Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 210521
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1121 PM MDT Wed Sep 20 2017

...Updated Aviation Discussion...


.UPDATE...

Broad/deep upper level low/trough with center over the Canadian
Prairies continues to elongate to the SW this evening as upper
level energy drops south into northern CA, maintaining a fast and
cold SW flow aloft across the Northern Rockies and MT. Upper
level jet and shortwave energy streaming NE across ID and WY will
continue to support showery precipitation across portions of SW
MT tonight with the most persistent precipitation occurring right
along the ID/WY border. Further north, a few showers could linger
through tonight along the continental divide with generally dry
conditions to the east over the plains. Snow levels currently
range from around 5500 ft across northern areas to around 7500
along the ID/WY border this evening, with only minor lowering
anticipated through tonight, keeping bulk of heavier precipitation
across far SW MT in the form of rain or rain snow mix at pass
levels through tonight with accumulating snowfall limited to the
higher peaks. Hoenisch


&&

.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.

&&

.AVIATION...
Updated 0521z.

Main areas of concern through Thursday evening are a surface front
wind shift moving south through the region as well as the
development of a significant rain and mountain snow event across
southwest MT. Best timing of a transition from west to north or
north-northeast winds will be in the early-mid afternoon for KCTB
and KHVR, with a late afternoon frontal passage wind shift at KGTF
and KLWT late afternoon, then the remainder of the terminals in the
evening hours. As for precipitation over SW MT, expect significant
mountain obscurations beginning early Thu morning toward KWYS,
expanding north and west through the afternoon and evening, with
cigs dropping to low MVFR. Snow levels will be dropping slowly
through the day and into the evening Thursday, with any transition
from rain to snow at KBZN and KEKS expected after 22/06z. CC

&&

.PREV DISCUSSION...
/ISSUED 545 PM MDT Wed Sep 20 2017/

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  38  54  34  44 /  10  10  30  50
CTB  35  50  31  43 /  10  30  20  40
HLN  37  54  34  43 /  50  10  60  80
BZN  37  51  34  42 /  30  20  80  80
WEY  32  42  26  39 /  90  70  70  60
DLN  33  47  31  39 /  20  30  90  80
HVR  40  57  34  49 /  10   0  10  30
LWT  39  52  33  42 /  20  10  50  60

&&

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

$$

weather.gov/greatfalls


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