Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 280246
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
846 PM MDT Sun May 27 2018

.SYNOPSIS...

Scattered showers and isolated thunderstorms will linger across
Southwest Montana into later this evening, then redevelop again on
Memorial Day. Across Central Montana, mostly dry conditions are
expected, although a few showers cannot be ruled out in the
mountains and near Lewistown. Temperatures will be close to, or
above, normal on Monday.

&&

.EVENING UPDATE...

A somewhat interesting forecast this evening. An extensive band of
convection extends from the Central Plains all the back to the
Northern Rockies, on the periphery of an upper level low over Utah.
On the northeast quad of that low resides a diffluent flow aloft
which is aiding in thunderstorm maintenance from Wyoming into SE MT.
As the upper low edges NE, so will the area of broad lift. At the
surface, this will lead to a strengthening moist, E/SE flow across
Montana. All of this should allow thunderstorms to continue to
advance further north into parts of southern MT overnight and
into Monday. As the band of showers/storms rotates north/northwest
towards our forecast area, though, it will encounter drier
mid/upper level air and likely some subsidence as well. The
assumption, then, is that the band will weaken/break apart
overnight as it approaches our area, but possibly not before a few
showers reach our far eastern counties (ie Madison/Gallatin up
through Fergus). The forecast has been adjusted some based on
these trends, although no significant changes were needed. For
Memorial Day, a moist and weakly unstable environment will allow
weak afternoon showers and thunderstorms to develop, mainly
focused along the Continental Divide and parts of SW MT (not
unlike today). Any thunderstorm that develops on Monday will be
capable of heavier rainfall in a short amount of time given the
higher moisture content and slow storm motions. Any sustained
updraft could also contain some hail up to dime size, but that
threat appears fairly low/unlikely. - Martin

&&

.AVIATION...
Updated 2342Z.

VFR conditions expected through Monday evening.

Isolated, weak SHRA/TSRA continue to push west across SW MT at this
time. This activity should begin to diminish over the next few hours
with the loss of daytime heating. However, there may be just enough
lift to keep a few SHRA around overnight, mainly along the MT/ID
border. The band of TSRA over WY this evening will continue to lift
north into SE MT by Monday. This band may approach KLWT/KBZN by
Monday afternoon. Otherwise, diurnally-driven SHRA and perhaps a
TSRA are expected again Monday afternoon, but mainly across SW MT. -
Martin

&&

.HYDROLOGY...

Mainstem river flooding to continue this week, especially
Southwest Montana. Isolated flash flooding risk continues this
week as well.

Ongoing snowmelt will allow for continued rises for several
mainstem rivers across Southwest Montana over the next several
days, and likely beyond. That snowmelt water will eventually
reach southern sections of Central Montana and river levels will
need to be closely monitored mid to late-week in that area.

Thunderstorm coverage will vary this week, but may become a bit
more widespread Wednesday/Thursday. Of note, high precipitable
water values will likely lead to efficient rainfall production
with any thunderstorm that develops this week and there will
remain at least an isolated potential of flash flooding. - Martin

&&

.PREV DISCUSSION...
/ISSUED 542 PM MDT Sun May 27 2018/

Tonight through Tuesday Night...An upper level low will be the
main weather maker through Monday as it slowly tracks from the
Great Basin to the Northern High Plains. The result will be more
chances for showers and thunderstorms through this evening into
the overnight, mainly across southwest Montana. Ridge of high
pressure briefly moves in for Tuesday with brisk northwesterly
winds behind a cold front moving southeast across the plains.
Mainly dry conditions will be a result, although cannot rule out a
few mountain thunderstorms. Overall temperatures will remain
above normal during the short term period with highs in the 70s to
low 80s and lows in the 40s and 50s. CC/Anglin

Wednesday through Saturday...A cooler and wetter weather pattern
commences Wednesday and may last through the weekend. Wednesday
sees a sharp cold front passage. This combined with modest
instability and ample shear could bring a decent chance of
widespread showers and thunderstorms. Some storms could be on the
stronger side, which will need to be monitored. Ridging across the
northern plains will cause this front to stall across the region
for Thursday into Friday. This combined with a broadscale trough
looks to bring widespread chances for showers to the area, and
with 700 mb temps below freezing some high elevation snow is still
possible. The mentioned trough ends up wrapping into a large
upper low and brings more widespread shower activity for Friday
night into Saturday. This low then either moves into Canada
leaving us with cool and breezy conditions or lingers across the
region leaving us with cool and showery conditions. Either
solution is a stark change from the current pattern. Temperatures
ahead of the front on Wednesday will be above normal, then we
return to near or even below normal temperatures Thursday into the
weekend. Some models are even showing high temps struggling to
get into the 60s late in the week, although didn`t go that cold
yet. Active weather may also bring increase water to already high
running rivers and streams. Anglin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  54  78  53  76 /  10  10  10  10
CTB  51  79  51  72 /   0  10  10  10
HLN  52  74  50  77 /  30  10  50  10
BZN  47  69  47  73 /  30  20  40  30
WEY  39  59  38  64 /  50  50  50  30
DLN  45  66  45  72 /  40  20  60  20
HVR  54  81  54  77 /   0   0  10   0
LWT  50  69  50  72 /  20  20  20  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

Flood Warning for the Sun River (near Simms) until further notice.

Flood Warning for the Missouri River (at Toston) until further
notice.

Flood Warning for the Gallatin River (at Logan) until further
notice.

Flood Warning for the Jefferson River (near Three Forks) until
further notice.

Flood Warning for the Big Hole River (near Melrose) until further
notice.

Flood Advisory in effect until 615pm Monday for Meagher County.

&&

$$

http://www.weather.gov/greatfalls


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