Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 160247
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
847 PM MDT Fri Sep 15 2017

.SYNOPSIS...

Widespread rain and mountain snow will continue to impact the area
through Saturday morning, then gradually taper off from west to
east Saturday afternoon. Travel will be difficult at times over
mountain passes through Saturday morning, along with wet and raw
conditions for those outdoors. Dry weather Sunday gives way to
unsettled conditions returning next week.

&&

.UPDATE...Widespread rain and snow continues across the region
this evening. Going forecast continues to have a good handle on
the situation, but had to make a few tweaks this evening. Biggest
change to the forecast was to remove elevation wording for the
Winter Storm Warning in Beaverhead and Madison Counties. As such,
all locations in these counties, including Dillon, Ennis and the
I-15 corridor are now covered by the warning. Made a few minor
updates to overnight low temperatures and added a mention for
patchy fog and freezing fog to many locations overnight. Forecast
area remains on-track to see precipitation taper off Saturday
afternoon and evening with Sunday expected to be a rather pleasant
day. mpj

&&

.AVIATION...
Updated 2344z.

Widespread precipitation and low clouds causing IFR/MVFR conditions
with areas LIFR continue through Saturday morning. Mountains will
be obscured through the period. Precipitation tapers off to
scattered showers with conditions improving from northwest to
southeast on Saturday. mpj

&&

.PREV DISCUSSION...
/ISSUED 544 PM MDT Fri Sep 15 2017/

Rest of today through Saturday night...A complex and dynamic system
continues to bring widespread rain and snow to much of central
and SW MT at this time. Numerous reports of 1-2" of rain have come
in for many areas, but lower amounts along the Hi-line where 0.25
to 0.50 inches has fallen. As for snow, many reports of 1 to 5
inches have been reported in the mountains, with isolated amounts
up to 10 inches. Webcams show mountain passes are mostly wet
right now, although there were slushy accumulations on the roads
reported earlier this morning. Through the day, roadways are
expected to remain mostly wet. However, later this evening and
tonight, roadways and air temps will cool as the sun sets, with
temps tonight possibly falling about 1-3 degrees colder than last
night. This should set the stage for the highest impact period
being tonight/SAT AM and therefore, no changes are planned at the
moment for the ongoing Winter Storm Warnings and the Winter
Weather Advisory.

Otherwise, the forecast generally looks good at this point. An axis
of moderate to, at times, heavy precipitation extends from west of
Great Falls south through Dillon. This axis will slowly shift east
through tonight, with precip winding down from west to east on
Saturday. Snow below 5000 feet will very much be rate-driven through
this afternoon. Dillon recently reported heavy snow, causing the
temp to fall to 33 degrees. These same conditions will be possible
in the 4500-5500 foot level through this evening where heavier bands
of precip develop and this will cause rapidly changing conditions at
times. For lower elevation areas like Great Falls, Helena,
Lewistown, Bozeman, etc some snow may mix in at times tonight as
the coolest temps are realized, but at this time, significant
impacts are not expected. We`ll closely monitor this potential,
though. MARTIN

Sunday through Sunday night...a fast moving upper level shortwave
embedded within a nearly zonal flow at H500 will traverse the
Northern Rockies and into the Northern High Plains during the day on
Sunday. This shortwave will be dry in nature as it traverses the
region, however, an increase in high cloud cover during the morning
and into the early afternoon hours can be expected. Beneath the
upper level shortwave, zonal/cross barrier flow at H700 will
increase throughout the day and into the nighttime hours. This will
lead to breezy, dry downsloping winds across the plains of North
Central Montana and along the Rocky Mountain Front. High
temperatures during the day on Sunday will likely be the warmest
temperatures of the upcoming week given the expected downsloping
winds. As the H500 shortwave exits the Northern Rockies Sunday
night, weak upslope flow along the Continental Divide will give way
for the chance for light rain showers and/or snow showers. -
Moldan

Monday through Friday...All models continue in good agreement
with a very strong upper level disturbance swinging across the NW
US in the beginning of next week. For this region, a fast-moving
cold front will eject eastward which will likely cause some rain
showers, or perhaps a few thunderstorms for central and southern
areas during the afternoon and evening hours on Monday.
Simultaneously, a deep lee-side surface low will begin to develop
just across the Canadian border to the north. This will lead to
increasing pressure gradient winds, coupled by increasing 700mb
winds aloft to create breezy conditions beginning on Tuesday. The
aforementioned upper level disturbance will then likely close off,
allowing for the surface low to stall just to the north which
will keep the area in an unsettled pattern through the rest of the
week along with continued windy conditions. Expect chances of
rain showers, persistent cloud cover and cool temperatures to last
well through the end of the week with temperatures ranging 5 to
15 degrees below climatological normal for this time of the year.
KLG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  35  48  33  62 / 100  80   0   0
CTB  33  51  30  63 /  80  30  10   0
HLN  35  51  33  66 / 100  80  10  10
BZN  35  48  30  62 / 100  80  10   0
WEY  29  44  23  56 /  90  60  10  10
DLN  31  47  30  63 / 100  50   0  10
HVR  39  53  33  66 /  50  60  20   0
LWT  34  46  31  63 / 100  90  20   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until noon MDT Saturday above 6500 feet for
Gallatin.

Winter Storm Warning until noon MDT Saturday above 5500 feet for
Broadwater...Cascade...Central and Southern Lewis and Clark...
Jefferson...Judith Basin...Meagher...Southern Rocky Mountain
Front.

Winter Storm Warning until noon MDT Saturday Beaverhead...
Madison.

Winter Weather Advisory until noon MDT Saturday above 6000 feet
for Northern Rocky Mountain Front.

&&

$$

weather.gov/greatfalls



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