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564
FXUS65 KTFX 171419
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
819 AM MDT Tue Sep 17 2019

.UPDATE...
No major changes were made this update. Temps and dewpoints/RHs
were updated for latest conditions and trends. POPs were adjusted
for current conditions and latest hi-res guidance, but overall the
trend for higher POPs on the western border of the CWA and for SW
MT seems to be holding true. -Kredensor

&&

.SYNOPSIS...

High temperatures today will be some 20 to 30 degrees cooler than
that of Monday thanks to a Pacific cold front moving across the
region. Precipitation chances will exist this morning and into the
mid-afternoon hours, before decreasing from west to east this
evening. Winds will be breezy and out of the west, with gusts
ranging from 20 to 40 mph.

&&

.AVIATION...
Updated 505 AM MDT Tue Sep 17 2019 (17/12Z TAFs)

VFR and near-MVFR conditions are generally expected throughout the
1712/1812 TAF period; however, periods of MVFR conditions are
possible beneath precipitation activity up until 00z Wednesday,
after which precipitation is quickly expected to diminish in
coverage. Mountain obscuration will continue through the morning and
into the afternoon hours, before improving this evening as drier air
works in across the region. Winds behind will become breezy and out
of the west this morning and into the afternoon hours, before
dissipating this evening. While confidence is at best low-to-
moderate at this time, patchy fog is possible tonight and Wednesday
morning after skies clear and winds relax, especially in areas that
see precipitation today. - Moldan

&&

.PREV DISCUSSION...
/ISSUED 505 AM MDT Tue Sep 17 2019/

Today through Wednesday night...primary forecast concerns are
precipitation chances and winds today.

Negatively tilted H500 trough axis extended from Southwest British
Columbia and into the Desert Southwest early this morning, with the
largest height falls (~60-80m) occurring over Southeastern
Idaho/Western Wyoming/Northern Utah. At the surface, a broad area of
low pressure was developing over Southern Saskatchewan/Eastern
Montana/the Western Dakotas, while a Pacific cold front was sliding
east across the Continental Divide. As the day progresses today
(Tuesday), the core of the H500 height falls is expected to slide
east northeast across Wyoming and over Southeastern Montana/the
Western Dakotas by this evening. This track will keep the best
synoptic ascent and forcing southeast of Southwest and North Central
Montana for much of the day, however, mid-level frontogenetic lift
will help to support some stratiform precipitation chances through
the early afternoon hours across most of the CWA. In addition, a few
convective showers will also be possible from late this morning and
into the mid-afternoon hours as lapse rates steepen from cooler air
moving in aloft. This will be especially true where any clearing
occurs. Winds will also be quite breezy from the west (wind gusts of
20 to 40 mph) this morning and into the evening hours as the front
crosses the remainder of the region and 6hr pressure rise of 3-6mb
overspread the Northern Rockies. High temperatures today will be
significantly cooler than the previous two days, with most locations
20 to 30 degrees cooler than Monday`s highs.

Transient H500 ridging will then overspread the Northern Rockies
during the overnight hours, which will allow for clearing skies and
reduction in winds. Depending on how much precipitation falls today
(which will provide additional boundary layer moisture to the
region), patchy to areas of fog could develop during the overnight
hours. High temperatures on Wednesday are expected to rebound closer
to climatological norms as the ridge transitions northeast over
Southern Canada and the H500 flow backs to the southwest ahead of
the next H500 longwave trough. At this time, Wednesday appears to
be the driest and "warmest" day of the next week.

Thursday through Tuesday...expansive Western CONUS trough and
northwest flow will bring below normal temperatures and
precipitation chances to the Northern Rockies through Saturday.
While subtle difference in the global models exist throughout this
period with the upper level pattern, it appears that areas along and
south of a Augusta, to Great Falls, to Lewistown line could see
precipitation amounts of 0.25"-0.5" from Thursday through Friday
night, with portions of Southwest Montana seeing upwards of 0.5"-1"
of QPF. With the cooler temperatures will also come falling snow
levels, which could lead to some accumulating snow (especially
during the overnight hours) to the higher peaks of Southwest
Montana. Transient H500 ridging and zonal flow moving across the
Northern Rockies Sunday and into early next week could bring
temperatures closer to normal, but model guidance begins to diverge
significantly over this timeframe and confidence is low. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  63  43  70  45 /  60   0  10  70
CTB  61  40  63  41 /  70   0  20  50
HLN  63  42  70  45 /  70   0  20  60
BZN  62  38  72  42 /  70  10  10  40
WEY  49  31  64  32 /  70  30   0  30
DLN  58  38  66  41 /  80   0  20  60
HVR  69  42  74  46 /  40  20   0  50
LWT  62  40  72  43 /  40  20   0  20

&&

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

$$

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