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 FXUS65 KTFX 170515

Area Forecast Discussion
National Weather Service Great Falls MT
1015 PM MST Sat Dec 16 2017

...Aviation Section Updated...


Made minor tweaks to grids to reflect latest observations. There
are no major changes in the current thinking, with the potential
for patchy fog development especially toward the Bozeman area
which received the most snow. Winds will begin to increase in the
next few hours as low pressure troughing develops along the Rocky
Mountain Front, which mean most locations in this area are already
at or near their forecast lows and will slowly rise through the
night. CC



High pressure will build in from the west tonight for clearing
skies and seasonably cold temperatures. Some patchy fog is
possible in valley locations tonight as skies clear. Dry
conditions on Sunday will give way to increasing winds and cloud-
cover Sunday night. Several weather systems will move across the
region next week for progressively colder conditions along with a
a chance for snow.


Updated 0515Z.

IFR/LIFR conditions mainly due to fog and mist will be possible
across portions of Southwest and Central Montana into the early
morning hours today, before drier air and mixing within the boundary
layer disperse the moisture. The KBZN and KLWT terminals are most at
risk of observing these IFR/LIFR conditions. Elsewhere across the
remainder of Southwest and North Central Montana, MVFR and VFR
conditions are expected. Some mountain obscuration is possible
through the morning hours across Southwest and Central Montana.
Breezy to gusty winds are expected during the afternoon hours on
Sunday across the plains of North Central Montana and along the
Rocky Mountain Front. Frequent wind gusts in excess of 25kts are
expected for the KCTB, KHVR, and KGTF terminals Sunday afternoon and
into the evening hours. - Moldan


/ISSUED 425 PM MST Sat Dec 16 2017/

Trough axis extending from western Canada south into the Great
Basin will shift east across the forecast area late this afternoon
while splitting. Drier air moves in relatively quickly this
evening from the west as an upper level ridge moves onto the coast
for clearing skies from west to east across the forecast area
while any lingering snow across SE portions of the forecast area
ends early this evening. This may set the stage for patchy fog
across portions of central and SW MT with SW MT valleys more
likely to see the fog development later this evening as winds
will be relatively light and precipitation has only recently
ended, leading to relatively high surface moisture. Further north
across central and eastern portions of north central MT, there
will be a window of opportunity for patchy fog development early
this evening as skies clear but before winds increase in response
to lee-side surface troughing developing in Alberta. The upper
level ridge axis shifts east across the Northern Rockies and MT
Sunday for generally dry conditions with surface winds increasing
across north central MT as winds aloft increase along with a
strengthening lee-side trough at the surface. The ridge flattens
Sunday night and Monday as a larger scale trough moves into
western Canada with upper level jet sagging south to along the
US/Canadian border by Monday afternoon. Winds continue to
increase Sunday night through Monday morning with greatest
potential for strong winds along the east slopes of the Rockies.
Previously mentioned upper jet will also stream moisture in from
the Pacific for orographically enhanced precipitation along the
continental divide sunday night into Monday. An E-W frontal
boundary is likely to set up initially along the northern border
early Monday which then sags south through central MT Monday
afternoon, providing a focus for additional precipitation across
central and north-central portions of the forecast area. There is
some potential for strong winds to spread east into central MT
Monday morning but confidence remains low as surface winds will
also be shifting to NW as the frontal boundary sags south.

Monday night through Saturday...Monday`s front will sink south
during the evening hours, then stall near or just south of Great
Falls. This should allow the stronger winds from Monday to begin to
decrease areawide, with precip mainly confined to SW MT. On Tuesday,
a fast-moving, but strong, upper level system will move into the Pac
NW which will push the stalled front back north as a warm front. As
the upper level system nears, lee-side surface low development is
expected near Cut Bank. This low will then quickly shift east
through Wednesday, allowing a cold front to surge south across the
area at the same time. This system will likely bring a widespread
light snowfall to much of the area. However, lift will be maximized
along and north of the surface low track, with higher amounts
expected in that area. The models differ some on the exact track of
this low, but at this time, the heaviest snowfall amounts and
greatest impacts look to be along the Rocky Mtn Front and east
across the Hi-line. The rapidly strengthening surface low should
allow a period of gusty winds over the plains, which may add to the
impacts due to the potential of blowing snow. Stay tuned for
continued updates on expected impacts with this winter system.

Beyond mid-week, the models continue to suggest the development of a
broad upper level trough over the central US, which would favor
continued colder conditions. How cold remains to be seen, but the
potential exists for some of the coldest air of the season,
especially with a fresh snowpack likely across much of the area. The
cold air will likely be accompanied by periods of light snow. MARTIN


GTF 23 40 35 44 / 10 10 0 30
CTB 20 40 29 38 / 0 10 0 40
HLN 13 31 27 42 / 10 10 10 30
BZN 6 25 20 38 / 30 10 10 30
WEY -3 18 10 24 / 20 10 30 50
DLN 8 28 22 38 / 10 10 10 20
HVR 18 41 29 39 / 10 10 0 40
LWT 16 37 30 40 / 20 0 10 50