Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
226 PM MDT Wed Jul 21 2021

.SYNOPSIS...

Scattered showers and thunderstorms will continue to develop this
evening, with a few stronger storms likely. Gusty westerly winds
may develop over the plains on Thursday and Friday, which will
bring fire weather concerns. Temperatures will generally remain
well above normal for the remainder of the forecast period.

&&

.DISCUSSION...

Satellite and radar imagery shows the expected development of
scattered showers and thunderstorms across Southwest Montana, and
also over the Northern Rockies and central Mountains. Mesoanalysis
indicates moderate instability with increasing shear located
upstream over NW Montana. Storm coverage will expand into the
evening areas, with the environment capable of supporting a few
stronger to severe storms. Faster storm motion compared to
yesterday should result in lower rainfall totals, but locally
heavy rainfall, hail to the size of quarters, and damaging wind
gusts are all possible from some of this evenings thunderstorms.

Thunderstorm activity dissipates overnight. Attention then turns
to an approaching cold front associated with a strong upper level
trough presently over the Canadian Rockies. The surface front will
move through the region Thursday morning, causing a shift to west
and northwest winds as dry air moves in behind the front. Although
technically a cold front, temperatures will still climb back into
the 80s to near 90, causing a period of critical fire weather
conditions over portions of North-central Montana Thursday
afternoon. Scattered showers are expected to develop once again
along and ahead of the front, with dry lightning potential outside
of thunderstorm cores.

While surface dewpoint temperatures have recently been in the 50s,
these will drop into the 20s by Friday with critically low
relative humidity dropping below 15 percent Friday afternoon.
Low level satellite water vapor imagery reveals the source region
for this dry air presently over Northern California into Southern
Oregon. Deep layer mixing under the stable but brisk westerly flow
will mix gusty winds to the surface Friday, once again causing
elevated to critical fire weather concerns over a larger area
Friday afternoon.

After being temporarily flatted Friday, the upper level ridge slowly
amplifies Saturday into next week again for continued very warm
and dry conditions. Some moisture may get entrained into the flow
around the ridge and move into at least southwest MT by the middle
of next week for isolated thunderstorm chances, otherwise little
or no precipitation is expected.

&&

.AVIATION...
1151 AM MDT Wed Jul 21 2021 (21/18Z TAF period)

An approaching shortwave combined with monsoon moisture and
instability will bring another round of shower and thunderstorm
development this afternoon and evening, mostly between 21/20Z and
22/04Z. Thunderstorm hazards for today will include cloud to ground
lightning, hail, strong gusty winds, and brief periods of heavy
rainfall. Periods of mountain obscuration are expected throughout
the next 24 hours. -RCG

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

&&

.FIRE WEATHER...

A cold front passing over the Northern Rockies Thursday afternoon
will cause increasing west winds while humidity falls below 20
percent. These are critical fire weather conditions and warrant a
Red Flag Warning. These dry and windy conditions may spread a bit
further into adjacent fire zones towards Great Falls and Helena.
Scattered thunderstorms are also possible Thursday afternoon along
and ahead of the front, possibly producing additional dry
lighting activity outside thunderstorm cores. Winds decrease
Thursday night, but with critically dry air moving into the area,
overnight RH will be poor to moderate. Relative humidity is
expected to drop below 15 percent on Friday, with gusty winds
returning in a deeply mixed boundary layer.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  57  91  50  88 /  50  20  10   0
CTB  52  82  44  82 /  30   0   0   0
HLN  61  92  56  90 /  40  20  10   0
BZN  56  92  52  90 /  30  30  10   0
WYS  44  82  42  83 /  10  30  10   0
DLN  54  88  49  88 /  40  30   0   0
HVR  59  92  50  89 /  60  10  10   0
LWT  59  90  53  87 /  60  40  30   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Fire Weather Watch from Friday afternoon through Friday evening
Central and Eastern Lewis and Clark National Forest Areas...
Chouteau and Fergus Counties...Helena and Townsend Ranger
Districts of the Helena National Forest...Lincoln Ranger
District of the Helena National Forest.

Red Flag Warning from noon Thursday to 9 PM MDT Friday Eastern
Glacier/Toole/Central/Eastern Pondera/Liberty...Hill and Blaine
Counties...Lewis and Clark National Forest Rocky Mountain
District-Rocky Mountain Front.

Fire Weather Watch from Friday afternoon through Friday evening
Deerlodge/Western Beaverhead National Forest...Eastern
Beaverhead National Forest.

Fire Weather Watch from Friday afternoon through Friday evening
Gallatin National Forest.

&&

http://www.weather.gov/greatfalls

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
632 PM MDT WED JUL 21 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0528 PM     TSTM WND GST     3 WNW TOWNSEND          46.33N 111.59W
07/21/2021  M61 MPH          BROADWATER         MT   MESONET

            GINM8 RAWS. TIME ESTIMATED FROM RADAR.

&&

HART

Special Weather Statement
National Weather Service Great Falls MT
632 PM MDT Wed Jul 21 2021

MTZ053-055-220100-
Broadwater MT-Gallatin MT-
632 PM MDT Wed Jul 21 2021

...SIGNIFICANT WEATHER ADVISORY FOR SOUTHERN BROADWATER AND NORTHERN
GALLATIN COUNTIES UNTIL 700 PM MDT...

At 632 PM MDT, Doppler radar was tracking a strong thunderstorm 7
miles northeast of Manhattan, or 19 miles northwest of Bozeman,
moving east at 30 mph.

HAZARD...Wind gusts up to 50 mph and pea size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
Belgrade, Three Forks, Manhattan, Logan, Trident, Menard, Madison
Buffalo Jump State Park, Missouri Headwaters State Park and Willow
Creek.

This includes the following highways...
 Interstate 90 between mile markers 278 and 296.
 Highway 287 between mile markers 96 and 97.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 4578 11170 4598 11145 4616 11085 4595 11079
      4579 11080
TIME...MOT...LOC 0032Z 250DEG 26KT 4594 11122

Ludwig

Special Weather Statement
National Weather Service Great Falls MT
635 PM MDT Wed Jul 21 2021

MTZ011>014-045-046-049-050-053-054-220100-
Broadwater MT-Chouteau MT-Cascade MT-Liberty MT-Eastern Teton MT-
Hill MT-Eastern Pondera MT-Judith Basin MT-
Central and Southern Lewis and Clark MT-Meagher MT-
635 PM MDT Wed Jul 21 2021

...SIGNIFICANT WEATHER ADVISORY FOR SOUTHEASTERN LEWIS AND CLARK...
SOUTHEASTERN PONDERA...NORTH CENTRAL BROADWATER...NORTHWESTERN
MEAGHER...WESTERN JUDITH BASIN...SOUTHEASTERN LIBERTY...EAST CENTRAL
TETON...SOUTHWESTERN HILL...WESTERN CHOUTEAU AND CASCADE COUNTIES
UNTIL 700 PM MDT...

At 635 PM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from 10 miles northeast of The Knees to 30 miles
northwest of White Sulphur Springs. Movement was northeast at 30 mph.

HAZARD...Wind gusts up to 50 mph and pea size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
Great Falls, Fort Benton, Belt, The Knees, Highwood, Black Eagle,
Carter, Neihart, Floweree, Eden, Stockett, First Peoples Buffalo Jump
State Park, Shonkin, Loma, Kenilworth, Ulm, Raynesford, Monarch,
Portage and Sluice Boxes State Park.

This includes the following highways...
 Interstate 15 between mile markers 267 and 290.
 Highway 200 between mile markers 0 and 20.
 Highway 87 between mile markers 4 and 63.
 Highway 89 near mile marker 16, between mile markers 26 and 71, and
between mile markers 72 and 92.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

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

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.

&&

LAT...LON 4847 11034 4679 11059 4671 11167 4809 11145
TIME...MOT...LOC 0035Z 243DEG 27KT 4812 11110 4684 11138

SC

Special Weather Statement
National Weather Service Great Falls MT
641 PM MDT Wed Jul 21 2021

MTZ048-220115-
Southern Rocky Mountain Front MT-
641 PM MDT Wed Jul 21 2021

...SIGNIFICANT WEATHER ADVISORY FOR NORTHWESTERN LEWIS AND CLARK AND
WEST CENTRAL TETON COUNTIES UNTIL 715 PM MDT...

At 641 PM MDT, Doppler radar was tracking a strong thunderstorm 29
miles northeast of Seeley Lake, or 47 miles southwest of Choteau,
moving northeast at 35 mph.

HAZARD...Wind gusts up to 50 mph and nickel size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

This storm will remain over mainly rural areas of northwestern Lewis
and Clark and west central Teton Counties.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

This storm 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.

&&

LAT...LON 4787 11296 4772 11263 4740 11305 4748 11306
      4750 11312 4754 11313 4757 11312 4760 11314
      4764 11315
TIME...MOT...LOC 0041Z 214DEG 30KT 4752 11309

SC