Area Forecast Discussion
Issued by NWS Great Falls, MT

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298
FXUS65 KTFX 150430
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
930 PM MST Sat Jan 14 2017

.DISCUSSION...

Surface high pressure remains situated over the interior NW US
with a lee-side trough of low pressure extending south along the
east slopes of the Rockies from AB into central MT. A dry NW flow
aloft will spread some areas of thin high clouds across the region
tonight with mainly clear conditions prevailing. Breezy winds
will continue along the east slopes of the Rockies which will also
limit temperatures from falling much tonight in these areas from
the Rocky Mtn front east to the I15 corridor in N-central MT.
Elsewhere, a strong low level temperature inversion continues to
produce large variations in temperatures by elevation with lower
elevation locations in north central MT and valley locations in SW
MT already falling to near and below zero with additional cooling
likely through tonight. Have made some minor adjustments to
minimum temperatures tonight based on observed trends so far this
evening. Otherwise going forecast is in good shape. Hoenisch

&&

.AVIATION...

With high pressure dominating, the main concern is valley fog
development overnight. The haze will continue at KHLN, with
potential fog at KBZN. VFR conditions will return area wide on
Sunday, with gusty winds developing at KCTB/KGTF. db

&&

.PREV DISCUSSION...
/ISSUED 407 PM MST Sat Jan 14 2017/

Tonight through Monday...Weak high pressure aloft will keep the
area dry and mostly clear again tonight, but a passing shortwave
trough will bring clouds to the area on Sunday. Breezy westerly
downslope winds will help keep temperatures mild along the east
slopes of the Rockies and into the Lewistown area throughout the
period. A strong surface pressure gradient will also linger along
the Rocky Mountain Front through the period, continuing the strong
westerly winds there. However, very little blowing snow has been
observed there, as the lingering snow there has likely become
somewhat crusted over. Have therefore cancelled the Advisory for
blowing snow.

Continued temperature inversions in the Southwest Montana valleys
will allow temperatures to fall well below zero again there
tonight, and patchy fog will likely redevelop as well. These
inversions will also continue the hazy conditions in the Helena
Valley, as well as in valleys of Broadwater and Jefferson
Counties. Winds have also been slow to mix down to the surface
elsewhere over the plains, so the likely decoupling of winds there
overnight will cause temperatures to fall below zero once again.
However, there may be some hope that temperatures will warm up
somewhat in these cooler areas Sunday through Monday, which may,
in turn, help alleviate the hazy conditions. The shortwave trough
mentioned above for Sunday has cold air advection and a weak cold
front, which may help mix down breezy westerly winds aloft and at
least weaken the inversions in place. Although snowpack in those
cooler areas will be slow to melt, making a rapid warm up
unlikely, persistent erosion of the temperature inversions should
help warm these areas closer to normal through Monday.
Coulston

Monday Night through Saturday...Overall a fairly quiet period,
with the best chance for widespread precipitation Thursday
afternoon/night. An upper level ridge of high pressure over the
region Monday night and Tuesday will produce mostly dry and mild
conditions over the northern Plains, with just a few showers over
Southwest MT/Rocky Mountain Front. The ridge moves east of the
area Tuesday night. Pacific moisture will advance up to the
Continental Divide and produce likely chances for precipitation
over the Northern Rockies. Northwesterly flow at mid-levels could
locally enhance snowfall over the southwest mountains Tuesday
night. Downslope winds will strengthen amid a strong pressure
gradient along the Rocky Mountain Front Wednesday morning,
possibly nearing high wind criteria. Temperatures will be be
warming up during this period...possibly reaching near 50 degrees
in the Fort Benton area. For Wed into Thursday morning, a
southwesterly flow aloft will develop, resulting in the mild air
continuing to reside over the region, with a few showers likely
over the Front Range and across Southwest MT. Strong downslope
winds continue Thursday morning. The precipitation should become a
bit more widespread late Thu afternoon into early Friday morning.
With an upper level disturbance combined with a Pacific cool
front moving through the region, this looks to be our best chance
for precipitation this week. Most areas could receive between 0.05
and 0.15 inches of precipitation. Some of the precip will fall in
the form of rain at lower elevations on Thursday before changing
over to mostly snow by Friday morning. Snow accumulations in the
mountains of 1 to 3 inches look reasonable at this time.
Temperatures cool off a touch for Fri/Sat as most of the
precipitation exits the region. PN/Brusda

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  20  37  21  40 /   0   0   0   0
CTB  18  37  19  40 /   0  10   0   0
HLN -11  23   1  28 /   0   0   0   0
BZN -11  20  -3  31 /   0   0   0   0
WEY -23  15 -12  24 /   0   0   0   0
DLN   3  30   7  31 /   0   0   0   0
HVR  -4  25   6  29 /   0   0   0   0
LWT  14  37  17  39 /   0   0   0   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

weather.gov/greatfalls



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