Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 221005

Area Forecast Discussion
National Weather Service Great Falls MT
405 AM MDT Tue May 22 2018


A broad area of low pressure gradually moving northeast from the
Great Basin will bring increasing chances for showers and
thunderstorms again today with greatest coverage across central
and southwest Montana. Some thunderstorms may be slow moving with
locally heavy rainfall. Coverage of showers will increase
Wednesday, especially across eastern portions of central Montana.
Temperatures remain near to above seasonal averages through
Wednesday with warmer conditions expected late this week into
early this weekend.



An upper level low centered over the Dessert SW US this morning will
slowly lift north across the Great Basin today and across eastern
MT Wednesday. Weak upper level ridging builds over the interior
western US in the wake of this system on Thursday and Friday
before another upper level low pressure system moves into the
Great Basin region this weekend.

Mid-level instability and moisture already in place across the
region will combine with increasing instability this afternoon for
an increase in showers and thunderstorms. With wind shear and
steering flow remaining relatively light with main concern,
similar to yesterday, continuing to be localized heavy rainfall
beneath slower moving storms late this afternoon and evening. On
Wednesday the passage of the upper low center just east of the
forecast area will keep greatest instability across eastern MT,
but N/NE flow around the circulation should provide a fairly
widespread coverage of showers and possibly a few thunderstorms
with heaviest precipitation amounts focused across eastern
portions of the forecast area where models are now suggesting a
more cohesive area of wrap-around precipitation may develop. While
this system lifts north to the Canadian Prairies Thursday,
lingering moisture and weak embedded energy in westerly flow could
still set off a few showers and thunderstorms Thursday afternoon
before more substantial drying moves into the region Friday under
a transient upper level ridge.

Models continue to trend toward a more unsettled scenario for the
Memorial day holiday weekend as incoming Pacific energy evolves into
another broad upper level low over the Great Basin region. Models
are also trending toward a slower progression of this system
east/northeast across the interior western US through early next
week, resulting in increased chances for precipitation in the
form of showers and thunderstorms Saturday through Tuesday.


Updated 0550Z.

VFR conditions are generally expected to continue through at least
06Z Wednesday, but brief periods of MVFR conditions may occur with
thunderstorms between 18Z and 03Z. A low pressure area over the
Great Basin will continue the mid level cloudiness through the
period over the area south of a KCTB to KHVR line, but along and
north of this line, it will remain partly cloudy. A few showers may
linger through 15Z south of that line, but showers will become more
widespread through 20Z with scattered thunderstorms possible once
again as the airmass becomes unstable. Brief heavy rain and erratic
wind gusts will be the main threats. The shower/storm activity will
gradually decrease after 01Z.  Coulston



Flood Warning for Big Hole River (NW Madison/NW Beaverhead Counties)
until further notice: Despite the river level falling Monday night,
the current forecast has it staying above flood stage and rising to
Moderate flood stage Thursday evening, where it could remain into
next week. No changes needed at this time.

Flood Warning for the Jefferson River at Three Forks (along the
borders of Jefferson, Madison, Gallatin, and Broadwater Counties):
The river level has reached flood stage Monday night, so the Watch
was changed to a Warning, in effect until further notice. It is
forecast to rise to Moderate stage Wednesday night, then Major stage
Thursday night.

Additional locations may approach or exceed flood stage through
Thursday due to continued warm temperatures and a chance of brief
heavy rainfall from thunderstorms. Further flood highlights may be
needed, and an Extended Streamflow Forecast (ESF) has been issued
addressing this. 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  74  50  74  52 /  40  20  60  30
CTB  74  47  77  49 /  20  10  20  30
HLN  70  50  73  49 /  40  40  70  50
BZN  73  48  66  46 /  40  60  80  60
WEY  65  38  56  37 /  40  60  80  50
DLN  68  45  65  44 /  30  50  80  50
HVR  78  51  78  53 /  10   0  50  50
LWT  66  49  65  49 /  60  50  90  70



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

Flood Warning in effect until 145 AM MDT Monday for the Jefferson
River near Three Forks.


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