Severe Thunderstorm Warning 89, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
536 PM MDT Sun Aug 18 2024

...Aviation Section Updated...

.SYNOPSIS...

Moist, southwest flow aloft will bring showers and thunderstorms
to the Northern Rockies through the first half of the upcoming
work week, with a few thunderstorms this afternoon and evening and
then again on Monday potentially being strong to severe. Main
threat from any severe storm today will be damaging winds, with
hail and damaging winds being the threats on Monday. High
temperatures will once again warm well above normal on Monday,
with most locations reaching the mid-80s to mid-90s.

&&

.DISCUSSION...

Rest of today through Tuesday...primary forecast concerns throughout
the period will be thunderstorms, some of which could become strong
to severe late this afternoon through the evening hours tonight and
then again on Monday, and above normal temperatures. H500 ridge axis
will remain just east of the Northern Rockies and over the Northern
High Plains through the period, with subtle shortwaves lifting
northeast and over Southwest through North Central Montana within
the overall moist, southwest flow aloft. Strong surface heating this
afternoon and then again on Monday, will produce high temperatures
well above normal and in the mid-80s to mid-90s on both days, which
combined with sufficient moisture and shear will help to support
isolated to scattered thunderstorms during the afternoon and evening
hours (today and Monday). Terrain based cumulus has already
developed early this afternoon over the mountains of Northern Idaho
and Southwest Montana, with showers and thunderstorms expected to
develop over the next 1-2 hours over these same areas. These showers
and thunderstorms will then lift northeast and over the remainder of
Southwest through North Central Montana through the late afternoon
and evening hours tonight before gradually exiting most of the CWA
around midnight tonight. A few showers and thunderstorms may linger
beyond midnight tonight along the Hi-Line in North Central Montana
and along the Idaho/Wyoming borders, but overall coverage will
decrease beyond the aforementioned midnight hour. Given the
marginally sheared environment through the evening hours tonight the
primary concern with any of the stronger thunderstorms will be gusty
and erratic winds and potentially a few damaging wind gusts. By
Monday slightly stronger shear and instability will support a better
(5% to 15%) chances for hail, potentially large, along with the
continued threat for damaging winds. Given these risks for hail
and/or damaging winds the Storm Prediction Center (SPC) continues to
maintain a Marginal Risk for severe thunderstorms across most if not
all of Southwest through North Central Montana today and on Monday.
Shower and thunderstorm development on Monday is expected to be
slightly further north and over Southwest and into portions of
Central Montana during the early afternoon hours, with this activity
then lifting northeast and across the remainder of the CWA through
the evening hours.

Wednesday through next Sunday...ensemble clusters predominately
favor broad troughing over the Pacific Northwest, Great Basin, and
Northern Rockies for much of the period; with the leading mode of
uncertainty being the exact position and/or timing of the H500
trough as it slowly digs southeast from the Eastern Pacific over the
Pacific Northwest and Great Basin before lifting northeast over the
Northern Rockies. The position and/or timing of the aforementioned
trough will ultimately impact when and how long "cooler"
temperatures arrive and persist across the Northern Rockies, in
addition to precipitation chances. Latest deterministic NBM guidance
supports high temperatures falling near to below normal for the days
on Wednesday and Thursday in wake of a series of weak surface cold
fronts, warming back above normal on Friday as a warm front lifts
northeast amongst increasing southerly flow aloft ahead of the main
trough axis moving overhead, and falling back near to normal over
the upcoming weekend as the trough shifts east of the Northern
Rockies. Daily chances (15% to 30%) for precipitation will also
exist across portions of the Northern Rockies throughout the period,
mainly along and near the cold fronts on Wednesday and Thursday,
over the higher terrain and lifting warm front on Friday, and across
all areas over the weekend. - Moldan

&&

.AVIATION...
19/00Z TAF Period

The primary concern this TAF period will be for showers and
thunderstorms progressing northeastward across the region overnight.
Gusty winds will be the main concern with these thunderstorms, but
some small hail will be possible early this evening. Another round
of showers and thunderstorms will develop Monday afternoon, once
again moving from southwest to northeast. -AM

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  55  93  55  89 /  50  20  40  10
CTB  54  89  54  84 /  40  20  40  10
HLN  56  93  53  92 /  60  50  40  20
BZN  54  91  49  91 /  10  40  20  20
WYS  44  79  40  80 /  30  50  30  10
DLN  52  86  48  85 /  10  30  10  10
HVR  63  96  60  93 /  50  20  50  10
LWT  57  91  53  90 /  20  30  40  20

&&

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

http://www.weather.gov/greatfalls

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