Special Weather Statement, NWS Great Falls, MT

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

.UPDATE...

Only update to current forecast was to add mention of gusty winds,
small hail, heavy rain, and frequent lightning to thunderstorms to
reflect current situation. The current forecast describes the
potential for lingering thunderstorms through midnight. The main
area of thunderstorms that moved east over the plains is currently
along a line from Havre south through Lewistown that is moving
east. Wind gusts of 30 to 45 mph have been common with areas of
heavy rainfall and at least small hail. The few storms that are
moving off the Rocky Mountain Front and across southwest Montana
seem to be weakening. -Coulston

&&

.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.

&&

.AVIATION...
645 PM MDT Wed Jul 21 2021 (22/00Z TAF period)

Moisture in the southwest flow aloft will bring scattered showers
and thunderstorms to much of the area along and south of a KMSO-KHVR
line through around 06Z, then again after 18Z. Wind gusts in excess
of 40 kt, periods of heavy rain with MVFR conditions, and lightning
will be the main threats. Otherwise, conditions will be VFR with
mostly light winds, but westerly winds will become gusty overall
after 18Z. -Coulston

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.

&&

.PREV DISCUSSION...
/ISSUED 645 PM MDT Wed Jul 21 2021/

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.

&&

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

MTZ011-044-045-220330-
Liberty-Hill-Toole-
914 PM MDT Wed Jul 21 2021

...A LINE OF STRONG THUNDERSTORMS WILL AFFECT NORTHEASTERN LIBERTY...
NORTHEASTERN TOOLE AND NORTHWESTERN HILL COUNTIES...

At 914 PM MDT, radar indicated strong thunderstorms were located
along a line extending from 18 miles northeast of Sunburst to 19
miles north of Joplin. Movement was northeast at 50 mph.

Winds in excess of 30 mph and pea size hail are possible with these
storms.

Locations impacted include...
Whitlash.

If outdoors, consider seeking shelter inside a building.

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

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

LAT...LON 4878 11078 4899 11156 4899 11035
TIME...MOT...LOC 0314Z 227DEG 45KT 4903 11157 4884 11085

SC

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

MTZ011-047-051-220400-
Fergus-Hill-Blaine-
936 PM MDT Wed Jul 21 2021

...A LINE OF STRONG THUNDERSTORMS WILL AFFECT FERGUS...BLAINE AND
EASTERN HILL COUNTIES...

At 936 PM MDT, radar indicated strong thunderstorms were located
along a line extending from 17 miles north of Chinook to 23 miles
south of Lewistown. Movement was east at 20 mph.

Winds in excess of 40 mph and half inch hail are possible with these
storms.

Locations impacted include...
Havre, Lewistown, Chinook, Harlem, Winifred, Grass Range, Hays, Fort
Belknap, Turner, Roy, Fergus, Forestgrove, Lodge Pole, Cleveland,
Zurich, Valentine, Hogeland, Hilger, Fred Robinson Bridge and Lohman.

This includes the following highways...
 Highway 200 between mile markers 75 and 123.
 Highway 191 between mile markers 1 and 2, between mile markers 1 and
42, and between mile markers 27 and 87.
 Highway 2 between mile markers 383 and 445.

If outdoors, consider seeking shelter inside a building.

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

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

LAT...LON 4727 10872 4710 10874 4708 10860 4675 10864
      4670 10957 4773 10961 4774 10953 4813 10953
      4813 10965 4890 10969 4896 10823 4844 10835
      4844 10842 4798 10843 4792 10889 4774 10890
      4765 10877 4756 10832 4732 10831
TIME...MOT...LOC 0336Z 278DEG 18KT 4883 10932 4673 10935

SC

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

MTZ015-055-220415-
Madison-Gallatin-
944 PM MDT Wed Jul 21 2021

...AN AREA OF STRONG THUNDERSTORMS WILL AFFECT NORTHEASTERN MADISON
AND CENTRAL GALLATIN COUNTIES...

At 943 PM MDT, strong thunderstorms were clustered 8 miles northeast
of Ennis, or 18 miles northeast of Virginia City, moving northeast at
35 mph.

Winds in excess of 40 mph and brief heavy rain are possible with
these storms.

Locations impacted include...
Bozeman, Belgrade, Three Forks, Manhattan, Ennis, Pony, Harrison,
Norris, Big Sky, Mcallister, Bozeman Hot Springs, Gallatin Gateway,
Cameron, Missouri Headwaters State Park, Logan, Four Corners,
Amsterdam, Jeffers, Church Hill and Willow Creek.

This includes the following highways...
 Interstate 90 between mile markers 278 and 304, and near mile marker
306.
 Highway 191 between mile markers 41 and 87.
 Highway 287 between mile markers 28 and 81, and between mile markers
95 and 97.

If outdoors, consider seeking shelter inside a building.

LAT...LON 4555 11217 4580 11173 4580 11171 4585 11164
      4602 11135 4553 11090 4552 11092 4536 11092
      4535 11104 4523 11105 4504 11163
TIME...MOT...LOC 0343Z 236DEG 31KT 4543 11161

SC