Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 190308
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
808 PM MST Sat Nov 18 2017

.UPDATE...

Only update to tonight`s forecast was to keep stronger winds going
until around midnight along the Rocky Mountain Front. 00Z NAM
models keep a core of moderate winds going until then, which has
matched up pretty well with observed winds there. It`s far from a
high wind situation, but it is significant enough to warrant
adjustment. Also adjusted lows a bit lower in spots that were
already near forecast lows, but it was not a widespread change.
Coulston

&&

.SYNOPSIS...

Strong winds will continue through the weekend, especially along
the Rocky Mountain Front on Sunday and spreading east into
western portions of north central Montana late Sunday and Sunday
night. A cold front descends southeast into the region on Monday
and will result in areas of light rain changing to snow.

&&

.AVIATION...
Updated 2340Z.

VFR conditions with mid and high cloudiness passing overhead will
continue through the next 24 hours as high pressure aloft spreads
over the western United States.

The main impact for this period will be wind and resulting
widespread mountain wave turbulence. The westerly flow aloft will
become increasingly strong through the period as a disturbance
flattens the high pressure ridge. The gusty westerly surface winds
currently over the plains of north central Montana will decrease
somewhat after sunset as they partially decouple. However, winds
close to the Rocky Mountain Front (west of KCTB and KGTF) will
continue to gust into the 40 to 50 kt range. Strong winds will
become widespread again over the plains after 15Z. After 18Z, Gusts
of 25 to 35 kt will be common over the eastern plains (KHVR and
KLWT), but winds will gust to between 40 and 55 kt over the western
plains (KCTB and KGTF). Gusts in excess of 60 kt are likely closer
to the Rocky Mountain Front, making isolated wind shifts (rotors)
and/or decreases (hydraulic jumps) possible, further causing
hazardous flying conditions. Southwest Montana terminals will
generally continue to have relatively light winds, except at KEKS
where the pressure gradient is favorable for wind gusts to 30 kt at
times out of the south.
Coulston

&&

.PREV DISCUSSION...
/ISSUED 440 PM MST Sat Nov 18 2017/

Today through Monday...Upper level ridge drifts east across the
region today and tonight for relatively quiet and dry conditions.
Substantial pressure gradient across the Rockies remains at the
surface for continued gusty winds along the east slopes of the
Rockies tonight with breezy west winds. The upper ridge begins to
flatten Sunday in response to shortwave energy dropping SE through
BC, which eventually shears east across southern Alberta Monday
along with an associated surface low, sending a cold front south
through the forecast area in its wake late Sunday into Monday.
Already strong west winds along the immediate east slopes of the
Rockies will increase further though the day on Sunday as winds
aloft increase under the flattening upper ridge. Models continue to
indicate 700MB flow at 60-80kts spreading out to the south and east
of the Rocky Mtn Front late Sunday and Sunday night, bringing
potentially strong to high winds to adjacent western portions of
north-central MT. A cooler airmass slides south into the region
Monday behind the shortwave and surface low traversing the Canadian
Prairies. Rain and snow showers are possible Monday as the front
progresses south through the forecast area with orographically
enhanced precipitation also likely to be focused along the
mountains and adjacent plains. CC/Hoenisch

Monday night through Saturday...A high pressure ridge aloft will
reside over our CWA Monday night through Wednesday. However,
multiple disturbances should traverse the ridge from west to east;
the most prominent of which should affect the region on Tuesday.
This ridge is then forecast to be broken-down by several stronger
disturbances on Thanksgiving and Friday. In addition, a Canadian
cold frontal passage now looks to occur late Thanksgiving into
Friday morning. Model guidance is in good agreement that a high
pressure ridge aloft will then build eastward into our region on
Saturday. Atmospheric profiles remain favorable for periods of
strong, gusty downslope winds along the Rocky Mountain Front. Right
now, Wednesday through Thanksgiving is the window of greatest
concern for these winds.

The aforementioned pattern will be conducive to periods of upslope
precipitation along the Continental Divide and the Southwest MT
mountains. Elsewhere, isolated snow and/or rain showers are possible
Monday night through Wednesday, courtesy of the aforementioned
disturbances traversing the ridge aloft. POPs also increase late
Thanksgiving night through early Saturday, especially over Southwest
MT, where one or more bands of steadier precipitation may form along
the Canadian cold front. This progressive weather pattern will
result in variable temperatures. At this time, Tuesday looks to be
the coolest day of the long term period, while Wednesday should be
the warmest. In fact, Wednesday`s highs look to reach the 50`s to 60
degrees at many lower-elevation locations. These forecast readings
are about 15-20 degrees above-normal.
Jaszka

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  34  52  39  46 /  10   0  10  40
CTB  30  49  32  37 /  10   0  20  40
HLN  19  47  30  45 /  10   0  10  40
BZN  18  47  27  44 /  10   0   0  50
WEY   8  32  18  33 /  10  10  10  70
DLN  17  43  27  44 /   0   0  10  50
HVR  24  52  31  38 /  10   0  10  30
LWT  26  51  34  42 /   0   0   0  40

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from Sunday afternoon through late Sunday night
Cascade...Central and Southern Lewis and Clark...Eastern
Pondera...Eastern Teton...Judith Basin...Toole.

High Wind Warning from 6 AM Sunday to 5 AM MST Monday Eastern
Glacier...Northern Rocky Mountain Front...Southern Rocky
Mountain Front.

&&

$$

http://www.weather.gov/greatfalls



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