Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
538 PM MDT Tue Jul 20 2021

...UPDATED AVIATION SECTION...

.SYNOPSIS...

A day to day chance for mostly afternoon and evening showers and
thunderstorms can be expected through Thursday. 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.

&&

.DISCUSSION...

Remainder of this afternoon through Wednesday...Primary concern
over the period will be scattered, mostly afternoon and evening,
showers and thunderstorms. These storms will produce lightning,
gusty winds, and brief heavy rains. Some localized flooding will
be possible especially this afternoon and evening as PWATs climb
near to above 1", which is 2 to 3 standard deviations above
normal. This will be especially true if a storm moves over a fire
burn scar area. Monsoonal moisture continues to stream
northeastward and over the region this afternoon and has lead to
the development of scattered showers and thunderstorms over the
southwest and will likely expand northeastward into Central MT
later this afternoon, mostly impacting areas roughly along and
southeast of a Helena to Lewistown line. Thunderstorm activity is
still possible northwest of this line , but confidence of storm
development lower for this area. As mentioned earlier, PWATs are
running anomalously high, which increases the potential for heavy
rainfall, especially where the training of showers and
thunderstorms occurs, and/or if storm develop over burn scars.
Shower and thunderstorm activity is expected to decrease in
coverage tonight, though some showers and perhaps a few storms
can't be completely ruled out. Another shortwave moves through the
region on Wednesday and will likely bring another round of
showers and storms moving from southwest to northeast. The same
thunderstorm concerns can be expected for Wednesday despite
slightly less moisture than today. -RCG

Wednesday night through next Tuesday...Primary concern over the
period will be the breakdown of the H500 ridge by the trough
currently moving towards the Pacific Northwest/BC (this morning)
during the Wednesday night through Thursday night timeframe. This
ridge breakdown will result in increasing westerly surface winds,
especially over Central and North Central Montana during the day
on Thursday, as a Pacific front moves across the Northern Rockies.
High temperatures throughout the period will remain on the warm
side, with most locations rising into the 80s and 90s. The
expected combination of winds, very warm temperatures, and low
relative humidity has lead to the issuance of a Fire Weather Watch
for zones 112, 113, 114, and 116 for Thursday afternoon and early
evening. Any new discrete lightning fire starts from today or
Wednesday could quickly grow in these condtions. Precipitation
chances are not expected to be as widespread as compared to the
first half of the work week, but isolated to scattered showers and
thunderstorms are possible on Thursday southeast of a Lincoln, to
Great Falls, to Chinook line. - Moldan/RCG

&&

.AVIATION...
538 PM MDT Tue Jul 20 2021 (21/00Z TAF period)

VFR conditions are mostly expected to continue with hazy skies
through the next 24 hours. However, scattered showers and
thunderstorms will move northeast to east over the area along and
south of a KGTF-KLWT line through southwest Montana through around
04Z, bringing periods of MVFR conditions in heavy rain, along with
erratic wind gusts. Some forecast models also indicate that a more
organized line of showers and thunderstorms may develop east of a
KCTB-KGTF line after 04Z, but confidence is low that this will
happen. Otherwise, scattered showers and thunderstorms may redevelop
after 20Z over the mountains along and southwest of a KCTB-KLWT
line, potentially causing periods of MVFR conditions. -Coulston

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

&&

.FIRE WEATHER...

While scattered to numerous showers and thunderstorms are
expected today, with most if not all being on the wet side, recent
heat/dryness will likely leave fuels receptive to new lightning
ignitions despite the higher humidity and wetting rain potential.
The ridge which has dominated the pattern as of late is then
expected to be broken down Wednesday night and through the day on
Thursday. Strong and gusty winds, especially during the afternoon
and evening hours on Thursday, are expected across the region due
to this ridge breakdown. This will likely bring elevated fire
weather conditions during the day on Thursday, especially
considering the potential for emerging holdover fires from
lightning earlier in the week. As a result, a Fire Weather Watch
has been issued for Thursday afternoon and evening for fire zones
112, 113, 114, and 116.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  61  92  57  92 /  30  30  30  20
CTB  56  88  53  84 /  20  40  20  10
HLN  61  92  61  93 /  50  60  30  20
BZN  56  92  56  92 /  50  30  30  30
WYS  45  82  46  83 /  50  30  20  40
DLN  54  88  54  89 /  50  30  30  30
HVR  63  95  60  94 /  10  40  40  10
LWT  61  89  59  91 /  20  40  40  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Fire Weather Watch from Thursday afternoon through Thursday
evening Eastern Glacier/Toole/Central/Eastern Pondera/Liberty...
Hill and Blaine Counties...Lewis and Clark National Forest Rocky
Mountain District-Rocky Mountain Front...Lincoln Ranger District
of the Helena National Forest.

Fire Weather Watch from Thursday afternoon through Thursday
evening Deerlodge/Western Beaverhead National Forest.

Red Flag Warning until midnight MDT tonight Gallatin National
Forest.

&&

http://www.weather.gov/greatfalls

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