Area Forecast Discussion
Issued by NWS Great Falls, MT

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590
FXUS65 KTFX 231110
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
410 AM MST Thu Nov 23 2017

UPDATED AVIATION

.SYNOPSIS...

A brisk westerly flow aloft will reside over the region for our
Thanksgiving into Friday. As a result, expect periods of strong
winds over the region, especially this afternoon and evening.
Additionally, periods of rain are expected over the western
mountains. The rain will gradually change to snow on tonight into
Friday, as a Pacific cool front moves through the region. In
addition, this front has the potential to produce a brief period
of heavy rain at lower elevations. Looking ahead to the weekend,
it will be mild and breezy through Sunday. Occasional rain and/or
snow showers are possible, especially in the mountains.

&&

.DISCUSSION...

Today and Saturday...An unusually-strong high pressure ridge aloft
will be broken-down by a Pacific trough today, allowing westerly
flow aloft to strengthen. This disturbance will cross our CWA this
evening through the early morning hours of Friday. A cold front
accompanying this trough aloft will cross our area from northwest to
southeast between this early evening and the predawn hours of Friday
morning. High pressure riding aloft will then become re-established
over the region later Friday through Saturday. However, a subtle
Pacific disturbance is progged to traverse the ridge Friday evening
into Saturday morning with little consequence.

Periods of upslope precipitation are expected along the Continental
Divide, especially the Rocky Mountain Front portion, through
Friday. Please see the hydrology section for further details. In
addition, the previously- mentioned cold front is expected to
trigger one or multiple bands of rain this evening into early
Friday morning. The rain should be moderate to heavy in many
locations as the front encounters weak instability amidst a plume of
abnormally-high humidity originating over the tropical Pacific, near
Hawaii. As the front advances southeastward, so too will the
moisture plume, allowing this plume to exit our CWA by daybreak
Friday. Latest model guidance suggests the best chance for this
brief period of heavier rain exists over Central and Southwest MT,
where low-level convergence and lift along the front should be
stronger than farther north. Colder air overspreading our area
behind the front will allow any lingering rain to change to snow
above approximately 4000 to 5000 feet MSL. At this time, no winter
weather highlights appear necessary for any of our mountain passes.
This frontal passage is also expected to be preceded and accompanied
by strong winds at the surface and aloft. Accordingly, high wind
warnings have been issued for the entire CWA. Please see these
statements for further details, including timing. Overall, quieter
weather is expected later Friday morning through Saturday as
stabilizing high pressure builds into our area.

Despite the cold frontal passage, temperatures are expected to
remain above-normal throughout the period due to the front`s Pacific
origin. The warmest readings will occur today, when afternoon highs
will be about 15-25 degrees above-normal. We still expect record
highs to be challenged in some places today, including Great Falls.
Jaszka

Saturday night through Thursday...Progressive upper level weather
pattern continues though next week, beginning with an upper level
ridge traversing the region Saturday night though early Sunday. A
lead shortwave moving over the ridge begins to break it down late
Sunday with moisture in an increasing SW flow aloft bringing some
precipitation to the western and SW MT mountains by Sunday evening.
Medium range models begin to get out of step with the arrival of
this lead energy, with the GFS bringing a more robust Pacific
frontal passage and moisture into the region as early as Sunday
afternoon while CMC and ECMWF models hold off until late Sunday
night into Monday for this with the arrival of more substantial
energy within the incoming upper level trough axis. In either case,
look for cooling to occur Monday with details on the timing of a
period of precipitation yet to be resolved. Cooling on Monday will
allow snow levels to lower below mountain passes with main impact
and focus for winter precipitation looking to occur near the
continental divide as well as the mountains of SW MT. Upper level
ridging then builds back into the Northern Rockies and MT on
Tuesday. Beyond Tuesday, models remain out of sync with regard to
individual shortwaves but are hinting at more of a NW flow pattern
vs previously advertised ridging.
Hoenisch

&&

.HYDROLOGY...
Updated 317 AM MST Thursday.

Periods of rain will continue over the northern Rockies today.
Additional rainfall of 0.5 to 1.5 inches is expected. This rainfall,
combined with melting snow, could lead to some minor flooding
problems on creeks/streams that flow out of the mountains. Flood
highlights are possible today should impacts develop. Cooler air
will move into the region tonight, changing the rain to snow and
diminishing the threat for flooding.
Brusda/Jaszka

&&

.AVIATION...
Updated 1110Z.

Mainly VFR and westerly flow aloft expected next 24-hours. Gusty
surface winds will impact all terminals, especially after 18Z/Thur.
Periods of precipitation and mountain obscuration will persist along
the Continental Divide. In addition, a cold front will sweep SE`ward
over the region between about 00Z and 12Z/Fri. This front will be
accompanied by a surface wind shift from southwesterly to westerly.
Periods of rain are expected along and behind the front, as are
periods of MVFR to IFR CIGS and VSBY. Elevations above ~4500 feet
MSL are most likely to experience rain changing to snow behind the
front.
Jaszka

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  64  36  46  33 /  20  20   0  10
CTB  58  34  44  30 /  20  30   0  10
HLN  60  31  43  26 /  30  60   0   0
BZN  62  30  44  22 /  20  70  10   0
WEY  44  27  39  22 /  30  70  30  10
DLN  62  30  44  25 /  20  50  10   0
HVR  65  34  50  27 /  10  10   0   0
LWT  63  33  44  28 /  20  20   0   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from 8 AM this morning to 5 AM MST Friday
Blaine...Cascade...Central and Southern Lewis and Clark...
Chouteau...Fergus...Hill...Judith Basin.

High Wind Warning until 11 PM MST this evening Eastern Glacier...
Eastern Pondera...Eastern Teton...Liberty...Northern Rocky
Mountain Front...Southern Rocky Mountain Front...Toole.

High Wind Warning from 2 PM this afternoon to 5 AM MST Friday
Beaverhead...Broadwater...Gallatin...Jefferson...Madison...
Meagher.

&&

$$

http://www.weather.gov/greatfalls



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