Area Forecast Discussion
Issued by NWS Great Falls, MT

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998
FXUS65 KTFX 202345
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
545 PM MDT Wed Sep 20 2017

Aviation Section Updated.

.SYNOPSIS...

Moist westerly flow aloft will continue to bring clouds and
showers western and southwest Montana. While breezy conditions
and continued cool temperatures...along with isolated showers are
expected over the plains. This pattern is expected to continue
tonight through Thursday. A larger disturbance may then bring
widespread rain and mountain snow on Friday.

&&

.DISCUSSION...

Tonight through Friday...North Central and Southwest Montana will
remain beneath the influence of a deep upper level low (currently
centered over southern Alberta) through the short term period.
Moist west to southwest flow aloft will continue over the forecast
area tonight and Thursday bringing periods of rain and snow to
the western and Southwest Montana mountains. The plains are
expected to remain dry and breezy; however, winds should diminish
overnight as surface pressure gradients relax due to the departing
low pressure system in Canada. Situation gets a bit more
interesting Thursday night into Friday as models suggest that the
upper level low splits with a secondary low developing over
southwest Idaho by late Thursday evening. The secondary low will
cause low and mid- level winds to become easterly across the
southern half of the forecast area. Orographic lifting of moisture
over the mountains will combine with upper level dynamics
associated with the low for widespread rain and snow, mainly south
of a line from KHLN-KGTF-KLWT. Snow levels appear to be low enough
for possible advisory-criteria snow accumulations at pass level
and above from the Little Belt Mountains southward to the Idaho
border. At this time, have opted to hold off from issuing any
highlights as models continue to trend the best QPF associated
with this storm system further south. As such, would prefer to see
another model run or two in order to see if models can stabilize.
Otherwise, much cooler temperatures are expected on Friday with
highs likely topping out in the 30s and lower 40s in Southwest
Montana, while North Central Montana should remain in the 40s. mpj

Friday night through Wednesday...Latest model guidance consensus has
remained on track in regards to the amplification and slow movement
of a longwave trough across the western US. However, discrepancies
in regards to how much moisture resides over the area is still in
question. Regardless, look for spotty shower chances through the
weekend with times of clouds and sun. Temperatures will begin to
moderate some as we get into next week as ridging begins to build
back into the Pac NW. We will also get a chance to dry out through
the middle part of next week as temperatures return to near, or
slightly above climatological normal. KLG
&&

.AVIATION...
Updated 2345z.

Upper level low pressure trough over southwestern Canada will
maintain a fast, moist, and unsettled westerly flow aloft across the
region this evening through late Thursday morning. Several
disturbances in the flow aloft will move across Southwest Montana
during the forecast period, triggering showers over western and
Southwest Montana. Occasional MVFR conditions can be expected at
KBZN, KEKS and possibly KHLN terminals due to precipitation with
widespread mountain obscurations anticipated. Mostly dry and breezy
conditions will continue across the plains. mpj

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  35  54  34  44 /  10  10  30  50
CTB  32  50  31  43 /  10  30  20  40
HLN  33  54  34  43 /  30  10  60  80
BZN  35  51  34  42 /  40  20  80  80
WEY  29  42  26  39 /  90  80  70  60
DLN  30  47  31  39 /  20  20  90  80
HVR  36  57  34  49 /  10   0  10  30
LWT  35  52  33  42 /  30  10  50  60

&&

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

$$

weather.gov/greatfalls



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