Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
1045 AM MDT Fri Aug 6 2021

.SYNOPSIS...
Scattered showers and thunderstorms are expected today over
Central and Southwest MT. A dry, but breezy day follows on
Saturday. Rain is likely over North-central Montana on Sunday, but
warm and dry conditions return by the middle of next week.

&&

.UPDATE...
Updated forecast is out. Main adjustment was to lower POPs over
Southwest MT this morning, as showers have exited the region,
while a few showers developed over Blaine county. Expect another
round of thunderstorms this afternoon, mostly south of a line from
Helena to White Sulphur Springs. Otherwise, temperatures look on
track for today. Brusda

&&

.AVIATION...
1045 AM MDT Fri Aug 6 2021 (06/18Z TAF period)

Overall VFR conditions are expected across much of the region
during the period, smoke will limit distant visibility and could
reduce to some MVFR conditions at times. Additionally, a few
thunderstorms are expected this afternoon/evening, mostly over
Southwest MT, with some short periods of MVFR conditions possible
in/near any thunderstorm activity. Distant mountains/passes will
be obscured at times by smoke. Brusda

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

&&

.FIRE WEATHER...
A weather system moving across the Northern Rockies today will
help scattered showers and thunderstorms develop this afternoon.
Winds turn westerly and increase on Saturday, creating elevated
fire weather conditions during the afternoon hours. A strong
Pacific weather system is likely to produce wetting rainfall on
Sunday into Sunday night. Embedded thunderstorm activity could
produce erratic gusty winds as a cold front passes through Sunday
afternoon. Anticipate increasing southwest winds ahead of the
front Sunday, switching to west-northwest and continuing through
Monday.

&&

.PREV DISCUSSION...
/ISSUED 540 AM MDT Fri Aug 6 2021/

A rather slow-moving upper level shortwave trough will continue to
make its way across Southwest Montana today. Showers and
thunderstorms have been ongoing overnight, and are expected to
continue today. Some of this activity may develop further north
into Central Montana as well as CAPE values are forecast to reach
near 1500 J/kg. Precipitation moves away to the east later
tonight. Westerly winds fill in behind the departing shortwave.
Dry and breezy condition follow on Saturday.

On Sunday, attention turns upstream as a potent, closed upper
level low and its attendant surface front cross the Northern
Rockies into North-central Montana. Surface convergence along the
front, located under a strong diffluent flow aloft will combine to
drive areas of moderate to heavy rain. Surface heating ahead of
the front could provide enough instability to generate some
stronger thunderstorms capable of producing locally heavy
rainfall and strong gusty winds. The highest rainfall amounts
reaching near, or above one inch appear most likely over the
Northern Rockies including Glacier NP, extending eastward across
the Blackfeet Nation and along the HiLine. This will be a
beneficial rain for most, but keep in mind the possibility that
isolated instances of flash flooding may be possible under a
stronger thunderstorm producing high rain fall rates. This passing
storm system will also cause a period of stronger winds Sunday
into Monday across the region.

Dry and warmer weather returns on Tuesday. Ensemble forecasts
favor a warming trend as upper level ridging strengthens over the
Pacific coast. About a third of model solutions develop a more
amplified pattern that could allow another upper level low to move
into Montana along with precipitation and its attendant surface
cold front. Confidence is lower than average for this time frame
next week, so continue to monitor forecast updates.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  88  58  88  58 /  30  20   0  10
CTB  86  57  82  54 /  20   0   0  20
HLN  86  58  90  60 /  40  20   0  10
BZN  79  50  86  53 /  90  40   0  10
WYS  68  40  74  41 /  80  60   0   0
DLN  73  48  82  51 /  90  20   0   0
HVR  90  61  88  59 /  20  20   0   0
LWT  84  56  86  57 /  40  30   0  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.

&&

http://www.weather.gov/greatfalls

Special Weather Statement
National Weather Service Great Falls MT
101 PM MDT Fri Aug 6 2021

MTZ008-062000-
Beaverhead MT-
101 PM MDT Fri Aug 6 2021

...Strong thunderstorms will impact portions of southwestern
Beaverhead County through 200 PM MDT...

At 100 PM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from 18 miles northwest of Lima to 10 miles north of
Leadore. Movement was east at 10 mph.

HAZARD...Winds in excess of 30 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...
Clark Canyon.

This includes Interstate 15 between mile markers 26  and 45 .

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If on or near Clark Canyon Reservoir, get out of the water and move
indoors or inside a vehicle. Remember, lightning can strike out to
10 miles from the parent thunderstorm. If you can hear thunder, you
are close enough to be struck by lightning. Move to safe shelter
now! Do not be caught on the water in a thunderstorm.

Persons in campgrounds should consider seeking sturdy shelter until
these storms pass.

&&

LAT...LON 4491 11344 4505 11264 4476 11248 4471 11310
      4477 11313 4482 11324 4479 11335 4481 11334
      4484 11339
TIME...MOT...LOC 1900Z 257DEG 10KT 4486 11282 4483 11330

MAX HAIL SIZE...0.25 IN
MAX WIND GUST...30 MPH

CMC