Area Forecast Discussion
Issued by NWS Great Falls, MT

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031
FXUS65 KTFX 101810
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1110 AM MST Sat Dec 10 2016

UPDATED AVIATION SECTION

.UPDATE...

An upper level disturbance will continue to progress across
northern portions of the CWA...while a cold front pushes across
the south for today. This will basically bring two areas of
snow...one to the north and one to portions of the south...with
little snow now expected in between. Winter Weather Advisories
are currently in effect across much of the CWA. The northern
Advisories along the Hi-Line will likely see the most snow today
of 1-2 inches with isolated higher amounts. Light snow will become
scattered across the south...with some blowing snow
possible...making for some impacts to travelers...so will keep
those advisories as well. The question debated was what to do with
advisories in the central. Light snow will still be
possible...however...downsloping air will make accumulations
light...especially across Cascade County. Current composite radar
showing snow trying to break through this downsloping air. Would
like to see how the rest of the morning plays out before canceling
these advisories. So in summary all advisories remain...although
some may be dismissed early...stay tuned. Other item to monitor
today will be high temps. Warm conditions ahead of todays front
will be found across the south...which was expected. If the arctic
air can get scattered out...some higher temps are possible as far
north as Great Falls. Bumped them up a little bit for the
update...and again will be monitoring closely how the morning
plays out. Anglin

&&

.AVIATION...Updated 1810Z.

Southwest Montana (KHLN KEKS KBZN): Conditions will remain VFR for
the next 24 hours with some passing mid level clouds and mountain
top obscuring snow showers through 00Z and again after 10Z.

North Central Montana (KGTF KLWT KHVR KCTB): A passing disturbance
will continue widespread mountain obscuring MVFR/IFR conditions in
occasional light snow through 21Z. Conditions will improve after
then. A frontal boundary will move northeast to a KCTB to KLWT line,
keeping KGTF under VFR conditions with increasing mid level clouds
and breezy southwest downslope winds. However, along and northeast
of that line, increasing moisture will overrun the cold airmass and
bring another round of low VFR/high MVFR ceilings with occasional
light snow after 12Z.

Note: The passing disturbance may also cause low level wind shear
between 1500 and 2000 feet AGL, mainly along and south of a KGTF to
KLWT line. North of this line, the stronger winds will likely be
more so in the 2000 to 3000 feet AGL level. These low/mid level
winds will decrease after 02Z.
Coulston

&&

.PREV DISCUSSION...
/ISSUED 435 AM MST Sat Dec 10 2016/

Today through Monday...A rather complex mix of ingredients
provides periods of light snow today. The northern edge of a long
plume of Pacific moisture is skirting across southwest MT with a
strong westerly flow aloft. Orographic lift and weak mid-level
warm advection will continue to sustain mainly mountain snowfall
over this area, with an additional 3-6 inches of new snowfall
today and tonight. There is just enough moisture and instability
that periods of light snow will also be possible within the SW
valleys. This westerly flow will scour out the coldest air and
lift temperatures near to just above the melting point. Snow
showers could turn back to rain for a short period this afternoon
at lower elevations, and in some areas of southwest MT will have
to monitor for possible freezing precipitation as temperatures
continue to warm aloft over the sub-freezing surface layer.
Another shortwave trough presently seen over western Washington is
expected to advance quickly east today and produce an area of
snowfall across the northern portions of our forecast area. The
westerly flow will limit snowfall amounts along the Rocky Mtn
front and areas around Great Falls. However, snowfall amounts
along central- and eastern portions of the Hi-line could reach
near two inches this afternoon with the passing shortwave. Winter
weather advisories remain in effect today, even though snowfall
amounts will be rather variable at lower elevations. Precipitation
ends for most areas late tonight with weak ridging aloft. Surface
winds increase from the southwest overnight as the Arctic airmass
retreats further from the area. High temperatures on Sunday will
be among the warmest we`ve seen in a week, yet still below normal
for the North-central plains. The warming is short-live as the
flow aloft turns more northwesterly, initiating a new surge of
Arctic air into the region starting Sunday evening. The associated
cold front will produce a period of snowfall enhanced by northerly
upslope winds. Initial guidance suggests around 2 inches of new
snowfall may be occur early Monday as folks are heading back to
work. Temperatures then fall through the day on Monday. PN

Monday night through Saturday...Medium range period starts off
with a deep upper level closed low over northern Manitoba and a
secondary low off the Pacific Northwest coast. This pattern puts the
state beneath an unsettled northwest flow aloft over Montana for a
slight chance to chance for light snow and flurries Monday night.
Temperatures will be quite cold across North Central Montana while
slightly milder conditions prevail across Southwest portions of the
state. As the Canadian closed low moves east and the Pacific low
slides south to southeast, Montana will be left between systems for
mostly dry conditions across central and northern portions of the
forecast area while a slight chance for light snow persists over my
southern zones through Wednesday night. Temperatures during this
time will warm a few degrees but should remain well below seasonal
averages. By Thursday the deep closed low will have moved into far
eastern Canada allowing a southward surge of energy through western
Canada to merge with the Pacific low and carving out a positive-tilt
trough across all of western North America. The bulk of the energy
and precipitation associated with this system is expected to remain
well south and west of Montana but models indicate that temperatures
will remain well below seasonal averages and PoPs should trend back
toward climatological normals for Thursday through Saturday. mpj

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  12   4  24  -1 /  40  20  10  70
CTB   5  -3   6  -7 /  60  20  30  70
HLN  33  14  32   7 /  40  10  10  50
BZN  32  13  31  16 /  50  30  10  40
WEY  32   6  21   8 /  80  60  40  60
DLN  36  13  30  15 /  40  20  10  40
HVR   7  -6   8  -5 /  80  50  50  60
LWT  24   7  27  -7 /  50  20  10  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 PM MST this afternoon
Broadwater...Cascade...Central and Southern Lewis and Clark...
Fergus...Jefferson...Judith Basin...Meagher.

Winter Weather Advisory until 5 PM MST this afternoon Blaine...
Chouteau...Eastern Glacier...Eastern Pondera...Eastern Teton...
Hill...Liberty...Northern Rocky Mountain Front...Southern Rocky
Mountain Front...Toole.

Winter Weather Advisory until 5 PM MST this afternoon
Beaverhead...Gallatin...Madison.

&&

$$

weather.gov/greatfalls



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