Area Forecast Discussion
Issued by NWS Great Falls, MT

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958
FXUS65 KTFX 111212
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
500 AM MST Sun Dec 11 2016

Aviation Section Updated.

.DISCUSSION...

Today through Tuesday...A few areas of freezing fog have
developed under clear skies early this morning. This fog appears
quite shallow and so expect it should erode shortly after sunrise.
Temperatures range from around 15 degrees above zero for SW MT to
around 15 degrees below zero along the HiLine. A strong low-level
temperature inversion remains in place above the colder air.
Mixing will erode the colder airmass, so that locations generally
southwest of a line from Great Falls to Lewistown will reach high
temperatures in the mid 20s to lower 30s.

A minor shortwave trough presently over eastern Washington will
bring increasing clouds today over the Northern Rockies and much
of N-central MT. Periods of light snow will continue today over
the Northern Rockies including Marias and Rogers passes. The
passing shortwave will initiate a new surge of Arctic air moving
south from Alberta by early this afternoon. A band of light snow
will develop tonight and move south along and behind the surging
cold front. Around 1 to 3 inches of snowfall is possible for most
areas of N-central MT. Locations along the Rocky Mountain Front
and northerly aspects of other mountain ranges should receive a
bit more snowfall due to enhancement of the shallow upslope flow.
Snow and and areas of blowing snow warrant winter weather
advisories. The most significant impacts will occur over Marias
and Rogers passes where 8-10" of new snow is possible. Other
tricky spots include Kings Hill Pass and I15 between Helena and
Butte. Snowfall tapers from north to south during the day on
Monday. Windchill values may fall below -20F Monday night, so will
monitor for possible windchill advisories. Tuesday is looking
fair and cold for most areas except for lingering snow showers
over the southwest mountains. PN

Tuesday night through Sunday...Medium range models are in decent
agreement early in the forecast period. Models continue to show a
deep closed exiting northern Manitoba to the east. However, the
secondary low off the Pacific Northwest coast is now more of a weak
disturbance that is deformed toward the southwest and helps to build
short wave ridging over the Great Basin. As before, this pattern
puts the state beneath an unsettled northwest flow aloft with mostly
dry conditions across central and northern portions of the forecast
area while a slight chance for light snow persists over my southern
zones Tuesday night and Wednesday. By Thursday, models continue to
indicate that a positive-tilt trough will develop along the West
Coast then swing it inland across the Great Basin Thursday night
through Friday. This ends up developing a moderately strong storm
system across the Great Basin and central Rockies with wrap around
moisture now expected to impact central and southern portions of
Montana more strongly than earlier indicated. As a result, I have
bumped up PoPs for this time period. For the weekend, models
continue to indicate that the bulk of stormy weather will move into
the eastern USA with a drying trend and moderating temperatures
expected for Montana. mpj

&&

.AVIATION...Updated 1200Z.

Freezing fog that had formed earlier at KCTB/KBZN has lifted. While
this fog has been removed from the forecast this morning, will need
to continue to monitor as there remains some chance it could return.
Mid-level clouds are increasing over the region as the next
disturbance nears the Continental Divide. Mountain obscuration
becomes increasingly likely. Snow is forecast to develop from north
to south late this afternoon and overnight through Monday morning.
All terminals reporting VFR conditions this morning. Expect a
transition to MVFR...then IFR and LIFR conditions by the end of the
period. PN

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  23  -1   6  -4 /  10  80  80  10
CTB   4  -6   6 -10 /  20  60  50  10
HLN  32   6  13   4 /  20  80  90  40
BZN  28  16  22   9 /  30  30  30  40
WEY  20   7  21   1 /  60  70  40  40
DLN  29  15  28   8 /  20  30  20  20
HVR   3  -4   5  -8 /  50  80  50  10
LWT  24  -7   3  -8 /  10  60  80  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 5 PM this afternoon to 11 AM MST
Monday Blaine...Chouteau...Eastern Glacier...Eastern Pondera...
Eastern Teton...Hill...Liberty...Northern Rocky Mountain Front...
Southern Rocky Mountain Front...Toole.

Winter Weather Advisory from 11 PM this evening to 11 PM MST
Monday Broadwater...Cascade...Central and Southern Lewis and
Clark...Fergus...Jefferson...Judith Basin...Meagher.

&&

$$

weather.gov/greatfalls



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