726
FXUS63 KLSX 201106
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
606 AM CDT Mon Jul 20 2026

.KEY MESSAGES...

- Hot and humid conditions with heat index values of 100 to 108
  F will envelop much of the area today and then locations
  along/south of I-70 on Tuesday.

- The highest chance of showers and thunderstorms through
  Tuesday will be overnight tonight into Tuesday morning north
  of I-70, including a conditional threat of a marginally severe
  thunderstorm or two and locally heavy rainfall.

- Much cooler conditions will arrive by Wednesday, accompanied
  by a period of dry weather through Thursday.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 326 AM CDT Mon Jul 20 2026

Light winds and mostly clear skies have allowed areas of fog to form
across the CWA this morning, especially in the vicinity of a weak
stationary front and moistened soils from Sunday`s
showers/thunderstorms. Near/east of the Mississippi River, this fog
has been aided by Canadian wildfire smoke or at least further
reducing visibilities. Visibilities down to 1/4 mi are possible, but
confidence is low that these will occur on more than a localized
basis before dissipation through sunrise.

An upper-level trough entering the Upper Mississippi River Valley
and a leading shortwave trough will encourage slow northeastward
advancement of the front as a warm front today, reaching west-
central/south-central IL through the afternoon and evening.
Therefore, wildfire smoke is likely to linger in these locations
through much of the day, mainly impacting air quality (see Air
Quality Alert). With more of the CWA in the warm sector and much
less showers/thunderstorms, more widespread high temperatures around
90 to the mid-90s F are forecast along with pooled dewpoints
yielding heat index values of 100 to 108 F in the Heat Advisory
area. Diurnal showers and thunderstorms are expected again this
afternoon, but the HREF membership indicates that coverage will be
much lower than Sunday and mainly across southeastern MO where a
capping inversion is weakest. Confidence is also low in any showers
and thunderstorms associated with the lead shortwave trough this
afternoon reaching the northern CWA. These showers and thunderstorms
are more likely to remain across eastern IA and northern/central IL.

An increasing amount of HREF membership has showers and
thunderstorms developing along a cold front across IA and WI this
evening, tracking south/southeastward with a veering LLJ overnight
into Monday morning including reaching some portion of the CWA north
of I-70. However, there is still uncertainty on how far
southwestward thunderstorms truly develop along the front/into the
LLJ and reach the CWA given northwesterly downshear Corfidi vectors
and a dependency on upstream convective evolution. If thunderstorms
do reach the CWA, 30 to 40 kt of deep-layer wind shear and 1000 to
2500 J/kg MLCAPE support organized thunderstorms, largely into
either an MCS and/or a few supercells with a threat of isolated
marginally severe hail and damaging winds. There would also be a
threat of locally heavy rainfall along the southwestern flank of
thunderstorms with PW around 2" and deep warm-cloud depths. However,
both of these threats are conditional with another reasonable
scenario still including thunderstorms remaining just
north/northeast of the CWA.

The cold front will traverse the CWA on Tuesday, reaching locations
along/south of I-70 during the afternoon and evening. In these
locations, high temperatures will once again warm into the 90s F,
accompanied by heat index values of 100 to locally near 110 F
depending on how much dewpoint lower with deep BL mixing. To the
north behind the front, stronger post-frontal CAA will lag frontal
passage with high temperatures still reaching around 90 F, but
dewpoints will be lowering more quickly and significantly limit heat
index values. With the upper-level trough passing just to the
northeast and mid-level heights remaining neutral, it is uncertain
how much development of showers and thunderstorms there will be
along the portion of the cold front traversing the CWA, with
stronger forcing and less prominent capping inversion to the
east.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Issued at 326 AM CDT Mon Jul 20 2026

The much cooler and drier airmass will finally reach the Mid-
Mississippi River Valley by Wednesday, remaining in place through
Thursday. In this airmass, seasonably cool high temperatures in the
80s F are forecast, accompanied by low dewpoints in the 50s and low-
60s F. A surface high pressure center will also dominate the region
with dry weather during this time, but a series of upper-level
shortwave troughs with northwesterly flow will induce rounds of
showers and thunderstorms along/north of a warm front, initially in
the Central Plains, Thursday through Saturday. However, global
ensemble model guidance indicates that the most favored corridor of
showers and thunderstorms will not reach the CWA until Thursday
night or Friday as the warm front progresses northeastward. Although
moisture will be increasing through this time, the CWA will likely
remain largely on the cool side of the front through Friday not to
mention potential precipitation and clouds keeping temperatures
seasonably cool.

Global model guidance is in agreement that an upper-level ridge will
be amplifying across the Front Range of the Rocky Mountains through
the end of the week, favoring the warm front to continue lifting
northeastward along with the more active track of shortwave troughs.
Therefore, confidence is high that temperatures will moderate to
average, if not above average through the weekend along with
increasingly dry conditions.

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 606 AM CDT Mon Jul 20 2026

Reduced flight conditions associated with fog and smoke will improve
through several hours after sunrise, but a brief period of MVFR
ceilings is likely in the early stages of diurnal cumulus
development. Otherwise, predominantly dry conditions are expected
today with only isolated showers and thunderstorms this afternoon,
mainly across southeastern MO and away from the TAF sites. There is
a 30 to 50 percent chance for showers and thunderstorms to move into
the region overnight tonight into Tuesday morning, with the highest
chance of impacts at KUIN. Light winds will prevail through the TAF
period.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 7 PM CDT Tuesday for MOZ041-047>051-
     059>065-072>075-084-085-099.
     Heat Advisory from noon today to 8 PM CDT this evening for
     MOZ018-019-026-027-034>036-042-052.
IL...Air Quality Alert until midnight CDT tonight for ILZ058>060-
     065-095>097.
     Heat Advisory until 7 PM CDT Tuesday for ILZ069-074-079-
     100>102.
     Heat Advisory from noon today to 8 PM CDT this evening for
     ILZ095-097-098.

&&

$$

SHORT TERM...Pfahler
LONG TERM...Pfahler
AVIATION...Pfahler