Severe Thunderstorm Warning 99, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
540 PM MDT Mon Aug 19 2024

Aviation Section Updated.

.SYNOPSIS...

Showers and thunderstorms will continue across North-
central, Central, and Southwestern Montana through Wednesday.
Isolated thunderstorms will produce strong to severe wind gusts and
large hail. An upper-level trough begins to move over the area on
Friday which will bring strong wind gusts on Friday and Saturday and
cooler temperatures for the weekend.

&&

.DISCUSSION...

This afternoon through Tuesday... This afternoon an
upper-level ridge remains over North-central, Central, and
Southwestern Montana. However an upper-level shortwave moves through
the upper-level flow and will bring showers and thunderstorms to the
area this afternoon. These showers and thunderstorms will continue
through about midnight tonight. Isolated thunderstorms will produce
strong to severe wind gusts and/or large hail. All thunderstorms
will produce frequent lightning and brief heavy rainfall. The Storm
Prediction Center (SPC) has issued a slight risk (15% chance within
25 miles of a point) for severe winds from Toole County to Western
Hill County down to western Jefferson to eastern Granite County. The
rest of North-central, Central, and Southwestern Montana has a
marginal risk (5% chance within 25 miles of a point) for severe
winds and a marginal risk for hail for northern Beaverhead to
northern Gallatin County up to Cascade County and west. This
afternoon through tonight there is a 15 - 40% chance for a tenth of
an inch of rain or greater. On Tuesday an upper-level ridge remains
over North-central, Central, and Southwestern Montana. However an
upper-level shortwave moves through the upper-level flow and will
bring showers and thunderstorms to the area Tuesday afternoon and
evening. Isolated thunderstorms will produce strong to severe wind
gusts. On Tuesday the SPC has issued a marginal risk (5% chance
within 25 miles of a point) for severe winds and large hail from
Bozeman to Butte north and west. On Tuesday there is a 5 - 20%
chance for a tenth of an inch of rain or greater across the area.
On Tuesday across Southern Beaverhead, Southern Gallatin, and
Southern Madison there will be elevated fire weather conditions
due to low RH and wind gusts greater than 25 mph.

Wednesday through Friday... On Wednesday the upper-level ridge
remains over North-central, Central, and Southwestern Montana. In
the afternoon through the early evening the ridge begins to shift
east and break down. This will cool down temperatures and bring
showers and thunderstorms to North-central, Central, and
Southwestern Montana on Wednesday. On Wednesday the SPC has issued a
marginal risk (5% chance within 25 miles of a point) for severe
thunderstorms west of a Havre to Great Falls to Bozeman Pass line.
On Wednesday there is a 15 - 45% chance for greater than a tenth of
an inch of rain in North-central Montana. On Wednesday across
Southern Beaverhead, Southern Gallatin, and Southern Madison there
will be elevated fire weather conditions due to low RH and wind
gusts greater than 25 mph. On Thursday an upper-level ridge
builds back in over North-central, Central, and Southwestern
Montana. Thursday afternoon/early evening an upper-level shortwave
moves through the upper-level flow. On Thursday it will be
pleasant and dry. On Friday the upper-level ridge begins to break
down and an upper-level trough begins to move into the area. This
will bring strong wind gusts primarily to north-south oriented
valleys of Southwestern Montana on Friday. This will continue to
be monitored for details. Due to warm air advection from the
upper-level trough temperatures will warm up on Friday. On Friday
the upper- level trough will bring isolated showers to North-
central, Central, and Southwestern Montana today.

Saturday through next Monday... On Saturday three of the four
clusters (86% of ensemble members) show an upper-level trough moving
over North-central, Central, and Southwestern Montana. The other
cluster (14% of ensemble members) keeps the upper-level ridge over
the area. For the upper-level trough scenario this would transition
North-central, Central, and Southwestern Montana to a wetter and
cooler pattern with breezy winds on Saturday. The upper-level ridge
scenario would keep status quo weather for the area. On Sunday the
clusters show an upper-level trough of varying strengths over North-
central, Central, and Southwestern Montana. This would transition
the area to a wetter and cooler pattern with windy conditions as the
trough moves directly over the area. On next Monday clusters,
ensembles, and deterministic models begin to diverge. Two clusters
(64% of ensemble members) show a weak upper-level trough over North-
central, Central, and Southwestern Montana which would keep the
cooler/wetter weather in place over the area. One cluster (25% of
ensemble members) has zonal flow over North-central, Central, and
Southwestern Montana which would keep it cool but dry across the
area. The other cluster (11% of ensemble members) has an upper-level
ridge over North-central, Central, and Southwestern Montana which
would warm up temperatures and bring unsettled weather back to the
area. This will be mentored for more details. -IG

&&

.AVIATION...
20/00Z TAF Period

Numerous showers and thunderstorms are spread across Central
Montana. Observations indicate cloud bases around or above 10kft.
Heavy rain, hail up to 1 inch diameter, and gusty winds to 45kt are
possible impacts at terminals including KHLN/KGTF/KLWT. Storms are
moving north, and may impact KCTB/KHVR with primary gusty wind
hazard through 04Z. Skies clear quickly behind the thunderstorms,
with quiet, VFR conditions prevailing overnight through early
Tuesday afternoon. Widely scattered showers and isolated
thunderstorms are expected again Tuesday evening, but there is not
enough confidence to add these impacts to the TAF at this time.

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  57  89  55  82 /  50  20  30  20
CTB  55  83  52  76 /  50  20  40  30
HLN  55  92  55  83 /  40  20  10  30
BZN  51  90  50  84 /  30  30  10  20
WYS  43  80  42  77 /  10  10  10  10
DLN  49  84  48  78 /  20  10   0  10
HVR  61  92  58  85 /  50  10  70  20
LWT  55  90  52  81 /  40  20  30  20

&&

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

http://www.weather.gov/greatfalls

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