111
FXUS63 KLSX 261939
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
239 PM CDT Thu Jun 26 2025

.KEY MESSAGES...

- Higher chances of showers and thunderstorms will exist tonight
  through Monday, which, along with associated clouds, casts some
  uncertainty in exact high temperatures and heat index values
  each day.

- More seasonable temperatures and humidity along with drier
  conditions will prevail Tuesday through mid-next week.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 238 PM CDT Thu Jun 26 2025

There has been little change in the weather pattern across the Mid-
Mississippi River Valley in the last 24 hours with the region at the
northwestern periphery of an upper-level ridge and a hot and humid
airmass residing below with heat index values in the upper 90s to
around 100 F across much of the CWA. Due to slightly warmer mid-
level temperatures/weaker mid-level lapse rates, diurnal showers and
thunderstorms have been more isolated and struggling to produce as
much lightning as Friday.

Although the aforementioned showers and thunderstorms will dissipate
around sunset, an additional cluster of showers and thunderstorms
across southeastern IA/northwestern MO this afternoon will track
southeastward this evening, reaching northeastern MO around 10 pm.
This cluster will likely be weakening by that time as instability
decreases and it becomes outflow dominant with weak wind shear (deep-
layer 10 to 20 kt). However, showers and thunderstorms will take the
longest to weaken across west-central to central MO where moisture
convergence will be maximized at the nose of a 40 kt southwesterly
LLJ. This evolution is also supported by most CAMs, hanging on to
this southwestern flank of the cluster until the LLJ weakens Friday
morning.

Following a relative lull during the morning on Friday, additional
scattered development of showers and thunderstorms will take place
mainly across east-central, southeastern MO and southwestern IL
during midday into afternoon near remnant outflow as instability
increases and an upper-level shortwave trough passes. Although
MLCAPE will reach 1500 to 3000 J/kg, weak deep-layer wind shear of
only 10 to 15 kt decreases confidence that any thunderstorms will
become severe, but a few weak microbursts could lead to gusty winds.
Cloud debris and greater coverage of showers and thunderstorms will
both lead to cooler high temperatures on Friday, the main limiting
factor for the going stretch of advisory level heat to continue.
That being said, afternoon heat index values will still approach 100
F in the St. Louis metro and southwestern IL, least likely to be
significantly impacted by early day outflow or precipitation/clouds
with temperatures reaching around 90 F. Showers and thunderstorms
will dissipate and/or depart to the east Friday evening with
propagation and decreasing instability.

Pfahler

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 238 PM CDT Thu Jun 26 2025

Over the weekend, model guidance is in agreement that upper-level
flow over the Mid-Mississippi River Valley will become quasi-zonal
and be navigated by one or more perturbations/shortwave troughs. The
presence of these features will be important in determining when/if
showers and thunderstorms over the weekend will be more than just
scattered and diurnal in nature. For this reason, despite low-level
WAA and southwesterly flow leading to warmer temperatures, there is
slightly lower confidence in high temperatures Saturday and Sunday,
with the NBM interquartile range varying 5 F from the upper 80s to
the mid-90s F. With dewpoints also continuing to be in the 70s F,
afternoon heat index values could reach around 100 F, especially in
the St. Louis metro.

A pattern change will take place early next week as an upper-level
trough deepens across the Upper Midwest and Great Lakes, accompanied
by passage of a shortwave trough and cold front through the Mid-
Mississippi River Valley. Between the weekend and Monday, ensemble
model guidance have the greatest membership with measurable rain on
Monday which aligns with when the greatest large-scale forcing will
be present. Deep-layer wind shear will remain weak but current
indications are that it will be slightly stronger and 15 to 20 kt,
which suggests the potential for at least a few strong thunderstorms
with 50 to 70 percent probabilities of 2000+ J/kg of MUCAPE. Another
impact of the pattern change will be post-frontal low-level
northwesterly CAA cooling temperatures closer to average and
seasonably lower humidity by Tuesday along with drier conditions.

Around the middle of next week into the 4th of July Holiday, model
guidance suggests that upper-level northwesterly flow in the wake of
the trough may evolve into more quasi-zonal flow once again. This
process will transition the seasonable, tranquil conditions back to
warmer and more humid conditions as low-level flow becomes southerly
with the a warm front eventually lifting through the region,
accompanied by opportunities for showers and thunderstorms.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1213 PM CDT Thu Jun 26 2025

Predominantly VFR flight conditions are expected through the TAF
period, but there are chances of showers and thunderstorms at times.
Through this afternoon showers and weak thunderstorms are expected
to be isolated, limiting confidence in any direct impacts. Tonight
into early Friday morning, a weakening cluster of thunderstorms is
forecast traverse northeastern MO, potentially surviving long enough
to reach KUIN, KCOU, and KJEF. Following a lull in precipitation,
redevelopment of showers and thunderstorms will take place midday
into afternoon on Friday with the greatest chances at St. Louis
metro terminals. Thunderstorms could contain brief downpours with
MVFR to IFR flight conditions and gusty winds.

Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 9 PM CDT this evening for Callaway MO-Cole
     MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson
     MO-Lincoln MO-Madison MO-Montgomery MO-Osage MO-Pike MO-
     Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis
     City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-
     Washington MO.

IL...Heat Advisory until 9 PM CDT this evening for Adams IL-Bond IL-
     Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey
     IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
     Pike IL-Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX