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Area Forecast Discussion (AFD)

Issued by Great Falls, MT (TFX)

FXUS65 KTFX 212011

Area Forecast Discussion
National Weather Service Great Falls MT
211 PM MDT Mon May 21 2018


A large low pressure system will bring widespread shower and
thunderstorms today through tonight. Brief heavy rain and
lightning will be the main concern for today. This low pressure
system then lingers across the area for Tuesday and again
Wednesday...continuing widespread chances for showers and
thunderstorms. Locally heavy precipitation will again be the main
threat with any thunderstorm. Temperatures will also be near to
slightly above normal...with highs in the 60s and 70s and lows in
the 40s and 50s. 



Today through Wednesday...Large upper level low starting to 
stream moisture across the area this afternoon...creating 
scattered showers and thunderstorms. Expect increasing coverage 
through tonight as this initial push of moisture moves through. 
Instability values are decent...although shear and upper level 
flow is weak. This could lead to slow moving thunderstorms and 
possibly periods of heavy rain. A brief lull in activity may then 
be found late tonight into Tuesday morning...then the next found 
of moisture surges in with the upper level low still impacting the
area. The result will be more rounds of showers and 
thunderstorms. Slightly better shear on Tuesday could lead to an 
isolated stronger storm...although periods of heavy rain will 
still be the main threat. Everything then peaks on Wednesday as 
the low moves almost overhead. Expect more persistent showers 
with a few thunderstorms...especially in the central and 
southwest. This is still a relatively warm airmass with this 
system. Expect near to above normal temperatures through at least 
Wednesday. Anglin

Wednesday night through Memorial Day...The main upper level low
moves out of the region for Thursday. However...unstable west 
northwest flow will still be found across the region. As a result
some scattered showers and thunderstorms are possible...mainly
across the mountains...with not as wide of coverage as we are
expecting to see earlier in the week. Models then trying to build
a ridge Friday into Saturday. Now they are showing the ridge never
fully gets established...although weak disturbances to ride up and
over. The result could be some isolated to scattered thunderstorm
activity. Models are also speeding up a strong trough and upper
level low. This would now bring stronger and more widespread
storms on Saturday...with lingering showers and thunderstorms
possible on Sunday. An upper low cold then linger across the area
into Memorial day providing chances for more shower and
thunderstorm activity. Overall the timing of these disturbances
will need to be monitored. Those with outdoor plans should stay
tuned to the latest forecasts...and prepare for perhaps unsettled
weather at times. Temperatures will also warm up through the long
term. Some areas may even see high temps in the 80s...with the
timing of the disturbances later in the week determining whether 
those warm temperatures stick around. Anglin


Updated 1800Z.

Shower and thunderstorm activity will start to become more
widespread this afternoon...brining brief MVFR conditions to area
TAF sites...with VFR conditions expected in between shower
activity. Thunderstorms could also bring gusty winds at times of
generally around 30 knots. Thunderstorm activity diminishes this
evening...however...shower activity may linger through the night.
MVFR to brief IFR conditions are possible with these lingering
showers. A brief break in activity is then possible for Tuesday
morning...returning VFR conditions. More showers and thunderstorms
are then expected by late Tuesday morning into Tuesday afternoon.



Flood Warning for Big Hole River (NW Madison/NW Beaverhead Counties) 
through 3 pm MDT Monday (today): The river gauge reading near 
Melrose has fallen somewhat Sunday night, but it remains just above 
flood stage. The current forecast has it staying above flood stage 
on Monday and rising to Moderate flood stage Thursday evening, where 
it could remain into next week. Will therefore change the warning 
wording to be Until Further Notice.

Flood Watch for the Jefferson River at Three Forks (along the 
borders of Jefferson, Madison, Gallatin, and Broadwater Counties): 
The river level has remained steady just below flood stage, but it 
is forecast to exceed flood stage Monday evening, likely due to 
snowmelt and shower/thunderstorm runoff, possibly reaching Major 
flood stage on Thursday. 

Additional locations may approach or exceed flood stage Tuesday or 
Wednesday and may require further flood highlights if Monday 
morning's forecasts are consistent with those from Sunday night. We 
are monitoring the Missouri headwaters area (including the Missouri 
River near Toston and the Gallatin River near Logan) and the Sun 
River (near Simms). Coulston


GTF  52  73  51  72 /  60  30  20  60 
CTB  47  76  48  75 /  20  20  10  30 
HLN  49  73  50  70 /  50  60  50  60 
BZN  48  72  48  66 /  30  60  60  70 
WEY  33  66  37  58 /  20  50  60  70 
DLN  43  69  46  65 /  40  40  60  60 
HVR  52  78  52  78 /  30  20  10  50 
LWT  48  68  48  68 /  50  60  40  70 



Flood Warning in effect until 300 PM MDT Sunday for the Big Hole 
River in Beaverhead and Madison counties.

Flood Watch until further notice for the Jefferson River in 
Broadwater...Gallatin...Jefferson and Madison Counties.