Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 261714
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1114 AM MDT Sat May 26 2018

...Morning Update and Updated Aviation Discussion...

.UPDATE...

Models seem to be coming into better agreement with showers and
thunderstorms developing along and near a cold front across our
area, with convection becoming better organized as it moves across
central Montana into eastern Montana. Behind the front, another
round of showers and weaker thunderstorms then develops over
central and parts of southwest Montana this evening and may
persist into the overnight.

Some concerns with thunderstorm activity this afternoon into the
overnight will be frequent lightning, the possibility for hail
reaching 1 inch in diameter or larger, and severe winds with gusts
in excess of 58 mph. Additionally, torrential downpours will be
possible with some of these storms, and a localized flash flood
threat is also possible, especially in steep terrain and on area
burn scars from last summer.

Due to the expectation of convective activity commencing by early
this afternoon, this may be the last Forecast Discussion update
until later this evening.

&&

.SYNOPSIS...

A cold front will bring widespread chances for showers and
thunderstorms today. Some strong to severe storms are
possible this afternoon and evening...with strong
winds...hail...and localized heavy rain possible. Thunderstorm
activity diminishes to just scattered showers tonight. More
showers and thunderstorms are possible for Sunday and
Monday...especially across southern Montana. Temperatures will
remain above normal today...and through the holiday weekend.

&&


.AVIATION...
Updated 1712Z.

Surface cold front continues to move south and east and will pass
through KHVR and KGTF in the early afternoon. Expect once this
upslope flow reaches the mountains shortly thereafter that
thunderstorm activity will initiate across the mountains and move
in a general east or east-northeast direction from near a KGTF-
KHLN line. A few showers and thunderstorms can not be ruled out
toward KCTB later today as well. This first area of thunderstorms
is expected to track east, but lingering moisture, lift, and
instability will mean showers and thunderstorms can not be ruled
out well into the evening or perhaps even overnight over central
parts of our area, mainly around KGTF but possibly also including
KHLN, KBZN, and KEKS. Some storms especially this afternoon and
early evening could produce erratic winds over 30 knots, heavy
rainfall with MVFR conditions,and hail. Mainly VFR conditions
return tonight...although lingering low clouds and scattered
showers could bring some MVFR conditions to southwestern sites.


&&

.HYDROLOGY...

Areal Flood Advisory for the Lincoln area will continue through
Monday morning. Heavy rain from yesterday`s evening thunderstorms
produced localized areas of flooding and increased run off,
especially near the Park Creek Burn Scar. Impacts from this event
should diminish later this morning, although additional
thunderstorms could return the threat throughout today.

The Sun River at Simms has risen above flood stage. The latest
forecast will keep the river at/or above flood stage for the next
several days, with a good rise expected later in the weekend into
early next week. Thus a flood warning is now in effect until
Thursday evening of next week for this section of the river.

Canyon Ferry is going to increase releases into the Missouri River.
The current forecast/plan is to keep the river below flood stage at
Ulm, Great Falls, and Fort Benton. However the river could rise to
near bankful in low areas. No flood watch will be issued at this
time, but any adjustments in the next few days could change that.

For the Missouri River near Landusky, including the James Kipp
Campground, the river is likely to go above flood stage sometime
next week with the increased releases. Flood highlights are likely
for this section of the Missouri River in the coming days.

The small stream flood advisory for Meagher County has been extended
until Sunday evening. Local DES reports minor problems continue.
With scattered thunderstorms through Saturday, minor problems are
expected to continue through much of the weekend.

Today there will be a minor to moderate threat for flash flooding
as forecasted thunderstorms could drop heavy rain at times.
Biggest concerns will come across burn scar areas from recent
fires and across any creeks or streams that are already running
high from increased run off. Those near these vulnerable areas
should stay alert to changing conditions today.

Current Highlights:

Small stream flood advisory for Meagher County until 615 PM Sunday.

Flood Warning until 715 PM Thursday for the Sun River in Cascade
County west of the city of Vaughn.

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

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

Flood Warning in effect until 1000 PM MDT Saturday for the Missouri
River near Toston in Broadwater County.

Flood Warning in effect until 1015 PM MDT Saturday for the Gallatin
River near Logan.

&&

.PREV DISCUSSION...
/ISSUED 545 AM MDT Sat May 26 2018/

Today through Tuesday...A weak wave riding over the diminishing
ridge is currently bringing some showers and isolated
thunderstorms across northern and central portions. These areas
have decent jet dynamics as well, which could continue the threat
through the morning hours. A brief lull in activity may then be
found early this afternoon before a cold front moves across the
region later this afternoon through the evening hours. Still a
marginal threat for severe weather today as this front meets
modest instability and shear. Best chances for severe weather look
to be from about Helena to Great Falls to Havre and eastward.
Gusty winds and hail will be the main threats. Other concern today
will be the high amounts of precipitable water. Heavy rain from
thunderstorms could bring a flash flood threat, especially to
already saturated areas, high creeks and streams from increased
runoff, and fire burn scar areas. Anyone with outdoor interest
should be alert today for adverse weather. Cold front exits to our
east tonight, briefly diminishing the shower and thunderstorm
threat. Large upper low in northern UT then brings the next round
of lift and moisture to the southern half of the CWA on Sunday,
while the northern half sees only isolated shower chances. Broad
scale trough then sets up for Memorial Day. This could bring more
rounds of showers and thunderstorms to the area, although coverage
is now expected to be scattered. Weak and likely brief ridging is
expected for Tuesday. Lingering moisture and instability could
still develop a few thunderstorms in the afternoon, with coverage
generally confined to the mountains. Temperatures through the
short term period look to be above normal with highs in the 70s to
low 80s, and lows in the 40s and 50s. Memorial day will see a
slight cool down, although temperatures will still be slightly
above normal. Anglin

Tuesday night through Saturday...Long term period still looks to
be on the more active side. Models agree that sometime Wednesday a
sharp cold front moves through brining a decent chance for showers
and thunderstorms to the region. Some stronger storms are possible
with this front especially given all the shear expected with the
front. Frontal boundary then stalls across the region and we see
a broad trough develop across the western U.S. The result could
be active weather for the region for Thursday into Friday.
Instability will be limited, so mostly expecting a shower and
isolated thunderstorm threat. Some models then developing a large
upper level low moving across the state later Friday through
Saturday. This would bring a more synoptic type widespread rain
event, with perhaps some higher elevation snow. This will have to
be monitored as high QPF values from these systems could add to
flooding concerns across the area. Temperatures in the long term
will start above normal. Wednesday`s strong front look to cool
down temps to near or slightly below normal readings on Thursday.
Temperatures then hover around normal into next weekend. Anglin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  79  52  77  52 /  50  70  20  20
CTB  74  48  78  49 /  50  50  10  10
HLN  78  51  73  51 /  50  70  60  40
BZN  78  48  68  48 /  40  40  70  70
WEY  70  39  61  37 /  30  30  70  70
DLN  74  46  65  46 /  50  50  80  70
HVR  83  53  81  52 /  30  30  10  10
LWT  78  51  71  50 /  40  50  30  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Numerous flood highlights are in effect until further notice.
Please Hydrology section above for additional details.

&&

$$

http://www.weather.gov/greatfalls


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