298
FXUS63 KLSX 261933
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
233 PM CDT Wed Aug 26 2026

.KEY MESSAGES...

- An isolated thunderstorm or two is possible (20 percent
  chance) through this evening, mainly in south-central/
  southwestern IL and southeastern MO. A thunderstorm could be
  strong to severe with damaging winds.

- Behind a cold front today, temperatures will be near to
  slightly below average before a heatwave build Sunday into
  next week.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 232 PM CDT Wed Aug 26 2026

Confidence continues to decrease in initiation of thunderstorms
across the CWA this afternoon and evening with some of the latest
CAMs have thunderstorms remain outside of the CWA and others showing
sparse coverage. This factor is mainly owing to the CWA being
between areas of large-scale ascent (upper-level trough to the north
and an MCV to the south), weak low-level convergence/forcing, and a
weak capping inversion that may not be overcome. That being said, an
isolated thunderstorm or two is still possible across south-
central/southwestern IL and southeastern MO this evening as a weak
cold front consolidates with a pre-frontal trough across that area,
providing a window of greater low-level convergence. With 20 to 25
kt of deep-layer wind shear, high DCAPE/"inverted-V" sounding
signatures, and 1500 to 3000 J/kg MLCAPE; one of these thunderstorms
could briefly become strong to severe with damaging winds.


&&

.LONG TERM /THURSDAY THROUGH WEDNESDAY/...
Issued at 232 PM CDT Wed Aug 26 2026

Behind the front, a surface anticyclone will expand into the Mid-
Mississippi River Valley Thursday and then arrive on Friday,
accompanied by a seasonably cool and dry airmass (850-hPa
temperatures and PW around the 25th climatological percentile)
infiltrating the CWA. This airmass will translate to high
temperatures in the low to mid-80s F, low temperatures in the upper
50s to mid-60s F, and lower humidity through Saturday morning.
Efficient nighttime radiational cooling will also promote fog
development each night/morning, predominantly in river valleys as a
combination of radiation and steam fog.

The surface anticyclone will quickly depart to the Great Lakes on
Saturday as it follows the most vigorous subsidence leading an upper-
level ridge amplifying and shifting into the Mid-Mississippi River
Valley. This process will facilitate southwesterly low-level
flow/WAA with 850-hPa temperatures surging to around 21 to 23 C
(above 95th climatological percentile) by Sunday, persisting into
next week. With full sunshine, these 850-hPa temperatures would
likely yield widespread high temperatures in the 90s F, including
favorable southwesterly flow and downslope warming off the Ozark
Plateau potentially resulting in values even approaching 100 F in
central MO and the St. Louis metro. Fortunately, dewpoints/humidity
will at least initially be lower than previous heatwaves we have
seen so far this summer but probabilities of 100+ F heat index
values do increase each day in response to a slow increase in
humidity/dewpoints. The details of the upper-level wave pattern next
week are still not uncertain but there has been an underlying trend
toward a stronger, more persistent ridge across the Mid-Mississippi
River Valley. As such, confidence is increasing in a longer duration
of heat through mid-next week. It is worth noting that there is a
still a preferred corridor for episodic showers and thunderstorms
around the periphery of the ridge, but if this trend holds there is
less concern for the CWA to be impacted by showers and thunderstorms
aside from far northern parts of northeastern MO and west-central IL
through Wednesday.

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1205 PM CDT Wed Aug 26 2026

Confidence remains low that any showers or thunderstorms will impact
TAF sites this afternoon as a cold front passes--slightly higher
chances are across southwestern IL and southeastern MO this evening.
River valley fog development is expected late tonight into Thursday
morning, including a period of MVFR to IFR visibilities at KSUS,
KCPS, and KJEF. Otherwise, dry and VFR flight conditions will
prevail with light winds.

&&

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

&&

$$

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