Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 160523

Area Forecast Discussion
National Weather Service Great Falls MT
1023 PM MST Fri Dec 15 2017

...Aviation Section Updated...


Main adjustment was to focus the current precipitation more so
over the area from Great Falls to Havre, then have it fill in
during the evening. The cold front has moved through most of north
central Montana as of the current time, and the resulting
northerly upslope winds will likely cause the main focus of rain
changing quickly over to snow to shift more so to the north- and
east-facing slopes overnight, including the Cut Bank, Great Falls,
and Lewistown areas. Moisture ahead of the approaching Pacific
weather system will spread into southwest Montana overnight and
into Saturday morning as the cold front moves into that area as
well. This will cause snow to become more widespread there during
this time. Precipitation will then decrease later Saturday morning
into Saturday afternoon as the Pacific system begins exiting the
area. Overall, expected snow amounts appear to be in good shape,
so will keep the Advisory as is.



Accumulating snow is expected to overspread the region today into
Saturday as a weather disturbance and cold front cross the area.
However, at lower elevations, precipitation may begin as rain
before quickly changing to snow this evening. High pressure will
then lead to quiet weather for most on Sunday, although a few
light snow showers may affect the mountains.


Updated 0523Z.

Mainly MVFR and/or IFR conditons are expected for much of the night
as light to at times moderate snow moves across the region from
north to south. As of 05z this evening, a cold front was generally
situated across Central Montana, or from about KLWT to KGTF. This
cold front will continue to slowly sag south across the remainder of
Southwest Montana, albeit slowly as it interacts with the
surrounding terrain. Snow is expected to accompany this front, with
the biggest impacts from falling snow expected south of a KGTF and
KLWT line. IFR conditions are expected at times with this snow,
especially in the heaviest bursts. In addition, light upslope flow
around the KCTB terminal will lead to MVFR/IFR ceilings.

Snow is expected to taper off from north to south starting Saturday
morning, with only scattered snow showers to be found by Saturday
afternoon (mainly across the southwest). However, MVFR cloud decks
may linger into the afternoon hours on Saturday. Expect the
mountains to be obscured during snowfall this evening and Saturday
morning/early afternoon. A return to VFR conditions across most
sites is expected Saturday evening. - Moldan/Anglin


/ISSUED 506 PM MST Fri Dec 15 2017/

Rest of today through Sunday...High pressure ridging aloft will
continue to be broken-down as a weather disturbance approaches
our area from British Columbia and the Pac NW. This disturbance
will then traverse the CWA on Saturday through early Sunday
morning. Thereafter, another high pressure ridge aloft will
build- in from the west. At the surface, a Canadian cold front
will cross the Hi- Line this afternoon and Central Montana this
evening. Thereafter, the front will continue moving
southeastward, albeit more slowly due to interaction with higher
terrain , and exit Southwest Montana by late Saturday morning.
The cold frontal passage will cause temperatures in North-
Central Montana to drop from above- normal today to near- normal,
especially with respect to highs, on Saturday through Sunday. In
Southwest Montana, recent cold air entrapment will contribute to
predominantly near-normal readings through the period.

The aforementioned disturbance and front will allow
precipitation, mainly in the form of snow, to overspread the
region from NW to SE the rest of this afternoon through evening.
Precipitation may begin as rain at lower elevations before
quickly changing to snow. This is especially true for North-
Central Montana. Winter weather advisories have been posted for
many of our counties where accumulating snow is likely to create
notable travel issues. This event will then taper-off gradually
on Saturday. Snow totals are expected to generally reach a trace
to 3 inches for the plains/valleys, while totals up to a foot are
still forecast for the mountains. After this event, quiet weather
is expected for most on Sunday, although a few light snow showers
are possible in the mountains.

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 from over southeast
British Columbia across the Montana/Alberta border, 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. The passage of this cold front will also cause
temperatures to fall to between 10 and 15 degrees below normal
Wednesday through Thursday. Beyond this, model solutions differ
somewhat through the remainder of the week with each keeping a
northerly flow aloft with generally low PoPs over the region.
Westerly winds will also develop into Friday, causing
temperatures to warm a bit closer to normal.


GTF  27  32  20  37 /  90  50  10  10
CTB  24  33  19  37 /  80  30   0  10
HLN  24  32  15  31 /  80  70  10  10
BZN  23  30   6  26 /  80  80  20  10
WEY  18  27   0  18 /  80  70  10  20
DLN  22  31   7  27 /  70  80  10  10
HVR  27  38  18  39 /  60  10  10  10
LWT  27  32  15  36 /  70  50  10  10


Winter Weather Advisory until 2 PM MST Saturday Beaverhead...

Winter Weather Advisory until 11 AM MST Saturday Cascade...
Central and Southern Lewis and Clark...Eastern Glacier...Eastern
Pondera...Eastern Teton...Judith Basin...Meagher...Northern
Rocky Mountain Front...Southern Rocky Mountain Front.


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