Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
634 PM MDT Sun Aug 1 2021

...Aviation Section Update...

.SYNOPSIS...

A weather system producing thunderstorms over western Montana this
evening will move eastward on Monday. The available moisture in
this system is well above average. Expect thunderstorms to produce
locally heavy rainfall rates Monday afternoon and evening. Flash
flooding is a possible outcome under a few of the heaviest rain
cores. Rain showers may persist through Tuesday morning. Fair and
drier weather returns Wednesday and Thursday.

&&

.DISCUSSION...

Surface observations across North-central Montana indicate
elevated to critical fire weather conditions occurring as
southeasterly winds mix to the surface with temperatures in the
upper 90s and relative humidity below 20 percent. In Southwest
Montana, slow moving thunderstorms are developing as monsoonal
moisture continues to advect into a shortwave trough now located
over eastern Washington. Locally heavy rainfall rates are possible
this evening under thunderstorm cores in northern Beaverhead
County. Short-term CAM ensembles keep the heaviest precipitation
over western Montana this evening.

The upper level ridge will weaken and shift east overnight. This
will allow the shortwave system to our west to move across the
Rockies and through N-central and Southwest Montana. Model
guidance continues to support increasing precipitable water
values. The HREF probability matched mean precipitation amounts
indicate a 50 percent chance or greater that 3-hour rainfall may
locally exceed 1 inch from Southwest Montana, northward up
through Central Montana. Flash flooding is a possible outcome
where heavier rain cores occur over complex terrain. Thus, a flash
flood watch is in effect for this area Monday afternoon into the
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.

Tuesday through next Sunday...Linger rain showers early Tuesday
will dissipate as the upper level shortwave weakens and moves away
to the east. Fair weather returns Tuesday night through Thursday
as an upper level ridge builds to our west and then drifts across
the Rockies. An unsettled weather pattern then returns across the
Northern Rockies through next weekend as a series of shortwaves
and longwave troughs move east from the Pacific Northwest. These
disturbances will help to bring temperatures back closer to or
below climatological norms, additional chances for precipitation,
and increasing westerly winds.

&&

.AVIATION...
635 PM MDT Sun Aug 01 2021 (02/00Z TAF period)

This evening...KBZN, KGTF, KLWT and north, remain VFR with
obscuration 6SM haze/smoke. KHLN, KEKS and KWYS, expect intermittent
MVFR conditions in light rain showers. Thunderstorms are occurring
over the mountains of southwest Montana, but not expected to occur
within airfield 10 mile aerodrome. Areas of gusty east southeasterly
winds are forecast outside of thunderstorms and showery
precipitation, as well.

Tomorrow...Expect rain showers and afternoon/evening thunderstorm
activity to cause periods of MVFR conditons. Light winds to breezy
east southeasterly winds are forecast across most of the area,
outside of thunderstorms and showery precipitation. Expect mountain
obscuration.
- Fogleman

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

&&

.FIRE WEATHER...

Elevated to critical fire weather condtions are occurring this
afternoon across northern portions of North-central Montana, where
deep layer mixing is producing southeast winds around 15 mph,
with 25 to 30 mph gusts and relative humidity below 20 percent.
Winds will decrease overnight, but with poor overnight recovery of
relative humidity. By Monday a surge of monsoonal moisture will
overspread the region, with increasing shower and thunderstorm
chances. Given the expected slow motion of thunderstorms on
Monday, locally heavy rainfall will be a significant threat and
could pose a risk for debris flows on burn scars.

&&

.HYDROLOGY...

Anomalously high (3 to 5 standard deviations above normal) PWATs
associated with a northward surge of monsoonal moisture will lift
north and over the Northern Rockies overnight through Monday
night. This surge of moisture will promote scattered to numerous
showers and thunderstorms developing across the region, with
potential for any shower or thunderstorm to bring heavy rainfall
given slow storm motions. While most of this rain will be
beneficial, some areas could see flash flooding, especially
in/near areas that have recently burned and urban areas. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  67  89  63  90 /  10  60  60  10
CTB  64  84  59  88 /  10  50  30  10
HLN  67  88  63  92 /  30  70  50  10
BZN  60  85  57  86 /  20  90  80  20
WYS  51  74  48  77 /  20  90  80  30
DLN  57  79  53  84 /  50  90  50  10
HVR  64  94  64  89 /   0  20  40  20
LWT  61  90  62  84 /   0  30  50  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 9 PM MDT this evening Chouteau and Fergus
Counties...Eastern Glacier/Toole/Central/Eastern
Pondera/Liberty...Hill and Blaine Counties.

Flash Flood Watch until 9 PM MDT this evening Beaverhead.

Flash Flood Watch from Monday afternoon through Monday evening
Beaverhead...Broadwater...Cascade...Gallatin...Jefferson...
Madison...Meagher.

&&

http://www.weather.gov/greatfalls

Special Weather Statement
National Weather Service Great Falls MT
310 PM MDT Sun Aug 1 2021

MTZ008-012145-
Beaverhead-
310 PM MDT Sun Aug 1 2021

...A STRONG THUNDERSTORM WILL AFFECT NORTHWESTERN BEAVERHEAD
COUNTY...

At 309 PM MDT, a strong thunderstorm was located 8 miles west of
Jackson, or 19 miles northeast of Salmon, moving north at 15 mph.

Winds in excess of 30 mph are possible with this storm.

This storm will remain over mainly rural areas of northwestern
Beaverhead County.

If outdoors, consider seeking shelter inside a building.

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.

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

LAT...LON 4548 11376 4554 11345 4527 11341 4523 11365
      4535 11374 4539 11373 4542 11378 4543 11376
      4544 11378 4545 11378
TIME...MOT...LOC 2109Z 164DEG 11KT 4533 11357

SC

Special Weather Statement
National Weather Service Great Falls MT
326 PM MDT Sun Aug 1 2021

MTZ008-012200-
Beaverhead-
326 PM MDT Sun Aug 1 2021

...A STRONG THUNDERSTORM WILL AFFECT NORTH CENTRAL BEAVERHEAD
COUNTY...

At 326 PM MDT, a strong thunderstorm was located 12 miles west of
Divide, or 26 miles south of Anaconda, moving north at 5 mph.

Winds in excess of 30 mph and pea size hail are possible with this
storm.

Locations impacted include...
Wise River.

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

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 4588 11318 4588 11310 4581 11300 4580 11293
      4578 11291 4579 11289 4571 11291 4570 11308
      4587 11320
TIME...MOT...LOC 2126Z 170DEG 5KT 4575 11301

RCG

Special Weather Statement
National Weather Service Great Falls MT
345 PM MDT Sun Aug 1 2021

MTZ008-012215-
Beaverhead-
345 PM MDT Sun Aug 1 2021

...AN AREA OF THUNDERSTORMS WILL AFFECT NORTHWESTERN BEAVERHEAD
COUNTY...

At 344 PM MDT, thunderstorms were clustered near Jackson, or 23
miles northeast of Salmon, moving north at 10 mph.

Winds in excess of 30 mph are possible with these storms.

Locations impacted include...
Wisdom and Jackson.

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

Cloud to ground lightning is occurring with these storms.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

LAT...LON 4564 11343 4528 11340 4523 11364 4525 11368
      4528 11368 4533 11374 4539 11373 4541 11378
      4543 11376 4545 11378 4550 11376 4557 11383
TIME...MOT...LOC 2144Z 178DEG 9KT 4541 11354

RCG

Special Weather Statement
National Weather Service Great Falls MT
458 PM MDT Sun Aug 1 2021

MTZ015-012330-
Madison-
458 PM MDT Sun Aug 1 2021

...A STRONG THUNDERSTORM WILL AFFECT NORTHEASTERN MADISON COUNTY...

At 457 PM MDT, a strong thunderstorm was located 12 miles southeast
of Harrison, or 20 miles northeast of Virginia City, moving northwest
at 15 mph.

Winds up to 40 mph are possible with this storm.

Locations impacted include...
Norris.

This includes Highway 287 between mile markers 58 and 75.

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

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 4547 11178 4569 11186 4570 11160 4564 11149
      4548 11153
TIME...MOT...LOC 2257Z 149DEG 11KT 4554 11167

RCG

Special Weather Statement
National Weather Service Great Falls MT
536 PM MDT Sun Aug 1 2021

MTZ015-052-020000-
Madison-Jefferson-
536 PM MDT Sun Aug 1 2021

...A STRONG THUNDERSTORM WILL AFFECT NORTH CENTRAL MADISON AND SOUTH
CENTRAL JEFFERSON COUNTIES...

At 535 PM MDT, a strong thunderstorm was located 8 miles southeast of
Silver Star, or 10 miles north of Sheridan, moving north at 10 mph.

Brief heavy rain and winds in excess of 30 mph are possible with
this storm.

Locations impacted include...
Silver Star, Waterloo and Cardwell.

If outdoors, consider seeking shelter inside a building.

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.

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

LAT...LON 4587 11190 4553 11202 4552 11227 4574 11237
      4575 11219 4577 11226 4579 11228 4583 11239
TIME...MOT...LOC 2335Z 176DEG 9KT 4561 11216

SC

Special Weather Statement
National Weather Service Great Falls MT
653 PM MDT Sun Aug 1 2021

MTZ014-020145-
Central and Southern Lewis and Clark-
653 PM MDT Sun Aug 1 2021

...A STRONG THUNDERSTORM WILL AFFECT SOUTH CENTRAL LEWIS AND CLARK
COUNTY...

At 648 PM MDT, the strong thunderstorm that moved north over the
Helena valley had weakened.

However, the brief heavy rainfall rate of 0.50 to 1.00 inches in one
hour could cause minor flooding of streets and low lying areas
through around 745 pm MDT. Any flooding that does occur should
likely diminish by 830 pm MDT.

Locations impacted include...
Helena, East Helena and Spring Meadow Lake State Park.

This includes the following highways...
 Interstate 15 between mile markers 191 and 198.
 Highway 12 between mile markers 40 and 53.

This rainfall may also make streets and highways slick during this
time, as dried oil on the roads over the recent dry spell will mix
with the rainfall. Use caution, and slow down while driving this
evening!

LAT...LON 4657 11184 4657 11210 4668 11212 4669 11185
TIME...MOT...LOC 0048Z 177DEG 11KT 4665 11198

SC