Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 212100
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
300 PM MDT Mon Apr 21 2014

.DISCUSSION...
Tonight through Friday...Quiet weather tonight will give way to a
more active weather pattern from Tuesday through Friday. Expect a
Pacific Cold front to move through the region on Tuesday. Showers
and thunderstorms will develop ahead/along the front. The best
chance for storms will be from 3 pm Tue until 9 pm Tue. It looks
like the strongest storms should be in the Judith Basin and Fergus
Counties area...with some gusty winds/small hail possible in this
region. The rain will change to snow in the mountains. A few
inches of snow are possible at pass level...but the more
significant snow fall should fall mainly at ridge tops...so no
winter highlights at this time. Temperatures cool off on
Wednesday...and then continue to remain cool/seasonable for
Thurs/Fri. Additionally...strong winds are possible by early
Wednesday morning over the Rocky Mountain Front and adjacent
plains...thus this area is under a high wind watch. For
Thursday...just a passing shower is possible...but for
Friday...another cold front will move through the
region...producing a better chance for precipitation across much of
the region...especially on Friday afternoon. Brusda

Saturday through Monday...Though the medium range models continue to
have similar overall trends, the details differ somewhat. However,
the 12Z runs of the GFS and ECMWF are beginning to trend toward each
other a bit, so have maintained a compromise forecast for this
period. For Saturday through Saturday night, an upper level trough
of low pressure will continue to move across the Pacific coast
states into the Rocky Mountain region. The resulting moist southerly
flow aloft will keep a very good chance of showers over the area,
especially in the mountains. Temperatures Saturday will stay warm
enough to keep any potential for snow (more likely, a rain-snow mix)
above 7000 feet. Temperatures will then cool enough Saturday night
to potentially mix snow with the rain at lower elevations as well.
The airmass in southwest Montana may also become somewhat unstable
Saturday afternoon, for a slight chance of thunderstorms there. The
upper level trough axis will then gradually move east across Montana
through the remainder of the period, for a decreasing chance of
showers. However, the main difference in the forecast models is just
how cold it will get. The GFS keeps a frontal boundary centered over
north central and southwest Montana, which keeps temperatures only 5
to 10 degrees below normal through the period. The ECMWF pushes the
boundary farther to the west and south, which allows the resulting
colder Canadian airmass to keep temperatures closer to 10 to 20
degrees below normal. As mentioned above, the models are trending
toward each other, as the GFS cools its temperatures slightly and
the ECMWF warms its temperatures slightly. The ECMWF seems to be
more of an outlier, so am leaning more towards the slightly warmer
GFS/consensus model solution for now and will continue to monitor
the progress of these solutions.
Coulston

&&

.AVIATION...
UPDATED 1735Z.
VFR conditions are expected to continue through at least 18Z
Tuesday. Areas of high cloudiness with ceilings of above 15000 feet
AGL will increase over the area through the period. However,
disturbances in the increasingly moist southwesterly flow aloft may
bring scattered to broken mid level clouds with ceilings of 8000 to
10000 feet AGL to the area between 00Z and 12Z, along with a few
mountain showers. Breezy southwesterly winds of 5 to 15 mph will
shift more easterly through 03Z.
Coulston

&&

.HYDROLOGY...
A Pacific cold front will move through the region on Tuesday. The
best potential area for any flooding to develop looks to be
Gallatin County. Thus a flood watch has been issued for this
region. The East Gallatin River is the river of main concern in
Gallatin County...but other rivers/creeks that are mountain feed
streams could possibly have problems. Showers and thunderstorms
are possible in other regions...but most precipitation amounts
will generally be less than a half inch or so...thus flooding is
not expected at this time. Should the precip amounts
change...additional flood highlights might be needed. Brusda

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  46  72  39  52 /   0  70  90  30
CTB  40  68  36  51 /   0  70  90  30
HLN  45  69  37  54 /   0  70  90  50
BZN  42  72  33  52 /  10  60  90  60
WEY  36  60  28  43 /  20  70  80  70
DLN  42  65  31  50 /  10  80  90  40
HVR  42  78  40  58 /   0  30 100  60
LWT  42  71  36  51 /   0  40  90  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

HIGH WIND WATCH from late Tuesday night through Wednesday
afternoon Eastern Glacier...Eastern Pondera...Northern Rocky
Mountain Front...Southern Rocky Mountain Front...Toole.

FLOOD WATCH from Tuesday evening through Wednesday afternoon
Gallatin.

&&

$$

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