Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 151501
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
801 AM MST Fri Dec 15 2017

.UPDATE...

Most of today`s forecast remains valid. However, freshened hourly
temperature grids through this early evening per latest
observations and model guidance. Freezing fog is dissipating in
the Southwest MT valleys, so allowed the freezing fog advisory to
expire. Any lingering fog is expected to dissipate by 10 AM this
morning. Gusty downslope winds over North-Central MT are still
expected to remain below warning criteria through this afternoon,
so issued a special weather statement to cover winds and expected
travel impacts.
Jaszka

&&

.SYNOPSIS...

Stubborn freezing fog is expected to persist in the valleys of
Southwest Montana through this morning, before finally eroding. A
rain snow mix is then expected to develop along the Hi-Line by
the afternoon hours today, with precipitation then spreading south
and east across the remainder of North Central and Southwest
Montana as snow. Accumulating snow is expected across most
locations during the overnight hours, with the highest
accumulations occurring across the mountains of Central and
Southwest Montana, and along the Continental Divide.

&&

.AVIATION...
Updated 1140Z.

Persistent LIFR/IFR conditions due to fog (some of which will be
freezing fog) will be experienced within the Bozeman and Helena
Valley`s (including KBZN and KHLN) through early this morning, before
dissipating around 15z this morning. Mountain obscuration is likely
in/near areas of fog this morning, especially where the fog is the
thickest/deepest. The remaining terminals across North Central
Montana can expect VFR conditons through the morning and into
portions of the afternoon. Breezy conditions are occurring, and are
expected to continue along the Rocky Mountain Front and onto the
plains of North Central Montana, generally affecting the KCTB, KGTF,
and KHVR terminals. Some turbulence due to mountain wave activity is
possible, especially closer to the Rocky Mountain Front.

A cold front is then expected slide from north to south late this
afternoon and through the evening hours. Accompany this cold front,
widespread precipitation is expected across the region, which will
lead to MVFR to IFR conditions. Mountain obscuration is likely due
to the falling precipitation and low ceilings. -  Moldan

&&

.PREV DISCUSSION...
/ISSUED 440 AM MST Fri Dec 15 2017/

Today through Sunday...an upper level disturbance over the Gulf of
Alaska early this morning, will quickly move southeast into the
Pacific Northwest/Intermountain West by this evening, crossing the
Northern Rockies through the day on Saturday. This disturbance will
in-turn help to break down the upper level ridge which has dominated
the Western CONUS as of lately. At the surface, a cold front will
begin to dive south and east out of Canada by as early as late this
afternoon across the Hi-Line. This cold front will continue to slide
south into Central Montana by this evening, and finally into
portions of Southwestern Montana by Saturday morning before
stalling.

The combination of increasing Pacific moisture associated
with the upper level disturbance, synoptic lift, and cold frontal
passage, will allow for a fairly widespread area of precipitation to
develop by the evening hours today. Precipitation is expected to
develop along the Northern Rocky Mountain Front and Hi-Line first,
before spreading east and south during the rest of the
evening/overnight hours tonight. Precipitation is expected to
initially begin as rain across the plains of North Central Montana,
before changing over to snow as colder air works into the region.
Across the higher elevations of the Continental Divide and Central
Montana, precipitation should be in the form of all snow, which will
allow for some decent snowfall accumulations. Areas like Marias and
Kings Hill Passes should expect snowfall accumulations of around 5
to 8 inches. For this reason, continued the Winter Weather
Advisories that were issued yesterday. Across Southwest Montana,
snowfall accumulations of 4 to 7 inches are expected across
Raynolds, Targhee, and Bozeman Passes, with generally 2 to 4 inches
expected across the lower elevations of Gallatin County. For this
reason, also issued a Winter Weather Advisory for all of Gallatin
County and elevations above 5000ft in Madison County. Precipitation
will slowly end across most of the region Saturday evening, as upper
level ridge tries to build into the region. - Moldan

Sunday night through Friday...Early portion of the medium range
period will be dominated by a moist and unsettled northwest flow
aloft over the region. This pattern will be favorable for an
occasional chance of precipitation with temperatures remaining near
or slightly above seasonal averages through Tuesday. Models remain
in good agreement bringing a vigorous shortwave trough inland
through the Pacific Northwest Tuesday night then across Montana on
Wednesday. However, they differ significantly in placement of the
associated surface low pressure system. By Wednesday morning the GFS
centers this low over southeast British Columbia while the ECMWF
places this system along the Montana/Wyoming border. As the surface
low moves off to the southeast, a strong Canadian cold front sweeps
southward through Montana with the ECMWF bringing this feature
through a good 6 to 12 hours faster than the GFS. Best chances for
accumulating snows for most of my CWA will be Wednesday into
Wednesday evening as this system sweeps through the region. Beyond
this, model solutions continue to diverge through the remainder of
the week with each keeping generally low PoPs over the region with
westerly winds redeveloping.
MPJ

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  48  23  35  21 /  10  90  40  10
CTB  44  19  33  19 /  60 100  30   0
HLN  30  17  34  15 /   0  90  40  10
BZN  31  21  32   6 /   0 100  60  20
WEY  26   6  26   3 /  10  90  70  20
DLN  37  19  30   8 /   0  80  40  10
HVR  45  23  37  16 /  30  90  10   0
LWT  50  25  33  14 /  10  90  50  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 5 PM this afternoon to 11 PM MST
Saturday Cascade...Judith Basin...Meagher.

Winter Weather Advisory from 11 PM this evening to 5 PM MST
Saturday Gallatin.

Winter Weather Advisory from 11 AM this morning to 11 AM MST
Saturday Northern Rocky Mountain Front.

Winter Weather Advisory from 11 PM this evening to 5 PM MST
Saturday above 5000 feet for Madison.

&&

$$

http://www.weather.gov/greatfalls



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