918
FXUS63 KLSX 081949
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
249 PM CDT Tue Jul 8 2025

.KEY MESSAGES...

- Scattered thunderstorms are expected to track southeast through
  the region today. A few thunderstorms could be strong, including
  the potential for isolated severe thunderstorms with winds gusts
  to 60 mph and locally heavy rain.

- Warm, seasonable temperatures will persist through the next
  several days along with periodic chances for thunderstorms to
  impact the region.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 138 PM CDT Tue Jul 8 2025

Thunderstorm potential will be the primary focus this afternoon
into this evening as an upper level trough and MCV (mesoscale
convective vortex) rotate southeastward through late this evening.
IR satellite shows a broad expanse of broken/overcast clouds
extending from northern MO through portions of IA/IL/WI. Clouds
are associated with the MCV, remnant of a convective complex that
developed over SD/NE last evening. Another area of clouds and
convection extend from eastern OK into southern IN. In between the
two areas, skies have remained mostly clear, allowing instability
to build ahead of the approaching MCV.

As of 18z, SPC mesoanalysis shows a what little CIN that exists
extending from Kansas City roughly just west of Quincy, IL. Where
skies have remained mostly clear, surface instability has climbed
with 2500 J/kg of SBCAPE roughly along and north of the I-44
corridor into southwestern IL along I-70. This not only jives
pretty well with guidance from prior runs, including this morning,
and maybe even on the higher end. HREF means showed SBCAPE of
2-2.2k with deterministic hi-rese guidance (RAP/HRRR) plotting
2.5-3k this afternoon. Moisture is no question with dewpoints in
the upper 60s to low-70s and PWATs around 1.7" per SPC
mesoanalysis. Thunderstorms will certainly have the moisture to
produce efficient rainfall with locally heavy rain, resulting in
ponding/nuisance flood in areas impacted by multiple cells. Though
soundings aren`t showing a particularly classic inverted-V,
50-100mb dewpoint depressions in the realm of 10-13C suggest
localized downbursts with marginally severe wind gusts (60 mph)
are possible. Hail cannot be entirely ruled out, but what does
fall is likely sub-severe with mid-level lapse rates at 5.5-6C and
competing updrafts as scattered convection become more numerous
in time.

Despite some slight spatial/temporal variability between
deterministic hi-res models, paintball plots (simulated reflectivity
>40 dBZ) show a general consensus that the best timeframe for
development runs from near KCOU to just south of KPPQ. IR satellite
is showing a notable uptick in development in this region,
increasing confidence that this will be the region for focused
initiation. Thunderstorms then move southeast through approximately
8-9 p.m. as thunderstorms progress southeast and weaken after
sunset.

Once thunderstorms clear to the southeast and dissipate late this
evening, clouds likely hang on for some time tonight before partial
clearing works into northeast MO. There is some hint at patchy fog
development running from central to northeast MO, but cloud cover
may not clear quick enough before sunrise, limiting fog potential.
This will be something to monitor through tonight, but even if fog
develops, it is not expected to be anything significant with the
limited time to take advantage of clearing.

By Wednesday, mostly dry conditions are expected with the low chance
that an isolated shower may develop. However, much of the potential
will reside outside the CWA. Should anything develop, the best
chance (30-40%) will line up along the southeast portion of the CWA
with chances quickly tapering to the north and dropping through the
day as surface high pressure builds in from the northwest.

Maples

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 248 PM CDT Tue Jul 8 2025

Ensembles and NBM guidance show a persistent, seasonably warm
pattern with relatively small spread through the extended range.
Variability is greatest on days with better convective potential
and even then, spread is considerably small in comparison to
events involving wholesale change.

The upper level ridge centered over the Desert Southwest is
expected to flatten and shift westward over southern CA and the
Baja Peninsula by Thursday. This results in southward shift in the
active 500mb vort track that has kept the northern CONUS active
over the last several days. Several shortwaves continue to round
the northern periphery of the ridge, progressing west to east
over the Intermountain West into the central Plains and mid-
Mississippi Valley. Though this looks like an opportunity for
multiple, periodic rounds of convective potential, surface
features and timing will be key in the magnitude and spread of
activity.

Surface high pressure moves into the Great Lakes but maintains
some influence early Thursday as the first such shortwave tracks
near the IA/MO border. This looks pretty close to scenarios we`ve
seen pan out over the last couple of days with afternoon/evening
convection initializing to the north and tracking southeastward at
the eastern periphery of the upper ridge. However, this round
forms at the nose of a 40-45 knot low level jet that set up over
the central Plains. Deterministic guidance (ECM/GFS) is fairly
good agreement with the placement of this system at this time,
which would have convective trends moving somewhere along the MS
River Valley into MO/IL Thursday morning. Usually convective
complexes of this nature will weaken as the low level jet fades
through the day and the complex become outflow dominant.
Nonetheless, it may be something to watch should timing/placement
shift over the next couple of days. Chances for showers and
thunderstorms are most favored over northeastern MO and west-
central IL in this scenario, as convection would tend to weaken
through the morning/early afternoon (if it lasts that long).

Friday largely looks dry as we await the next uptick in
thunderstorm potential, which may be the best shot over the course
of the week. This will come in the form of a deeper trough that
closely follows Thursday`s lead shortwave. This shortwave give
rise take on a slight negative tilt ahead of an amplified,
positively tilted trough that extends southwestward into the
northern Plains. As the shortwave approached, the surface low is
ejected northeast through the central Plains into the Upper
Midwest. A cold front that trails the system slow pushes southeast
late Friday into early Saturday. While this may be the best
opportunity for rainfall provided surface convergence underneath
mid/upper ascent, where the system track and when it moves through
will play a huge role in precipitation potential. If the upper
trough and shortwave track too far north, this would place the
mid- Mississippi River Valley in territory in which the cold front
relies on diurnal instability and convergence. While diurnal
convection would result in beneficial rainfall, thunderstorms
potential would struggle to survive through the diurnal lull.

Therefore, while there are multiple opportunities for rainfall,
there are enough caveats to questions just how much of it will
come to fruition to benefit the region with widespread potential.


Maples

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1232 PM CDT Tue Jul 8 2025

The primary focus will be the potential for showers and
thunderstorms this afternoon into this evening, a few of which
could be strong to marginally severe. The primary threats will be
locally heavy rain and gust winds, including isolated gusts
approaching 60 mph in the strongest thunderstorms. A few showers
have already developed around KUIN with potential trending
southward as more robust convection initialized across central and
east-central Missouri into southwestern Illinois. Considering the
slight difference in hi-resoultion guidance and scattered nature
of convection, TEMPO groups were utilized to convey the period of
greatest impact. This may be adjusted in later amendments should
initialization of activity be departed from current thoughts. The
strongest cells are likely to result in MVFR ceilings/visibility
for brief periods.

Outside of the thunderstorm potential, VFR conditions are
expected. A few showers and thunderstorms may linger into this
evening, but generally speaking, thunderstorms will move south of
the terminals and weaken after sunset. Light/variable surface
winds will persist into Wednesday.

Maples

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX