Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 150528

Area Forecast Discussion
National Weather Service Great Falls MT
1025 PM MST Thu Dec 14 2017

...Aviation Section Updated...


Main changes to going forecast were to increase fog coverage
overnight in the southwest valleys and generally lower low
temperatures. The dense freezing fog has remained in place from
the Helena Valley southeast over Canyon Ferry Lake and into the
Bozeman Valley. This has already caused delays and cancellations
at the Bozeman Yellowstone Airport, and it may do the same at the
Helena Regional Airport as low clouds may lower further and become
dense fog there. Many of the valley areas in southwest Montana
have cooled close to their dewpoint temperature, so more areas of
fog are likely. How dense it will become remains uncertain, so
will hold off on expanding the Advisory for now. Have also lowered
lows there around 10 degrees to reflect the rapid cooling.
Temperatures over the plains are also cooling close to previously
forecasted lows, so have lowered them generally up to 5 degrees,
as breezy westerly downslope winds will help keep temperatures
from falling too much more.



Stubborn freezing fog is expected to persist in the valleys of
Southwest Montana through Friday morning. Accumulating snow will
then overspread the region from northwest to southeast late Friday
morning through early Saturday morning. Precipitation will fall
as rain or a rain/snow mix at lower elevations on Friday before
changing to snow that evening. The snow will then end gradually
later Saturday morning through early Sunday morning.


Updated 0525Z.

Persistent LIFR visibilities due to fog (some of which will be
freezing fog) will be experienced within the Bozeman and Helena
Vallies (including KBZN and KHLN) this evening...and likely into the
morning hours before slowly dissipating. MVFR conditions due to fog
may also develop across a few other southwestern valleys later
tonight, but confidence is not as high as here (mainly affecting the
KEKS terminal). Mountain obscuration is likely in areas of fog,
especially where the fog is the thickest/deepest. Any fog that does
develop is expected to linger until around 15Z...but may end earlier
if winds can mix down. The remaining terminals across North Central
Montana should expect VFR conditons, with some high clouds through
tonight. Breezy conditions are expected along the Rocky Mountain
Front and onto portions of the plains through the night and into
tomorrow, 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 to bring rain changing to snow...and
possibly brief sleet...from north to south late Friday afternoon
through the evening hours. MVFR to IFR conditions will be possible
with any precipitation. Mountain obscuration will likely occur as
the precipitation moves across the region. - Moldan/Anglin


/ISSUED 511 PM MST Thu Dec 14 2017/

511 PM MST update...A quick update was made to extend the
Freezing Fog Advisory across southwestern portions through the
night. Satellite imagery showing fog remains from the Gallatin
Valley northward along Highway 287/12 to the Helena Valley. With
ridging over head...and light winds expected all night...will keep
the Freezing Fog Advisory through the night. There is the
possibility winds may increase just enough later
tonight...possibly ending the fog early. This will have to be
monitored through the night. Anglin/Coulston

Rest of today through Saturday night...High pressure ridging aloft
will break-down on Friday as a disturbance continues approaching
from the Gulf of Alaska Region. This disturbance will then cross
our area on Saturday and be followed by high pressure aloft
building-in from the west by early Sunday morning. At the surface,
a Canadian cold front will cross the border and enter the Hi-Line
Friday afternoon. This front is then expected to stall along the
Rocky Mountain Front and Central Montana mountains by Friday
evening. It will then remain there through Saturday before
dissipating thereafter. Above-normal temperatures will prevail
over North-Central Montana ahead of the cold front, then cool
somewhat behind the front for Friday night through Saturday night.
With recent cold air entrapment in the valleys of Southwest
Montana, overall cooler readings are expected there throughout the

Predominantly fair weather is expected to continue through Friday
morning. However, patchy freezing fog should persist in the
Southwest Montana valleys before dissipating toward noontime on
Friday. In addition, gusty downslope winds are expected to develop
over North-Central Montana overnight tonight into Friday, but
model guidance indicates peak gusts should remain below high wind
criteria. The aforementioned disturbance will allow snow to
overspread our region from northwest to southeast late Friday
morning through early Saturday morning. Thereafter, the snow will
taper-off gradually and end area-wide by early Sunday morning. At
lower elevations, precipitation will fall as rain or a rain/snow
mix for a time on Friday before changing to snow that evening.
Snow totals are still expected to reach a trace to 3 inches for
the plains/valleys, with totals of up to a foot in the mountains.
Issued winter weather advisories for Marias Pass and Kings Hill
Pass, where confidence is high accumulating snow will impact
travel. Will let future shifts decide if advisories need to be
expanded in areal coverage.

Sunday through Thursday...An unsettled weather pattern will reside
over the region, with a gradual shift towards colder temperatures
over North Central MT by late in the period. Breezy west winds
develop on Sunday allowing the I-15 corridor to warm up a bit from
Great Falls to Cut Bank, otherwise expect another round of light
snow to move through the region late Sunday night through Monday. A
few snow showers linger into Tuesday, otherwise the next major
weather system will be the cold front pushing southward from Canada
into Montana Wednesday into Thursday. A much colder air mass will
follow on the heels of this system under northerly upper level flow
late Wednesday through the end of next week.


GTF  32  45  27  33 /   0  10  70  50
CTB  31  42  24  33 /   0  40  70  40
HLN  14  37  24  30 /   0   0  60  40
BZN   9  36  21  29 /   0   0  70  50
WEY   1  25  16  24 /   0   0  80  50
DLN  10  37  22  30 /   0   0  40  30
HVR  21  42  25  36 /   0  40  60  20
LWT  27  47  25  30 /   0   0  70  40


Winter Weather Advisory from 5 PM Friday to 11 PM MST Saturday
Cascade...Judith Basin...Meagher.

Freezing Fog Advisory until 8 AM MST Friday Broadwater...Central
and Southern Lewis and Clark...Gallatin.

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


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