Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 180532
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1031 PM MST Sat Feb 17 2018

...Updated Aviation Discussion...

.SYNOPSIS...

A strong storm system tracking east across the region tonight
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. Periods of snow will
increase over southwest Montana as well tonight with a brief but
intense snowband accompanied by gusty winds expected this evening.
Gusty north winds will develop tonight as the low pressure center
moves off to the east and much colder air pours south into the
region from Canada.

&&

.UPDATE...

Winter storm continues to take shape over the region with most
locations in north-central and central Montana reporting snowfall
this evening. Latest Montana DOT reports indicate that several
roads have severe driving conditions in this region as well. As of
8 PM MST, temperatures across southwest Montana remain above
freezing so have updated the diurnal temperature trend to account
for this. Have also made minor tweaks to PoPs/QPF/Snow Amounts
based on latest short term model consensus tools and observations.
Overall, the forecast remains fundamentally unchanged for most of
the forecast area tonight into Sunday morning and all existing
highlights will remain in place. Biggest change noted was lower
snow amounts in the valleys of southwest Montana due to the
current warmer temperatures. mpj

&&

.AVIATION...
Updated 0531z.

A cold front that just passed KBZN in the last hour will continue to
push south and east out of the TFX forecast area through 12z.
Widespread light to moderate snow will continue MVFR/IFR conditions
across north central and central Montana through 12Z. Northerly
winds will also cause widespread blowing and drifting of snow. Snow
will then decrease from the northwest through 12Z, for a gradual
improvement toward VFR/MVFR conditions, but IFR conditions will
still be possible in the KHLN and KLWT areas. Snow will be more
showery in nature across southwest Montana, which will cause
VFR/MVFR conditions to quickly worsen to IFR in those showers.
Strong mid-level westerly winds will also cause mountain wave
turbulence and low level wind shear, and they will likely mix down
to the surface there at times with the showers, causing gusts of up
to 35 kt mainly through 12Z. 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   0   2 -15  -7 / 100  80  60  40
CTB  -2  -2 -20  -4 / 100  70  40  30
HLN   9  11 -11   1 /  90  70  60  30
BZN  16  18 -10   1 / 100  80  60  30
WEY  18  24 -13   9 / 100  90  80  40
DLN  23  24  -9   1 /  80  70  70  30
HVR   0   4 -18  -5 / 100  70  30  30
LWT   1   3 -13  -4 / 100  70  50  40

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 AM MST Monday Beaverhead...
Broadwater...Central and Southern Lewis and Clark...Gallatin...
Jefferson...Madison.

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...Toole.

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

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

&&

$$

http://www.weather.gov/greatfalls



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