions of the
region, especially the Hi-Line, where clearing skies toward
daybreak may contribute to fog formation per model sounding data.
Latest model guidance suggests tonight`s low temperature forecast
remains valid, although a window of increasing cloud cover may
cause temperatures to be static for a while. However, many
locations should have clearing skies toward daybreak Wednesday,
resulting in renewed nocturnal cooling around that time.
Jaszka

&&

.SYNOPSIS...

Clouds increase across central and southwest Montana overnight,
producing isolated snow showers over mountain areas. Expect low
temperatures ranging from around -5 to -20 Fahrenheit. Skies clear
over North-central Montana Wednesday, with lingering light snow
showers over the far Southwest. High temperatures Wednesday will
range from around zero to 20 above zero.

&&

.AVIATION...
Updated 0500z.

Expect primarily VFR conditions for all terminals overnight tonight
and through the day tomorrow. Some areas of patchy freezing fog may
develop across portions of the Hi-Line later this morning, possibly
making it into the Milk River Valley near HVR. Expect low CIGs with
the possibility of limited visibility at times that may lower flight
cateogires, but will continue to closely monitor through the early
morning hours. Otherwise, only a few areas of mid-level cloud cover
across the area with some low CIGs hanging tough around EKS, but
should lead to rising CIGs at or after daybreak. KLG

&&

.PREV DISCUSSION...
/ISSUED 301 PM MST Tue Feb 20 2018/

Tonight through Wednesday Night...Continued cold but otherwise
fair weather prevails across the forecast area. A weak disturbance
aloft will spread higher clouds across the region tonight, with
just a slight chance for a few light mountain snow showers.
Chances for mountain snow showers continue through Wednesday
across Southwest Montana. Weak surface troughing along the Rocky
Mountain Front has produced wind gusts to near 35mph. Expect
isolated areas where snow drifts across roads occurring now
through Wednesday night. With increasing clouds and slightly
warmer temperatures aloft, low temperatures will not be quite as
cold tonight. However, some areas across central Montana and
portions of far southwest Montana could experience wind chill
values below -20F. Confidence is low that wind speeds will exceed
10 mph alongside these temperatures, but nonetheless, wind chill
advisories have been posted as some locations are expected to
reach wind chill advisory criteria. Regardless of our criteria,
anyone spending time outside needs to bundle up and be prepared
for another very cold night. Skies clear over the north on
Wednesday, while partly to mostly cloudy conditions continue over
the Southwest. PN

Thursday and Friday...Expect increasing cloud cover with more
light snow chances, primarily across SW areas during the day on
Thursday. By Friday, this system will swing across the central
Rockies to our south, with a stretched area of upstream vorticity
and weak troughing to follow across our area. Regardless of
notable upper- level dynamic support in place, moisture will
remain rather starved, as PW`s range close to daily minimum values
per sounding climatology (~0.08" to 0.10"). With that said, going
to continue limiting any northward extent of snow potential and
keep chances primarily to south-central and southwestern MT,
specifically localized to any enhancement caused by terrain.

A strong surface pressure gradient develops along the Rocky
Mountain Front Thursday night into Friday. Wind speeds aloft do
not support a high wind event, but once again, the potential for
significant drifting and blowing snow will need to be evaluated
along and west of Highway 89 in Glacier, Pondera and Teton
counties.

Saturday through Tuesday... Once this trough passes, we return to
a zonal pattern beginning on Saturday. However, as this
transition progresses, heights will tightly pack across the area
which will lead to increasing winds aloft. H700 winds between 40
to 50kts will increase beginning in the morning hours on Saturday,
lasting throughout the day where afternoon mixing will aid in
downward transport, leading to breezy or windy conditions. ATTM,
not expecting this to cause any major impacts, as winds aloft are
not supportive for excessive windy conditions. Like clockwork, the
next shortwave trough arrives across the Pac NW on Sunday which
will drive Pacific moisture across the area in the form of
increasing cloud cover. This will be followed by increasing snow
chances along the Divide, eventually spreading out into the
Plains. Most long-range model guidance is lacking much in the way
of moisture supportive for any significant accumulations like
we`ve been seeing with previous systems. However, confidence of
lighter snow and associated accumulations looks good, and will
continue to monitor this system in the upcoming days. KLG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF -12  15 -10  10 /   0   0   0   0
CTB -17  13 -11  12 /  10   0   0  10
HLN  -9  17  -5  17 /   0  10   0  10
BZN -10  17  -5  19 /   0  10  10  10
WEY -17  13 -11  18 /  20  30  30  20
DLN -10  12  -5  14 /  10  10  10  20
HVR -18   5 -19   6 /   0   0   0   0
LWT -13  15  -7  14 /   0   0   0  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Wind Chill Advisory until 11 AM MST Wednesday Beaverhead...
Gallatin...Madison.

Wind Chill Advisory until 11 AM MST Wednesday Blaine...
Chouteau...Fergus...Hill...Liberty...Toole.

&&

$$

http://www.weather.gov/greatfalls



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