Area Forecast Discussion
Issued by NWS Great Falls, MT

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689
FXUS65 KTFX 230948
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
348 AM MDT Sat Sep 23 2017

.SYNOPSIS...

Rain and snow showers will gradually decrease through the morning
hours today, with only spotty rain and or snow showers possible
across the higher elevations during the afternoon hours. High
temperatures will remain well below normal through the weekend,
with most locations falling below freezing during the overnight
hours tonight. Patchy fog will also be possible across portions of
Southwest and North Central Montana tonight.

&&

.DISCUSSION...

Today through Monday...main forecast concerns over the short term
are lingering spotty precipitation chances, overnight low
temperatures (tonight and Sunday night), and the potential for fog.

An impressively large and elongated positively-to-neutrally tilted
H500 trough over the Wester CONUS, will make little eastward
progress through much of the day on Sunday. While much of the energy
with this system will remain over the Great Basin and Central
Rockies where the main H500 closed low will be located, and to the
east over the Central and Northern High Plains where better jet
dynamics reside; Southwest and North Central Montana will still be
under its influence. This will lead to well below normal high
temperatures today and Sunday, and spotty chances for rain/snow
showers (mainly during the day today over the mountains and far
eastern zones due to weak upslope flow and a passing embedded
shortwave within the main trough). By this evening, BUFKIT soundings
show drier mid-level air working in across North Central and
Southwest Montana. This drier air should promote clearing skies,
which will allow temperatures to fall below freezing across most
areas (with the exception of the Helena Valley and portions of
Chouteau, Hill, Blaine, and Fergus Counties). With antecedent
moisture from Friday/Friday nights rain/snow (and snow melt do to
relatively warm ground temperatures) combined with clearing skies
and light winds, believe that patchy fog will be possible during the
overnight hours tonight and into the morning hours of Sunday across
a good portion of Southwest and North Central Montana. By Sunday
evening, the H500 closed low over the Central Rockies will begin to
open and lift northeastward into the Northern High Plains of Eastern
Montana/Western North Dakota. As this occurs, H700 to H500 flow over
North Central and Southwest Montana will become increasingly
northwesterly. This northwest flow will begin to advect in "milder"
Pacific air in across the region for the day on Monday, with
temperatures warming/moderating (albeit marginally) across North
Central Montana do to the downsloping winds. Winds along the Rocky
Mountain Front will become breezy during the afternoon hours on
Monday as H700 cross barrier flow increases. - Moldan

Monday night through Saturday...By the beginning of next week,
models are in good agreement with the development of broad ridging
over the Pac NW, with the slow moving trough over the Northern
Rockies finally deamplifying and moving off to the NE. This will
place us in a NW flow aloft between the exiting trough and
amplifying ridge to the west. As far as day-to-day details, models
slightly differ on the idea of small embedded shortwaves swinging
across the state along the NW flow. Moisture will be modest, given
PWAT anomalies above normal areawide which may be enough to squeeze
out a few isolated showers Tuesday and Wednesday afternoon.
Regardless, the bigger story will be moderating temperatures. As we
get into Thursday, the aforementioned ridge across the Pac NW will
build eastward across a large majority of Northwest US. This will
send temperatures back to near climatological normal, if not
slightly above normal with widespread temperatures in the 60`s and
70`s by late week for many, followed by dry conditions. Models begin
to display noticeable discrepancies as we get into next weekend,
with the general idea of the ridge slowly breaking down. EPS and
GEFS guidance suggests continued above normal height anomalies which
would side against any cold/winter weather issues. However due to
low confidence in this time frame, I`ve decided to keep temperatures
near normal next weekend with only slight rain chances. But this
will likely change as models become in better agreement in the
upcoming few days. KLG

&&

.AVIATION...
Updated 0547z.

Look for continued low clouds and relatively calm winds across all
terminals through the overnight hours. Latest regional radar
composite imagery is showing the next wave of precipitation moving
into southern MT and will continue drifting northward through the
early morning hours. This may cause intermittent periods of light
rain/snow for EKS, BZN and LWT with IFR/MVFR conditions at times. We
will begin to see improving conditions after 12z for all terminals
with steadily rising ceilings and widespread VFR through the
afternoon hours. KLG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  51  30  56  33 /  10  10   0   0
CTB  51  29  58  33 /  10  10   0   0
HLN  53  34  56  34 /  10  10  10   0
BZN  50  30  52  31 /  10  10  20  20
WEY  45  26  45  25 /  10  10  20  10
DLN  45  28  48  28 /  10  10  10  10
HVR  54  33  59  34 /  10  10   0   0
LWT  47  32  51  32 /  20  10  10  10

&&

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

$$

weather.gov/greatfalls



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