Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
1152 AM MDT Mon Aug 2 2021

...AVIATION SECTION UPDATED...

.SYNOPSIS...

Southwestern Montana and portions of central Montana will see
numerous to widespread showers and thunderstorms this afternoon
and evening. Locally heavy rain is possible with these storms and
could produce flash flooding and debris flows, especially in
recently burn area. Rain showers may persist through Tuesday
morning. Hot and dry weather returns Wednesday and Thursday.

&&

.UPDATE...

The morning update has been published. The leading edge of
anomalously high precipitable water values are moving eastward
through North-central MT, with the Great Falls sounding observing
a 1.30 inch PWAT. Today's forecast remains on track for scattered
to numerous showers and thunderstorms to develop this afternoon
and persist into the overnight hours, while slowly moving off to
the east and northeast. Only change to the forecast was to raise
pops over the eastern portions of the plains to account for light
shower activity this morning. -RCG

&&

.AVIATION...
1152 AM MDT Mon Aug 02 2021 (02/18Z TAF period)

Monsoonal moisture will continue to bring broken/overcast mid and
upper level clouds to southwestern and north central Montana today
through Tuesday morning. Thunderstorm development is already ongoing
over the southwest and will become more pronounced by 19Z. It is
expected to be most widespread south of a KHLN to KGTF to KLWT line
with a more isolated to scattered brand of showers to the north of
the line. Some storms could produce heavy rain with gusty and
erratic winds. VFR conditions should prevail for much of the region
but precipitation will bring IFR/MVFR conditions with mountain
obscurations expected through the period. There should be some west
to east improvement by 03/04Z, but showers and some thunderstorms
will linger over into the overnight hours. -RCG

Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.

&&

.HYDROLOGY...

Monsoonal moisture continues to move northward through the region
today and tonight with numerous showers and thunderstorms expected
for western and southern portions of the forecast area. There is a
significant potential for any shower or thunderstorm to bring
heavy rainfall given slow storm motions and anomalously high
precipitable water values. Some areas could see flash flooding,
especially in/near areas that have recently burned and in urban
areas. A Flash Flood Watch is in effect for all of southwestern
Montana and a portion of central Montana from noon through
midnight. mpj

&&

.PREV DISCUSSION...
/ISSUED 345 AM MDT Mon Aug 2 2021/

The strong upper level ridge that has been over the region will
see a partial breakdown today as a disturbance aloft rides
northward along it's leading edge. This system will interact with
abundant monsoonal moisture with precipitable water values several
standard deviations above normal. Numerous showers and
thunderstorms will affect mainly southern and western portions of
the forecast area this afternoon and tonight. Some of these storms
could produce locally heavy rainfall and a Flash Flood Watch
remain in effect for this area through late this evening. Note
that while most locations are likely to see some beneficial
rainfall, the total amounts will be highly variable from one
location to another, ranging from less than a tenth, to over an
inch of rain.

Some rain showers could persist into Tuesday but a trend toward
mostly dry and warmer conditions is expected through Thursday as
an upper level ridge rebuilds over the Northern Rockies. Unsettled
weather and cooler temperatures return Friday and continue
through the weekend as an upper trough moves from the Pacific
Northwest into the Northern Plains. mpj

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  62  92  56  93 /  60  10   0   0
CTB  60  89  54  90 /  30  10  10  10
HLN  60  93  58  94 /  50  10   0   0
BZN  56  88  53  91 /  80  20   0   0
WYS  46  79  42  82 /  80  30   0   0
DLN  53  85  51  88 /  50  10   0  10
HVR  64  92  59  93 /  40  30  10  10
LWT  62  86  56  89 /  50  30  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from noon MDT today through this evening
Beaverhead...Broadwater...Cascade...Gallatin...Jefferson...
Madison...Meagher.

&&

http://www.weather.gov/greatfalls

Special Weather Statement
National Weather Service Great Falls MT
1252 PM MDT Mon Aug 2 2021

MTZ052-053-021915-
Broadwater-Jefferson-
1252 PM MDT Mon Aug 2 2021

...AN AREA OF STRONG THUNDERSTORMS WILL AFFECT WEST CENTRAL
BROADWATER AND NORTHEASTERN JEFFERSON COUNTIES...

At 1251 PM MDT, strong thunderstorms were clustered 11 miles
northeast of Boulder, moving northeast at 5 mph.

Periods of heavy rain and winds in excess of 30 mph are possible
with these storms.

Locations impacted include...
Boulder, Wickes, Jefferson City, Elkhorn State Park, Alhambra and
Corbin.

This includes the following highways...
 Interstate 15 between mile markers 164 and 181.
 Highway 12 near mile marker 56.

If outdoors, consider seeking shelter inside a building.

These storms may intensify, so be certain to monitor local radio
stations and available television stations for additional
information and possible warnings from the National Weather Service.

Torrential rainfall is also possible with these storms, and may
cause localized flooding. Do not drive your vehicle through flooded
roadways.

LAT...LON 4610 11204 4632 11222 4657 11179 4624 11150
TIME...MOT...LOC 1851Z 238DEG 6KT 4632 11192

SC