787
FXUS63 KLSX 190306
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1006 PM CDT Fri Jul 18 2025

.KEY MESSAGES...

- Additional rounds of thunderstorms are possible (30 to 60
  percent chance) through Monday, mainly across northeastern MO
  and west-central, southwestern IL. There is a low threat of
  damaging winds and flash flooding Saturday morning.

- A dangerous, prolonged heat wave will begin Saturday along/south
  of I-70 to southwestern IL, including the St. Louis metro. Due
  to precipitation and clouds, this heat wave may not begin to the
  north until Monday. This heat wave will persist through at least
  mid-next week.



&&

.SHORT TERM...  (Through Saturday)
Issued at 302 PM CDT Fri Jul 18 2025

A subtle mid-level convectively induced perturbation in quasi-zonal
flow and the northward advancement of a weak front will continue to
support scattered showers and thunderstorms, mainly along/south of I-
44 and I-70 through the afternoon and evening. Although scattering
of low stratocumulus has enabled 1500 to 3000 J/kg of MLCAPE to
develop among 15 to 25 kt of deep-layer wind shear, weak mid-level
lapse rates and warm mid-level temperatures are anticipated to limit
updraft strength and the threat of any thunderstorms becoming severe
with damaging winds. The aforementioned front will become rather
diffuse tonight, but a modest 25-kt southwesterly LLJ will
facilitate weak WAA and moisture transport sufficient for isolated
to scattered showers and thunderstorms at times, per around 20 to 30
percent of HREF membership.

The upper-level flow pattern across the central CONUS will to
transition to broadly anticyclonic through Saturday around the
initial amplification of a broad ridge while being navigated by a
parade of "ridge-running" shortwave troughs. The first of these
features will facilitate the development of an MCS across the
Northern Plains/Upper Midwest this evening, that will track
southeastward in the Great Lakes and possibly the Mid-Mississippi
River Valley. CAMs vary greatly in the evolution, timing, and track
of this MCS, but there is a loose consensus for it to reach the
locations north of I-70 between Saturday morning and early
afternoon. This timing of this MCS being out of phase with the
nocturnal LLJ decreases support for its western flank to backbuild
and result in a threat of flash flooding. Additionally, the
abatement of the LLJ will also likely lead to a weakening trend of
the MCS during the morning, but 20 to 30 kt of deep-layer shear may
help it stay organized and prolong weakening. Although not
completely out of the question, nearly all CAMs have the MCS failing
to restrengthen as instability increases again (1500 to 3000 J/kg
MLCAPE) with afternoon redevelopment mainly occurring to the
east/northeast of the CWA. Therefore, the confidence in flash
flooding and damaging winds Saturday morning and afternoon remains
low but they threats are still present if thunderstorms end up
timing/evolving more favorably.

Although the MCS and at a minimum its clouds will have some impact
on temperatures across the CWA on Saturday, the latest HREF
probabilities of 100+ F heat index values are 60 to 90 percent in
locations along and south of I-70 in MO to the Mississippi River,
including the St. Louis metro. Therefore, confidence in the upcoming
heat wave beginning on Saturday has sufficiently increased enough
for an Extreme Heat Warning to be issued in those areas with high
temperatures in the mid-90s F and dewpoints in the 70s F.

Pfahler

&&

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 302 PM CDT Fri Jul 18 2025

Model guidance are indicated that the upper and low-level pattern
will remain similar through Sunday with the opportunity for
additional rounds of thunderstorms, including organization into
MCSs, persisting. Undoubtedly, subsequent thunderstorms will have an
influence on the timing of additional shortwaves and effective
baroclinic zones, but ensemble model guidance probabilities of
measurable rainfall reaching 20 to 50 percent Saturday night through
Monday morning north of I-70, suggesting that those areas have the
potential to be impacted. The details are very uncertain, but the
threat of severe thunderstorms and heavy rainfall/flash flooding
will need to be monitored going forward. Similarly, confidence
remains highest in 100+ F heat index values existing on Sunday in
the same areas as Saturday with more influence from precipitation
and clouds to the north.

Monday through Wednesday, confidence increases that heat and
humidity will expand across the entire CWA as an upper-level ridge
amplifies more significantly with its axis over the Mid-Mississippi
River Valley. Around 20 to 30 percent probabilities of showers and
thunderstorms still persist into Tuesday across the eastern portion
of the CWA as a backdoor cold front approach before lifting back
northward. However, with the building ridge potentially proving
hostile to thunderstorms and probabilities of 100+ F heat index
values increasing through Monday and Tuesday, a Extreme Heat Watch
has been issued for the rest of the CWA outside of the warning.

These heat headlines have been issued to address the dangerous
length of the upcoming heat wave, but magnitude (110+ F) could still
be eclipsed at times as high temperatures warm well into the 90s to
nearly 100 F along with dewpoints in the 70s F. The ultimate end of
the heat and humidity is not yet clear with NBM temperature
distributions increasing each day after Wednesday. This spread is
due to some support for a gradual de-amplification of the ridge at
the end of next week into the weekend which could at least open up
opportunities for showers and thunderstorms, per around 20 to 30
percent of ensemble model membership. For this reason, the Extreme
Heat Warning and Watch are in effect through Wednesday evening, but
extensions may be needed in the future.

Pfahler

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 958 PM CDT Fri Jul 18 2025

VFR conditions will generally prevail through the TAF period. The
chance for isolated to scattered showers and thunderstorms
increases overnight for areas along and north of I-70, including
KUIN and the St. Louis metropolitan terminals. Late Saturday
morning into afternoon, a thunderstorm complex is forecast to
enter northeast Missouri/west- central Illinois and dive
southeast. The exact timing on this feature is still uncertain,
but I gave my best estimate. Whether the complex holds up long
enough or extends southwest enough to impact the St. Louis
terminals is still uncertain as well, but I kept the PROB30 group
for it in and pushed back the timing since confidence is too low
to remove it.

Jaja

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Extreme Heat Watch from Monday afternoon through Wednesday
     evening for Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-
     Ralls MO-Shelby MO.

     Extreme Heat Warning from noon Saturday to 7 PM CDT Wednesday
     for Audrain MO-Boone MO-Callaway MO-Cole MO-Crawford MO-
     Franklin MO-Gasconade MO-Iron MO-Jefferson MO-Lincoln MO-
     Madison MO-Moniteau MO-Montgomery MO-Osage MO-Reynolds MO-
     Saint Charles MO-Saint Francois MO-Saint Louis City MO-
     Saint Louis MO-Sainte Genevieve MO-Warren MO-Washington MO.

IL...Extreme Heat Watch from Monday afternoon through Wednesday
     evening for Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-
     Fayette IL-Greene IL-Jersey IL-Macoupin IL-Marion IL-
     Montgomery IL-Pike IL-Washington IL.

     Extreme Heat Warning from noon Saturday to 7 PM CDT Wednesday
     for Madison IL-Monroe IL-Randolph IL-Saint Clair IL.

&&

$$

WFO LSX