Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
1205 PM MDT Thu Jun 21 2018

.AVIATION UPDATE...

Forecast remains on track. Thunderstorms are already starting to
develop ahead of an upper level disturbance...giving us a sneak
peak of what is to come today. So far looks like slow moving heavy
rain type storms are likely. Expect thunderstorm to become more
numerous this afternoon and continue through the evening. Best
chances for the stronger storms still looks to be in the southwest
and portions of the east where currently clear skies will bring
ample instability. Some hail and strong winds are possible with
these storms...although with high Pwats today the main threat will
be heavy rain and possible flash flooding. Temperatures will
continue to warm to near normal readings today...with some 80s
possible. Anglin

&&

.SYNOPSIS...
Showers, thunderstorms, and localized flash flooding are possible
today through Saturday as multiple weather disturbances move through
the area. In addition, some thunderstorms may be severe this
afternoon and early evening with quarter-size hail and damaging wind
gusts. An overall drying trend is expected Saturday night into
Sunday as high pressure builds over the area. Low temperatures will
mainly be in the 50s, while highs will mainly reach the 70s or
80s through this weekend.

&&

.DISCUSSION...
Today through Saturday...An upper level disturbance will move
from west to east across the region this afternoon/evening.
Isolated thunderstorms this morning will become more widespread
and stronger this afternoon. Some storms could strong/severe late
this afternoon through early this evening. The airmass remains
unsettled for Fri/Sat, with scattered showers and thunderstorms
continuing. Some storms could produce heavy rain today through
Saturday. There is a marginal risk of flash flooding today through
Saturday, mainly south of a line from Helena to Lewistown.
Afternoon temperatures will be seasonable through the period,
generally within a few degrees of normal. Brusda

Saturday night through Thursday...Shortwave trough exits our CWA to
the east Saturday night, giving way to shortwave high pressure
ridging for Sunday. Thus, an overall drying trend is expected as
shower and thunderstorm probabilities decrease. On Monday, a
longwave trough aloft and embedded disturbances are progged to
slowly overspread our CWA from the west and break-down the shortwave
ridge. In addition, a Pacific cold front looks to cross our CWA from
west to east Monday night, perhaps resulting in some showers and
thunderstorms. Appreciable instability and vertical wind shear may
support a few severe thunderstorms and this potential scenario will
continue to be monitored. The longwave trough aloft and embedded
disturbances continue moving generally eastward over our region
Tuesday through Wednesday. At this time, models suggest showers and
thunderstorms will be isolated, but would not be surprised if the
forecast trends wetter with future model runs. By next week
Thursday, subtle high pressure ridging aloft is progged to build
over our area behind the departing longwave trough, resulting in
fair weather. Low temperatures trend near or slightly-above normal.
Highs will be within several degrees of normal (i.e. mid 70's to
lower 80's) for most days. The exception will be Monday, when strong
warm advection ahead of the aforementioned longwave trough should
contribute to highs reaching well into the 80's to lower 90's for
most. - Jaszka

.AVIATION...
Updated 1805Z.
Showers and thunderstorms will this afternoon and move
northeastward. Some storms could be severe during the
afternoon/evening hours. Storms will be decreasing by 06z Fri but a
chance of rain showers will redevelop in many areas
around 12Z Friday.

&&

.HYDROLOGY...
Lewis and Clark County...Flood warning will continue for the
entire county. Flooding is slowly decreasing across the county but
it will take time for all the flooding to end.

Teton County...Flood Warning will continue for the whole county
for potential further runoff. Flooding is decreasing in the
western portion of the county with peak flows expected this
morning in the eastern portions of the county on the Teton River.

Cascade County...Flood Warning will continue. The Sun River has
generally crested, and will slowly fall over the next few days.
The Missouri River has crested at the city of Cascade, and should
crest later today near moderate flood stage around Ulm.

Jefferson River...between Whitehall and Trident...Flood warning
continues. Minor flooding is expected into Friday.

Meagher County...Flood warning continues for flooding around
Martinsdale. This warning will be evaluated later this morning.

Marias River south of Shelby...Flood Warning continues. Minor
flooding is expected to continue through today.

Pondera...Flood advisory continues into this evening.

Broadwater County...Flood advisory continues into this morning for
minor flooding around Toston.

Big Hole River around Melrose...Flood Advisory continues for minor
flooding through today. The river is showing a falling trend, so
the minor flooding should not last too long in this region.

Fergus County...A flood watch is currently out for the potential
of heavy rain and new flooding or flash flooding on Thursday.
Flooding may then continue into the weekend along the Missouri
River...especially at the James Kipp Campground.

Region wide...scattered showers and thunderstorms will affect the
region for the next several days. There is the potential that some
thunderstorms could produce brief periods of heavy rain and
additional flooding or even flash flooding. A flood watch remains
in effect for portions of North Central and Southwest MT. Any
heavy rainfall over any areas currently under a flood warning or
advisory could make the situation worse or extend the flooding
problems. Anglin/Brusda

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  78  58  76  53 /  30  20  50  30
CTB  78  56  74  52 /  30  20  60  30
HLN  75  55  75  52 /  50  40  40  20
BZN  75  50  70  48 /  60  60  60  30
WEY  70  42  62  39 /  40  40  60  30
DLN  71  47  72  45 /  60  60  60  20
HVR  82  55  81  55 /  20  20  50  50
LWT  75  52  70  51 /  50  70  60  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

Numerous flood highlights remain in effect until further notice.

&&

http://www.weather.gov/greatfalls

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