Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 160542

Area Forecast Discussion
National Weather Service Great Falls MT
1042 PM MST Wed Nov 15 2017

...Update to Aviation...


Made a few updates to the forecast for tonight into Thursday.
Have lowered forecast lows, added areas of freezing fog to the
plains of north central Montana, and issued an additional Winter
Weather Advisory for the northern Rocky Mountain Front.

Lower lows and fog: Temperatures have fallen rapidly after sunset
to near/just below forecast lows. Have therefore lowered lows
around 5 degrees. The stationary front remains along the east
slopes of the Rockies, and am expecting this to gradually move
east overnight, allowing westerly downslope winds to spread east
behind it and cause temperatures to warm later tonight over the
plains. Lighter winds and smaller temperature/dewpoint spreads to
the east of this front have also allowed areas of freezing fog to
form. Am expecting this to persist overnight into Thursday morning
over the eastern TFX plains, but the development of westerly
winds should help dissipate the fog along and west of Interstate
15 after midnight.

Additional Winter Weather Advisory: Latest model runs continue the
likelihood of 2 to 4 inches of new snow on Marias Pass, in
addition to persistent westerly wind gusts of 35 to 45 mph there.
The falling and blowing snow will create hazardous driving
conditions through Thursday morning, so have issued a Winter
Weather Advisory for snow and blowing snow for midnight tonight
through 3 pm MST Thursday. Will continue to monitor the situation
for a possible extension of this advisory, given the potential for
additional snow late Thursday night into Friday.



A Pacific cold front will cross the area Thursday and Friday,
bringing gusty winds to the plains and snow in the mountains. The
valleys of Southwest Montana will see some snow as well, mainly
Thursday evening through Friday.


Updated 2342Z.

Moist southwest flow aloft with broken mid- and high-level clouds
prevailing early in the forecast period. Pacific cold front brings
increasing likelihood for mountain snow and obscurations across
western portions of the state with a few showers possible over the
plains between 09Z and 18Z. Better chance for precipitation at lower
elevations spreads southward across central and southwest Montana
after 15Z with the passage of the cold front through this area.
Gusty west winds expected behind the front, especially over the
plains. VFR conditions expected to prevail at most forecast
terminals but occasional IFR/MVFR conditions possible with
precipitation. Coulston/mpj


/ISSUED 445 PM MST Wed Nov 15 2017/

Tonight through Friday...Surface analyses reveal a trough
extending south-southeast from Glacier County toward Judith Gap.
On the north side of this trough, a stable cold airmass remains in
place along the HiLine with areas of freezing fog and light
easterly winds. Temperatures should hold steady or rise overnight
from west to east in this area as the cold air continues to
retreat away to the east. To the south and west of this trough,
temperatures were mild with gust southwest winds produced by a
locally strong pressure gradient. Satellite imagery shows
increasing clouds across southern Montana, leading a plume of
moisture that extends all the way upstream into the Pacific ocean
offshore northern California. This moisture, combined with
increasing southwest flow aloft will begin to produce valley rain
and mountain snow up to the Continental Divide later tonight.
Snow amounts of 4-6 inches are possible on Marias Pass late
tonight through Thursday. Potential travel impacts will be
localized to the pass, so a winter weather advisory for more
widespread impacts would not be warranted at this time. Snow also
develops late tonight over the higher terrain of far Southwest
Montana. This will include snow in the West Yellowstone area
tonight. The forecast here is complicated by the fact that the
Pacific cold front is not due through this area until late
Thursday night. Hence, daytime temperatures are forecast to rise
into the mid- to upper-30s on Thursday, causing a change back to
rain at and below pass level. Of greater concern is what impacts
develop with the passing cold front overnight Thursday into
Friday. High resolution model guidance suggests the potential for
a short period of heavy snowfall rates with the passing front.
This could spread a few inches of wet snow across southwest
Montana early Friday, including the Bozeman area, just in time for
the Friday morning drive time. Conditions could be similar to
those observed earlier this week on Tuesday morning. A winter
weather advisory is posted for these potential impacts Thursday
night through Friday. The most likely storm total snow amounts for
the West Yellowstone are range from around 4-7 inches. However,
8-11 inches appear possible, so will need to monitor closely for
potential higher impact levels. PN

Friday night through Wednesday...Weather system expected to move
through the region on Friday will move rapidly eastward Friday
night with northwest flow aloft setting up over Montana. This is
expected to set up a dry and windy period for Montana with
temperatures rebounding back to near or above seasonal averages
through the weekend. Models starting to come together for early
next week, although differences continue into mid week. Sometime
Monday an initial cold front looks to push across the area
bringing chances for rain and snow across the region. The
progressive European solution pushes this front through quickly
then builds a ridge over the region for Tuesday through Wednesday.
The GFS is quite a bit different. Bringing another shot of cold
air, lingering snow chances across much of the CWA for Tuesday and
Wednesday. For pops decided to linger at least some slight chance
pops during this time period. Temperatures are even more
uncertain as the Euro`s ridge sees temps return to near normal
levels, while cold winter like temperatures are the result of the
GFS. Decided to let the model blends handle temperatures for this
update, although precipitation chances and temperature trends will
need to be monitored. mpj/anglin


GTF  31  46  27  37 /   0  10  10  10
CTB  18  37  22  34 /  10  10   0  10
HLN  25  45  24  35 /  10  30  30  20
BZN  22  47  23  33 /  10  30  50  50
WEY  25  38  21  29 /  90  90  80  70
DLN  27  46  24  32 /  10  20  40  40
HVR  21  41  19  35 /   0  10   0  20
LWT  30  49  26  36 /  10  10  10  20


Winter Weather Advisory from 5 PM Thursday to 5 PM MST Friday

Winter Weather Advisory from midnight tonight to 3 PM MST
Thursday above 5000 feet for Northern Rocky Mountain Front.


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