Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 211128

Area Forecast Discussion
National Weather Service Great Falls MT
528 AM MDT Mon May 21 2018



A low pressure system currently moving into the Great Basin will
send a series of weak weather disturbances as well as increasing
moisture north into the region through the next several days.
Showers and thunderstorms are likely to develop today and again
each afternoon and evening through Wednesday with greater coverage
of thunderstorms over southwest and central Montana. Some storms
could produce locally heavy precipitation.



An upper level trough extends south though the western US this
morning with closed low forming over southern CA and moist
southerly flow with embedded disturbances lifting north through
the eastern Great Basin into MT. Upper low tracks slowly NE into
NV Tuesday, then opens/weakens while lifting NE across MT
Wed/Thurs. An upper level ridge amplifies over the interior
western US Fri and Sat before another trough moves in from the
W/SW late next weekend.

A weak weather disturbance lifts N/NE across the region today in
moistening southerly flow aloft. While a few showers are possible
this morning, thunderstorm development should commence this
afternoon over the higher terrain of central and SW MT with
coverage increasing late this afternoon and evening as the upper
level wave moves across the region. Cloud cover could be a
somewhat limiting factor to convective potential today over
western areas. With wind shear and steering flow fairly weak (10-
20 kts)and available moisture (PWATs) relatively high, main
concern with convection this afternoon will be locally heavy
precipitation, especially this evening as flow aloft further
weakens. Similar concerns are anticipated with convection Tuesday
and Tuesday night with greatest concentration of
shower/thunderstorm activity focused over the southern half of the
forecast area. Wednesday holds the best potential for widespread
showers and thunderstorms across most of the forecast area as the
remnant mid-level circulation from the SW US drifts NE across MT
with relatively deep easterly flow aloft, though main forcing
looks to track just east of central MT. System exits to the NE
Thursday with drier conditions developing Friday into early this
weekend. Temperatures remain slightly above seasonal averages
through much of this week with stronger warming late this week
into the weekend, potentially bringing some the warmest
temperatures of the season so far by Saturday. Looking ahead to
the later portion of the upcoming holiday weekend, models are
coming into somewhat better agreement on a fairly vigorous trough
to shift east across the region during the Sun/Mon timeframe.


Updated 1128Z.

VFR conditions are generally expected for the forecast area through
at least 12Z Tuesday. However, brief periods of MVFR ceilings and
mountain obscuration are possible with heavier showers in some
thunderstorms between 18Z and 06Z. A moist southwest flow aloft will
spread mid and high level cloudiness with a few mountain showers
over the forecast area through 18Z. A disturbance in this flow,
paired with increasing instability, will then spread showers and
scattered thunderstorms northeast over the area from 18Z through
00Z. Some storms could produce wind gusts of up to 35 kt. Storms
will likely decrease from the west after 02Z as the airmass becomes
less unstable, but scattered showers will likely linger overnight
with more widespread lower VFR ceilings.  Coulston



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 Warning for the Lincoln area (west central Lewis and Clark
County): No significant changes to the ongoing flood situation there
have been reported recently. Again, this warning is set to expire
this afternoon, but it may need to be extended if, after
coordinating with local officials, the flooding remains significant.

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  75  52  73  51 /  40  60  30  20
CTB  76  47  76  48 /  20  20  20  10
HLN  74  50  73  50 /  50  50  60  50
BZN  73  48  72  48 /  30  30  60  60
WEY  65  34  66  37 /  30  20  50  60
DLN  69  44  69  46 /  50  40  40  60
HVR  81  53  78  52 /  20  30  20  10
LWT  74  49  68  48 /  60  50  60  40



Flood Warning in effect until 345 PM MDT Monday for the Lincoln area
of west-central Lewis and Clark County.

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.


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