985
FXUS63 KLSX 311031
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
531 AM CDT Mon Aug 31 2026

.KEY MESSAGES...

- A Heat Advisory is now in effect for the entire area through
  Friday, with daily heat index values peaking around 100
  degrees.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 231 PM CDT Sun Aug 30 2026

Broad upper-level ridging is encompassing the eastern 2/3rds of the
CONUS per morning water vapor imagery as an expansive surface high
sits over the Southeast. With the CWA on the northwestern periphery
of the surface high, winds are southerly, signaling that warm air
advection continues. As daytime heating commences, the pressure
gradient will tighten and cause surface winds to shift to out of the
southwest. This will lead to adiabatic warming via downsloping off
the Ozark Plateau, further boosting our temperatures to reach some
of the warmest highs we`ve experience this summer. Widespread values
in the upper 90s are forecast, with a couple locations potentially
hitting 100. Compared to previous heat waves this summer, humidity
will be lower, with dew points in the mid to upper 60s yielding heat
index values around 100 degrees this afternoon.

With the pattern holding through at least tomorrow, the forecast for
Tuesday is nearly identical to today. With a similar degree of heat
forecast through the end of the workweek, a Heat Advisory is in
effect for the entire area, including the counties that had been
under an Excessive Heat Watch. An Excessive Heat Warning may be
needed later in the week for portions of the CWA, especially after
dew point trends for Tuesday and Wednesday become clearer.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 231 PM CDT Sun Aug 30 2026

Little change is expected to the upper-level pattern and our local
weather through the extended forecast period when compared to the
short term forecast period. The ridge will gradually amplify toward
the end of the workweek, with guidance converging on a common
placement of the ridge axis just east of the Great Plains. This will
keep the thermal ridge mostly in place over the Middle Mississippi
Valley and 850 mb temperatures steady. Toward the end of the
weekend, some guidance shows that a backdoor front may push the
thermal ridge westward and take the edge off of the heat, but a
majority of guidance currently shows it continuing.

An uninterrupted fetch of Gulf moisture into the area supports dew
points increasing through the end of the week, though by high much
remains uncertain. Guidance consensus is that the CWA will be on the
northern periphery of greater dew points through the period, with
slight changes in the placement of the upper-level ridge dictating
the exact placement of this moisture. Confidence is high that
surface winds will be southwesterly daily, leading to a local
decrease in dew points thanks to downsloping off the Ozarks. These
factors combined with a seasonal dearth in evapotranspiration lead
to low confidence in the NBM`s dew points. Current values lead to
pockets of heat index values just over 105 degrees, and if that is
realized, a targeted Extreme Heat Warning may be needed. If dew
points trend lower than currently forecast, the Heat Advisory will
continue through the end of the week area-wide.

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 530 AM CDT Mon Aug 31 2026

Dry and VFR flight conditions are expected through the TAF
period at all local terminals.

&&

.CLIMATE...
Issued at 305 AM CDT Sat Aug 29 2026

Record Highs...Record High Lows (value/year):

August 31
KSTL: 103/2013...79/2013
KCOU: 103/1913...78/1947
KUIN: 100/2011...76/1924

September 1
KSTL: 104/2011...82/2011
KCOU: 102/1913...76/1913
KUIN: 104/2011...78/2011

September 2
KSTL: 101/2011...80/2011
KCOU: 100/1913...77/1913
KUIN: 103/1913...80/1913

September 3
KSTL: 101/2011...80/1913
KCOU: 103/1939...75/1939
KUIN: 103/1913...78/1913

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from noon today to 8 PM CDT Friday for MOZ018-
     019-026-027-034>036-041-042-047>052-059>065-072>075-084-
     085-099.
IL...Heat Advisory from noon today to 8 PM CDT Friday for
     ILZ058>060-064-065-069-070-074-079-095>102.

&&

$$

SHORT TERM...Elmore
LONG TERM...Elmore
AVIATION...Elmore
CLIMATE...Elmore