Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 181745
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1045 AM MST Sun Feb 18 2018

Updated Aviation Discussion

.SYNOPSIS...

A strong storm system continues to track across the region and will
continue to spread snow across much of the area with steadiest
snow and greatest snow accumulation expected across western
portions of north-central Montana extending into southwest
Montana. Snow will gradually end through Monday but temperatures
will plummet below zero with dangerous wind chills likely through
at least early Tuesday, especially over the plains.

&&

.DISCUSSION...

Winter storm continues to affect the region though main upper
level forcing has shifted south and east of the area this
morning. Arctic frontal boundary is slowly pushing through the
remainder of SW MT towards ID border with an area of steadier and
at times moderate snow along and south of the I90 corridor before
precipitation intensity decreases there this afternoon. Further
north, the cold airmass is in place and deepening with low-mid
level north/northeast upslope flow maintaining some light snow
showers or flurries through most of today. Also seeing some weak
energy moving east across the rockies, producing somewhat more
persistent light snow across western portions of north- central
MT. With little or no additional snow accumulation expected for NE
portions of the forecast area, have cancelled the winter storm
warning for areas along the Hi-Line from Toole east to Blaine
counties. Winter storm warning/advisories continue through the
rest of the forecast area where additional though light snow
accumulation is expected, with greatest new accumulations of 3-5
inches occurring across SW MT. Blowing/drifting snow issues are
now mostly confined to areas along the continental divide where
the cold airmass is spilling over to the west as well as some of
the N-S oriented valleys of SW MT. As snow diminishes this
afternoon and evening, impacts will shift toward cold temperatures
and wind chill tonight. Temperatures in the single digits and
winds as high as 10-15 mph will create wind chill values of 10
below to 20 below zero at times. Wind chill values lower further
tonight as temperatures dip below zero across most of the forecast
area with best chance for wind chill values of 20 below to 30
below zero across the northern tier of the State. Winds continue
to diminish however along with the lowering temperatures but wind
chill statements may be considered for portions of the forecast
area tonight in areas that see winds around 10 mph. Hoenisch

&&

.AVIATION...
Updated 1800.

Upper level trough continues to shift east into the Northern Rockies
and MT through Monday afternoon for overall low cloud ceilings
mountain obscuration and areas of light snow or snow showers. Areas
of light snow will affect most terminals into this evening with
lowest vis/cigs in somewhat steadier snow across SW MT terminals.
MVFR to IFR conditions prevail with occasionaly LIFR conditions in
areas of steadier snow with some gradual improvement likely late
tonight through Monday morning. Hoenisch

&&

.PREV DISCUSSION...
/ISSUED 407 AM MST Sun Feb 18 2018/

Today through Monday night...Upper level wave continues to move
south and east across SW MT this morning into the afternoon. This
will help push the Arctic front farther south to the MT/ID border
by this afternoon. The best frontogenetic lift also shifts south
through the day today, so this is where the enhanced snowfall
amounts will be found today. Meanwhile, enough instability and a
deep cold dome is in place for snow bands to impact much of the
Rocky Mountain Front today, especially the southern half, and
extending south through southwest MT to the north of the Arctic
front. Meanwhile, plains locations east of Interstate 15 will see
only residual light snow into Monday as cold air deepens across
the area. For these areas, a downgrade from the warning to an
advisory or even a cancellation will be possible by this
afternoon. However, an advisory or possibly even a warning for
wind chills may be needed as temperatures continue to drop across
the region.

The longwave trough will linger over our area through Monday,
bringing continued widespread cloudiness and residual light snow.
By Monday night, some partial clearing will begin to occur, with
temperatures continuing to plummet across the entire region into
the 10 below to 25 below range, with wind chills in the 30s below
to perhaps as low as 50s below range. CC

Tuesday through Saturday...Forecast models are in generally good
agreement with the overall pattern solution of a deep, positively-
tilted low pressure over the western and central United States
through the period. The resulting north-northwest flow aloft will
keep the forecast area very cold through at least Tuesday night
with very little chance for snow. A series of disturbances will
then likely bring an increasing chance of snow to the mountains
for Wednesday through Saturday, but increasing westerly downslope
winds over the plains will hinder significant precipitation
development there. These winds will also help warm temperatures
gradually through this part of the period, but the likely
widespread snow coverage from recent storms will likely prevent
significant warming and keep temperatures 5 to 10 degrees below
normal (forecast highs mostly in the 20s and forecast lows mostly
in the 5 to 15 above zero range). CC/Coulston

PREV DISCUSSION...
/ISSUED 500 PM MST Sat Feb 17 2018/

Tonight through Monday...Strong shortwave and surface low now
moving onshore in far SW BC will continue ESE tonight with main
focus for widespread and at times moderate to heavy snow
continuing to be focused across N-central MT and particularly NW
portions f the forecast area from I15 west to the Rocky Mtn Front
where synoptic and frontogenic lift will combine with low level
upslope flow. Further east, snow amounts have been reduced slightly
from previous forecasts but still support 5-8inches across much of
north-central MT east of I15 through Sunday night with bulk of the
snowfall coming tonight followed by periods of lighter snow. From
Helena south through SW MT the airmass south of the surface to mid
level low will be slightly unstable with strong winds aloft tonight.
Snow showers will likely organize into bands later this afternoon
and evening with hi-res models supporting a more intense E-W band of
snow likely to develop around the I90 corridor later this evening
then sink south with the passage of the mid level shortwave. With
potential for convective elements in the snow band, some stronger
wind gusts to 50mph or more are not out of the question, resulting
in a brief but intense snow squall, potentially moving through the
Bozeman area between 9pm and Midnight. Otherwise, total snow
accumulation across SW MT remains the same and generally less than 5
inches except over the higher mountain ranges. Winds also increase
across north-central MT overnight as as the surface low moves east
and a much colder airmass surges south from AB. By sunday morning,
temperatures across north- central MT should be in the single digits
and remain steady with wind chill values in the teens and twenties
below zero and north winds around 10-15 mph. Upper trough continues
to deepen over the western US Sunday and Monday as additional
shortwave energy drops south along the coast. Cold cyclonic flow
aloft over the region with high pressure at the surface will
maintain cold temperatures through Monday though cloudy conditions
persist with periods of light snow. Hoenisch

Monday Night through Saturday...Forecast models are in generally
good agreement with the overall pattern solution of a deep,
positively-tilted low pressure over the western and central United
States through the period. The resulting north-northwest flow aloft
will keep the forecast area very cold through at least Tuesday night
with very little chance for snow. Lows Monday night will fall well
into the teens and 20s below zero with 5 to 10 mph winds potentially
causing wind chills in the -30s and -40s below zero. A series of
disturbances will then likely bring an increasing chance of snow to
the mountains for Wednesday through Saturday, but increasing
westerly downslope winds over the plains will hinder significant
precipitation development there. These winds will also help warm
temperatures gradually through this part of the period, but the
likely widespread snow coverage from recent storms will likely
prevent significant warming and keep temperatures 5 to 10 degrees
below normal (forecast highs mostly in the 20s and forecast lows
mostly in the 5 to 15 above zero range).
Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF   2 -14  -3 -20 /  90  60  40  10
CTB   3 -18  -1 -19 /  90  50  20  10
HLN  10 -10   1 -18 /  90  60  40  10
BZN  16  -9   2 -20 / 100  80  40  10
WEY  24 -12  10 -26 / 100  90  30  20
DLN  24  -8   4 -15 / 100 100  40  10
HVR   6 -18  -3 -22 /  50  50  30  10
LWT   3 -12  -2 -21 /  80  80  50  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 5 AM MST Monday Central and Southern
Lewis and Clark...Eastern Glacier...Eastern Pondera...Eastern
Teton...Northern Rocky Mountain Front...Southern Rocky Mountain
Front.

Winter Weather Advisory until 5 AM MST Monday Beaverhead...
Broadwater...Central and Southern Lewis and Clark...Gallatin...
Madison.

Winter Storm Warning until 5 AM MST Monday Cascade...Chouteau...
Fergus...Judith Basin...Meagher.

Winter Storm Warning until 5 AM MST Monday Jefferson.

Ice Jam Flood Warning continues until 915 PM MST for the Missouri
River near Townsend in central Broadwater County.

&&

$$

http://www.weather.gov/greatfalls



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