Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 201800

Area Forecast Discussion
National Weather Service Great Falls MT
1200 PM MDT Thu Oct 20 2016


Broad moisture streaming into MT ahead of the next shortwave trof
is creating widespread mostly cloudy to overcast skies across the
Hiline, parts of central MT and far southwest MT, while an area of
slightly drier air from Missoula to Livingston is bringing more
sunshine to areas around Helena and Bozeman this morning. All
areas will be mostly cloudy by midday as additional upstream
moisture arrives, but still expect little in the way of
precipitation despite all the cloud cover. Short-term models indicate
the main area of valley rain/mtn snow showers will be along the
Rocky Mtn Front, with some very isolated showers developing over
parts of the Hiline and northern Chouteau County later this aftn.
Precip amounts will be low (trace to less than 0.1 inch).

Surface winds will begin increasing along the Rocky Mtn Front and
over the central plains later this morning as the shortwave gets
closer and the surface pressure gradient increases. Current
projections of 40-45 mph gusts for the plains and southwest
valleys and 60-70 mph gusts in the higher elevations of the Rocky
Mtn Front still look good for this aftn/eve. Only forecast
changes this morning were to match cloud coverage to latest
satellite/webcam imagery.


Updated 1800Z.

VFR conditions expected. Widespread moisture accompanying an
approaching upper-level trough will generate widespread mid-level
cloud decks across central/southwest MT. Clouds will be generally
BKN-OVC from KGTF north to US/Can border and from KBZN south to the
MT/ID stateline, with SCT-BKN in-between those areas (KHLN/KLWT).
Other than -SHRA/-SHSN along the Rocky Mtn Front, little precip is
expected as the shortwave moves through; only mention of showers is
near KHVR for late aftn/early eve.  Surface winds will continue to
increase into the aftn hours as the pressure gradient tightens with
the shortwave`s arrival.  Widespread west-southwest gusts of 25-30
kts expected over the central plains and southwest valleys, with 55-
60 kts above ridgetops. Winds will steadily decrease later this


/ISSUED 605 AM MDT Thu Oct 20 2016/

Today through Tonight...A shortwave ridge over our CWA this
morning will move eastward as a fast-moving shortwave trough
approaches from the Pac NW. The axis of this shortwave trough
should reach the Continental Divide in southern BC/AB and north-
central Montana by this early afternoon, when another shortwave
trough should be moving onshore in southwestern BC and western WA.
Simultaneously, a lee surface trough will form/strengthen over the
plains of southern AB and north-central MT. This surface trough,
combined with stronger winds aloft accompanying the two
disturbances, will permit the development of gusty southwesterly
to westerly surface winds along the Rocky Mountain Front eastward
to about a Havre to Stanford line. The aforementioned pair of
disturbances should then reach western MB and western SK/northeast
MT, respectively, by daybreak Friday. Thus, the gusty surface
winds should begin to ease this evening and especially Friday
morning as the lee surface trough weakens. Peak gusts are
expected to be below high wind warning-criteria for the Rocky
Mountain Front and plains to the east. Otherwise, expect partly to
mostly cloudy skies during the period, with rain showers and
higher-elevation snow showers mainly impacting the mountains of
north-central and central Montana. Snow levels should rise from
near 5000 feet this morning to near 7000 feet MSL by daybreak
Friday. Highs this afternoon will mainly reach the 50`s or lower
60`s, while lows tonight will reach the mid 30`s to lower 40`s for

Friday through Saturday...The period begins with a shortwave ridge
building eastward into the region on Friday, allowing surface
winds to continue to ease across the area. Skies will become
partly cloudy, with dry weather for most. The exception will be
the Glacier Park area, where some rain and higher-elevation snow
showers may linger. Another Pacific shortwave trough will then
impact our region Friday night through Saturday afternoon, with
yet another shortwave ridge building-in thereafter for Saturday
evening. Cloud cover will increase Friday night into most of
Saturday, with the potential for widespread rain showers and
mountain snow showers, especially north of I-90, before quieter
weather and partial clearing commence Saturday evening. Snow
levels will primarily be within the range of 7000-8000 feet during
the period, but snow levels closer to 6000 feet MSL are expected
in/near Glacier Park. No significant or impactful snow amounts are
expected. Friday night will also have lows mainly in the mid 30`s
to lower 40`s, while Saturday afternoon highs reach the 50`s for
many locations. These temperatures will be near-normal to about 10
degrees above normal. Jaszka

Saturday Night through Thursday...Medium range models continue to
show good consistency Saturday night through Monday, but less so
after Monday. For Saturday night through Monday, an upper level low
pressure trough off the Pacific Northwest coast and an upper level
high pressure ridge centered over the Great Plains will keep the
forecast area under a southwest flow aloft. This will keep showers
over the western mountains, while lower elevations remain dry.
Temperatures will also gradually warm through this period from near
normal Saturday night/Sunday to around 5 degrees above normal for
Sunday night/Monday (lows 30s to lower 40s/highs 50s to lower 60s).

The forecast becomes a bit more muddled after Monday. The longwave
trough off the coast is forecast to eject a series of weak shortwave
troughs over the Pacific Northwest into Montana Tuesday through
Wednesday. However, there is some discrepancy with regards to the
timing of these disturbances moving over the area. The longwave
trough is then forecast to move onshore and at least into the
Pacific Northwest through Thursday. The GFS model keeps it in the
Pacific Northwest, while the ECMWF moves it into Montana. With this
model discrepancy, have gone with an unsettled pattern with an
attempt at timing the best chances for precipitation. Am therefore
going with somewhat higher than climatological average chances of
precipitation for this mid week period. This increase in cloudiness
and precipitation will help cool temperatures to between 5 and 10
degrees below normal (lows 30s/highs 40s to lower 50s).


GTF  59  42  60  40 /  10  20   0  20
CTB  53  36  54  36 /  20  20  10  10
HLN  61  38  60  38 /  10  10  10  20
BZN  58  37  60  37 /  10  10  10  10
WEY  42  27  51  29 /   0  10  10   0
DLN  57  34  58  35 /  10  10  10  10
HVR  56  38  57  38 /  20  30   0  10
LWT  58  40  59  39 /  10  20  10  10



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