-- Discussion containing changed information from previous version are highlighted. --
144
FXUS65 KTFX 210530
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1130 PM MDT Thu Jun 20 2024

UPDATED AVIATION SECTION

.SYNOPSIS...

Showers and a few stronger thunderstorms will be around again
today, with Friday bringing a better chance for strong to severe
thunderstorms across Central and North-central Montana. A warming
and drying trend commences for this weekend, with Sunday looking
to be the warmest day of the forecast period. Breezy winds develop
across the plains late Sunday into Monday as a mainly dry Pacific
front moves across the region.

&&

.UPDATE...

Shortwave energy will maintain slight chances for lighter end
shower and thunderstorm activity overnight tonight, though
thunderstorms should increasingly become less of a concern with
the loss of diurnal heating. Another round of more numerous
showers and thunderstorms can be expected again on Friday, some
will be on the stronger side with strong winds and hail in
addition to heavy downpours and lightning. - RCG

&&

.AVIATION...
-- Changed Discussion --
21/06Z TAF Period

VFR conditions are mostly expected through at least 22/06Z across
North Central (KCTB, KHVR), Central (KLWT, KGTF, KHLN), and
Southwest (KBZN, KEKS, KWYS) Montana, unless otherwise mentioned.

A weakly unstable southwesterly flow aloft will mainly move mid- and
high-level cloudiness over the area through at least 15Z; a few
light showers or weak thunderstorms are also possible, but
confidence that any will directly impact terminals is low, so have
not mentioned them in the TAFs. However, the airmass will become
increasingly moist and unstable after 15Z as a shortwave low
pressure trough approaches from the west. This will bring at least
scattered showers and thunderstorms to the area along and east of a
line from the Continental Divide southeast through the KHLN and KBZN
terminal areas. Some storms may become strong to possibly severe at
times over the plains between 18Z and 02Z, mainly east of a KCTB to
KGTF line, where heavy rain and lower ceilings could cause periods
of MVFR conditions. Showers and storms will start to decrease from
the west after 00Z as the shortwave moves through the area, which
will also bring some mountain wave turbulence as the westerly flow
aloft increases. -Coulston

Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.
-- End Changed Discussion --

&&

.PREV DISCUSSION...
/ISSUED 219 PM MDT Thu Jun 20 2024/

The rest of today... Strong ridging across the eastern CONUS will
keep broad troughing in place across the western CONUS through the
next 36 hours or so. An embedded wave within the west to
southwesterly flow aloft across the Northern Rockies will combine
with sufficient instability to result in showers and a few
stronger thunderstorms across primarily Central and North-central
Montana this afternoon and evening. A corridor of slightly higher
instability (On the order of 750-1,000 m2/s2 or so with locally
higher amounts up to 1,500 m2/s2) looks to set up along and
adjacent to the 87 corridor between Great Falls to Grass Range.
Marginal yet sufficient shear will be in place in this area to
allow for a few stronger storms to develop, with a marginal risk
for severe thunderstorms (Greatest risk is for hail). Storm motion
appears to be rather slow so far this afternoon, yielding concern
for localized flooding where thunderstorms are most persistent.
Showers and thunderstorms move off to the northeast and or
diminish tonight. There is less concern for patchy fog tonight,
given mid and high level clouds that will be in place across the
Northern Rockies.

Friday... Broad troughing will still be in place, but begin to shift
eastward with an embedded shortwave crossing the Northern Rockies
late morning through the evening. Meanwhile at the surface, high
pressure well off to the east (Across the northern Great Lakes) will
result in an easterly component to wind across the plains. Higher
dew point temperatures (Upper 40s to mid 50s) will be advected in as
a result. All of these ingredients will combine with relatively
steep mid level lapse rates and vertical wind shear to result in
several thunderstorms tomorrow, primarily across Central and North-
central Montana. Straight hodographs suggest hail and wind will be
the main concern with the strongest thunderstorms. The timeframe of
greatest concern will be around/just after noon until just after
sunset.

An additional concern will be for heavy rain with thunderstorms.
Storm motions look to be quick enough (On the order of 20 mph or so)
to mitigate most concerns for flooding or flash flooding, but the
risk will exist regardless.

Saturday into next week... Upper level ridging further east across
the eastern CONUS will begin to gradually break down Friday night
into Saturday, allowing for the upper level troughing that has been
over the Northern Rockies to shift eastward, away from the region.
This will allow an upper level ridge to build in from the west this
weekend, propelling temperatures above average by Sunday as the
ridge axis starts slipping east of the region. A compact upper level
low off the coast of BC will open up into a wave and shift
eastward beginning Sunday afternoon. The best upper level support
for precipitation with this system looks to remain north of the
border Sunday into Monday, resulting in the only impact being
breezy westerly winds across the plains.

Ridging is favored into Tuesday, but confidence beyond Tuesday
decreases. Ensembles favor an eventual breakdown of the ridge, but
are struggling to time it at this range. -AM

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  75  49  76  49 /  20  20  70  20
CTB  74  44  72  44 /  20  20  70  20
HLN  80  52  83  53 /  20  20  40  10
BZN  76  46  81  48 /   0   0  30  10
WYS  72  39  76  41 /   0  10  20  10
DLN  73  44  80  47 /   0   0  10   0
HVR  77  51  77  50 /  20  30  80  40
LWT  71  46  72  47 /  30  20  80  40

&&

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

$$

http://www.weather.gov/greatfalls