Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 181753
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1053 AM MST Thu Jan 18 2018

UPDATED AVIATION

.UPDATE...

Most of the forecast through sunset remains valid. However,
cooled this afternoon`s highs by several degrees in the Helena
and Bozeman Valleys. Expect light low-level winds and limited
boundary layer mixing to allow cooler air to remain trapped in
these areas ahead of the approaching Pacific cold front. Thus,
model soundings suggest precipitation may fall as freezing rain
for a time in these areas later this afternoon and early evening,
if surface temperatures remain 32F or colder. Also trimmed the
areal extent of the winter weather advisory for the Southern Rocky
Mountain Front and our central counties based on forecast
snowfall and expected magnitude of impacts. Thus, the advisory no
longer includes Great Falls and Lewistown.
Jaszka

&&

.SYNOPSIS...
Expect light snow to develop this morning over the Rocky Mountain
Front, with a mix of rain and snow developing late Thursday
afternoon and Thursday evening over Central and Southwest MT.
Afternoon temperatures will be seasonably mild in many areas, but
gradually cool down into the 20s and 30s over the weekend and
into early next week.

&&

.AVIATION...
Updated 1753Z.

Westerly to southwesterly flow aloft and abundant cloud cover are
expected over the area as a disturbance approaches from the Pac NW
and another advances SE`ward from northern Alberta to Manitoba.
Periods of upslope precipitation, primarily in the form of snow, and
mountain obscuration are expected along the Continental Divide
through the period. In addition, a Pacific front, most pronounced
just above the surface, is expected to cross the region from NW to
SE late this afternoon through Friday morning. CIGS and VSBY are
expected to primarily range from MVFR to LIFR due to snow along and
behind this front. However, lower elevations may experience a brief
period rain and/or freezing rain before colder air allows a change
to snow following the front`s passage. The best potential for
precipitation and significant deterioration in flight conditions
along/behind the cold front exists across Central and SW MT, south
of a KGTF to KLWT line.
Jaszka

&&

.PREV DISCUSSION...
/ISSUED 410 AM MST Thu Jan 18 2018/

This morning through Friday Night...It`s been a warm night across
much of north-central and southwest Montana with the exception of
a few pockets of colder temperatures in sheltered valleys. This
complicates matters tremendously when it comes to precipitation-
type forecasting for later today.

A well-defined cold front moves southeast across the region this
afternoon into the overnight. As the front reaches the Missouri
River just north of the mountains, precipitation will develop from
near Great Falls and extending south as the front moves through
central and southwest Montana. I have raised high temperatures
for today considerably with this morning`s forecast package, such
that in most locations this initial band of precipitation will
fall as rain late this afternoon and/or early this evening. In
areas that still have snow on the roads (such as side streets in
the cities) this will make for potentially hazardous travel
conditions with rain falling on hard-packed snow. Another concern
will be the potential for rain in some sheltered valleys
(especially near Helena) with temperatures that are above
freezing, but falling on surfaces that have been frozen for the
last few days due to temperatures that have been well below
freezing, essentially causing freezing rain. As the front moves
south through this evening and overnight, the precipitation will
change to all snow. Another complicating facet of this storm is
the progression of a second wave of this storm Friday morning,
with models disagreeing where the axis of heavier snow will fall,
with some depicting the heaviest snow amounts over the central
mountains while others have it falling over primarily the
southwest Montana mountains. Thus, it is likely that snow will end
for some areas well before the current 11 am Friday expiration
time given this uncertainty.

Snow lingers over the southwest mountains through much of Friday
and Friday night with minor additional accumulations.

Saturday through Thursday...Forecast models are in fairly good
agreement with the solution through this period, which lends itself
to good confidence in the forecast. The upper level low pressure
trough over the Montana plains on Saturday will move east of the
forecast area through Sunday and a shortwave high pressure ridge
moves overhead. This will transition the weather pattern to a more
progressive one through Tuesday, as disturbances bring a chance of
snow mainly to the mountains. Lower elevations will likely remain
dry with breezy southwest winds over the plains. The best chance for
precipitation at lower elevations during this time will be
associated with a shortwave trough passage Sunday night into Monday,
but amounts appear to remain light at this time. Temperatures will
also remain a few degrees below normal through this time. A large
and broad upper level low pressure trough will then move southeast
over the western United States Wednesday through Thursday, which
will cool temperatures further below normal (highs mostly 20s; lows
mostly single digits and teens) and bring a chance of snow to most
of the area. Again, however, the main threat for measurable snow
will remain in the mountains, where exact amounts are uncertain at
this time.  Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  52  29  42  22 /  40  60  10  10
CTB  48  27  41  22 /  10  10   0  10
HLN  35  25  39  19 /  50  90  20  10
BZN  40  30  38  14 /  10 100  70  40
WEY  35  28  31   6 /  60 100 100  80
DLN  47  30  38  14 /  20  60  40  20
HVR  49  28  41  18 /  20  30   0  10
LWT  54  28  41  17 /  10 100  20  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 3 PM this afternoon to 11 AM MST
Friday Beaverhead...Broadwater...Central and Southern Lewis and
Clark...Gallatin...Jefferson...Madison...Meagher.

Winter Weather Advisory from 3 PM this afternoon to 11 AM MST
Friday Cascade...Fergus...Judith Basin...Southern Rocky Mountain
Front.

Winter Weather Advisory until 9 PM MST this evening Northern
Rocky Mountain Front.

&&

$$

http://www.weather.gov/greatfalls



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