333
FXUS63 KLSX 130201
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
901 PM CDT Sat Jul 12 2025

.UPDATE...
Issued at 856 PM CDT Sat Jul 12 2025

A small but nearly stationary thunderstorm has developed along a
stalled frontal boundary across portions of Reynolds and Iron
county this evening, with rates near or even slightly higher than
2 inches per hour estimated by radar. While this storm is isolated
and compact, it has persisted for nearly two hours in
approximately the same location, and may continue to do so for
another hour or two before dissipating. While this rain is falling
over a limited area, the vast majority of this water is likely
falling in a small portion of the Black River basin, very near and
upstream from the Highway K Bridge. While the river itself may not
reach official flood stage, a rapid rise is likely, and may pose a
threat to those recreating along the river.

Additional cells have also begun to develop nearby, and we will
need to continue to monitor the potential for localized flooding
issues in other spots across the Ozarks throughout the evening
until the boundary sinks farther to the south, or until
instability wanes after dark.

BRC

&&

.KEY MESSAGES...

- Scattered showers and thunderstorms are forecast each day through
  late next week. There will be plenty of dry time.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 225 PM CDT Sat Jul 12 2025

Recent surface analysis shows a pre-frontal trough pushing through
southeast Missouri and southern Illinois this afternoon. This
feature is quickly traversing these areas and is forecast to stall
just beyond our border. As such, tonight will be mostly dry as
ongoing convection is shoved out of the area. Portions of northeast
Missouri and west-central Illinois will see a relatively cool night
as well with lows dropping into the mid to upper 60s. This is due to
a cold front ushering lower dewpoints into these areas.

Rain chances return early tomorrow morning through tomorrow
afternoon as a shortwave trough over the southern Plains ejects
vorticity lobes into Missouri and Illinois. These lobes will
interact with a stalled front to our south and produce showers and
thunderstorms. Although showers are forecast areawide, the highest
concentration of thunderstorms will generally be relegated south of
I-70 where moisture and instability will not have been tampered with
by the drier air to the north. With around 1500 J/kg of MLCAPE
present, I cannot rule out a few thunderstorms capable of gusty
winds, but strong thunderstorms are unlikely. Bulk shear values of
up to 15 - 20 kts will support quick-lived pulse convection as
opposed to organized thunderstorms. As we`ve seen earlier this week,
cores will likely collapse before bulking up enough to produce
strong winds. Otherwise, PWATs still in the 1.75 - 2" range will
support locally heavy rainfall underneath these thunderstorms.
Isolated flash flooding is a possibility in locations that
experience multiple thunderstorms and/or have received abundant
rainfall recently.

Thunderstorm coverage will taper off through the evening with a dry
forecast in store by midnight. A similar night to tonight is
expected with widespread mid to upper 60s forecast for most.

Jaja

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 225 PM CDT Sat Jul 12 2025

There is a chance for showers and thunderstorms each day from Monday
through late week, but plenty of dry time is expected. Coverage and
location of convection will be dictated by the position of the
aforementioned stalled front and the behavior of mid-level
shortwaves and vorticity maxima.

Global models begin to diverge on the pattern aloft as early as
Monday which has implications regarding our rain chances for the
week. What they do agree on is the presence of a near-cutoff low in
the Oklahoma/Arkansas vicinity on 12z Monday. This feature is
progged to head northeast through the mid-Mississippi Valley through
the day on Monday, and that`s where the similarities end. Where
exactly the feature starts the day, how strong it will be, how fast
it traverses the mid-south, and where it tracks all differ. Most
deterministic guidance favors a weaker, slower, and more equatorward
tracking low which would result in rain chances confined to south of
I-70 during the afternoon. However, there is some guidance that
favors a stronger, faster, and more poleward shifting low. This
solution would cause more locations to see showers and thunderstorms
with them beginning earlier in the day. Although our forecast
reflects the drier solution, both are possible at this juncture.

Thunderstorm chances increase Tuesday as the cutoff low pushes the
stalled surface boundary northward and back into our CWA. The low
will continue to provide upper level support for convection during
the day, but the details are muddled. The greatest chance will be in
the afternoon when conditions are most favorable, but exact timing,
location, and coverage all depend on the location and strength of
the boundary. This will, in turn, be determined by convection on
Monday and the behavior of the mid-level pattern. The same can be
said for Wednesday and so on. This uncertainty is reflected in a
growing NBM interquartile high temperature spread from Tuesday
onward. By Thursday the spread is 8 degrees. This is a notable
spread in July when the pattern is typically quiet and climatology
forecasts are generally within a couple of degrees of being correct.

Otherwise, high temperatures for next week are forecast to land
generally in the mid-80s to low-90s each day with morning lows in
the 60s and 70s.

Jaja

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 604 PM CDT Sat Jul 12 2025

The primary concerns during the 00Z TAF period will be the
potential for some patchy fog overnight tonight, followed by
showers, lowering ceilings, and a few thunderstorms tomorrow
morning through the afternoon. While a few very weak showers
continue to percolate in the area this afternoon, mostly near UIN,
VFR conditions are in place to start the period. Very light winds
and residual humidity overnight may allow for some patchy fog
development in low lying areas, most likely at SUS/JEF/CPS, with
lower probability chances at COU and UIN as well. Showers are
likely to move into central Missouri in the morning, followed
shortly thereafter in the St. Louis area, and will continue on and
off through the mid to late afternoon. A few thunderstorms are
also possible, but are not likely to produce much more than
lightning, bursts of heavy rain, and occasionally gusty winds.
MVFR ceilings are also likely with these showers with a lower
chance for dips to IFR levels as well. Visibility reductions due
to heavy rain will also be possible.

BRC

&&

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

$$

WFO LSX