Wentzville, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

518
FXUS63 KLSX 192022
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
322 PM CDT Sat Jul 19 2025

.KEY MESSAGES...

- Additional rounds of showers and thunderstorms are expected at
  times in portions of the area through Monday night. A few
  thunderstorms could be severe with isolated damaging winds and
  flash flooding Sunday evening and night along and north of I-70.

- The coverage of prolonged, dangerous heat wave will increase on
  Sunday along/south of I-70 into southwestern IL. After a
  potentially cooler day Monday for some areas, confidence is high
  that the heat will expand areawide on Tuesday.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 321 PM CDT Sat Jul 19 2025

A slowly decaying MCS continues to traverse the CWA this afternoon,
with sporadic thunderstorms lagging behind its outflow boundary with
a lack of strong low-level heating/instability and convergence. More
vigorous thunderstorms have developed across southern IL where
stronger heating has resulted in MLCAPE of 1500 to 2000 J/kg, but
these thunderstorms are largely propagating away from the CWA. A few
additional showers and thunderstorms cannot be ruled out this
evening if a modest southwesterly LLJ develops before the outflow
boundary completely dissipates. The MCS and decks of altocumulus
that developed ahead of it have lead to significantly cooler
temperatures today than previous anticipated with the exception of
southeastern MO and far southwestern IL. Therefore, areas near the I-
70 corridor in the Extreme Heat Warning that began today will fail
to see high temperatures reach 90 F, let alone heat index values
reaching 100+ F.

Later this evening into tonight, another round of showers and
thunderstorms is expected to initiate at the nose of the LLJ across
IA. The majority of HREF membership keeps the activity to the
northwest of the CWA but 30 to 50 percent does bring a weakening MCS
and/or develops altocumulus-based showers (similar to this morning)
into northeastern MO and west-central IL Sunday morning into mid-
day. During the afternoon and evening, a front, reinforced southward
by these showers and thunderstorms, is forecast to be draped across
the CWA somewhere north of the I-70 corridor by the afternoon. There
is a lack of discernible large-scale forcing in the upper-level
quasi-zonal flow overhead, leaving development along this front
uncertain through the afternoon and only supported by a small
handful of CAMs. With less clouds and precipitation through peak
heating, confidence is much higher in heat index values reaching
100+ F on Sunday along/south of I-70 with HREF probabilities of 80
to almost 100 percent.

Probabilities of thunderstorm initiation increase along and north of
I-70 Sunday evening as a 35-kt west-southwesterly LLJ begins to form
and interacts with the front. With progs of the LLJ depicting its
nose closer to the CWA, confidence is higher that initiation will be
in or near the CWA, contrasting previous rounds of showers and
thunderstorms that have been developing upstream. With 20 to 30 kt
of deep-layer wind shear and MLCAPE of 1500 to 3000 J/kg
organization of thunderstorms into a cluster or MCS with isolated
damaging winds would be favored. With some slight low-level
hodograph curvature a transient supercell or two is also possible.
With anomalous PW over 2", deep warm cloud depths, and the
environment capable of allowing thunderstorms to train and
backbuild; there could be a transition to a locally heavy rainfall
and isolated flash flooding threat overnight. There is some support
for this threat in the latest HREF LPMM with streaks of 2 to 4",
mainly across IL. Although these threats do exist, it is important
to note that CAMs vary significantly in the location, coverage, and
evolution of thunderstorms, decreasing overall forecast
confidence.

Pfahler

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 321 PM CDT Sat Jul 19 2025

Although model guidance remains consistent on an upper-level ridge
amplifying over the Mid-Mississippi River Valley on Monday, there
are also signs of the aforementioned front remaining near the CWA,
both reinforced by rounds of showers and thunderstorms as well as a
surface anticyclone across the Great Lakes. As a result, 30 to 50
percent of ensemble model guidance have additional rounds of showers
and thunderstorms lingering across the northern and eastern portion
of the CWA, lingering from Sunday night and/or redeveloping Monday
evening with another LLJ. This factor leads to the potential for
temperatures to be slightly cooler on Monday than Sunday,
complicating the continuity of the heat wave in the Extreme Heat
Warning area and decreasing confidence in the onset of the Extreme
Heat Watch area being on Monday. Adjustments to these headlines will
likely be needed going forward, including delaying the onset.

Confidence in more widespread heat and humidity (temperatures in the
90s F and dewpoints in the 70s F) with increasing coverage of the
heat wave is bolstered on Tuesday with ensemble model probabilities
of 100+ F heat index values nearly CWA-wide as the axis of the upper-
level ridge is more firmly overhead. Probabilities showers and
thunderstorms linger but gradually taper from 30 percent to 10
percent across IL as the front gradually dissolves instead of
exiting and mid-level temperatures becoming increasingly hostile to
thunderstorm development. The ultimate end of the heat and humidity
is still not completely clear hinging upon when upper-level ridging
relaxes and allows fronts to approach and showers and thunderstorms
to occur in the CWA, providing some relief. NBM temperature
distributions start to increase after Thursday, but there has been a
trend toward similar temperatures persisting. Therefore, the going
Extreme Heat Warning and Watch will likely be need to be extended.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1238 PM CDT Sat Jul 19 2025

A decaying complex of thunderstorms will continue southeastward
through the area this afternoon, featuring a wind shift and gusts
along with sporadic showers and thunderstorms. Beyond this complex,
largely dry and VFR flight conditions will prevail through Sunday,
but there is another higher chance of showers and thunderstorms at
KUIN Sunday morning. That being said, most showers and thunderstorms
are expected to remain north and northwest of all terminals tonight.

Pfahler

&&

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

     Extreme Heat Warning until 7 PM CDT Thursday 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 Thursday
     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 until 7 PM CDT Thursday for Madison IL-
     Monroe IL-Randolph IL-Saint Clair IL.

&&

$$

WFO LSX