602
FXUS63 KLSX 030949
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
449 AM CDT Thu Sep 3 2026

.KEY MESSAGES...

- An Extreme Heat Warning is now effect until Saturday evening as
  the dangerous late season heatwave continues.

- Temperatures are forecast to "cool" slightly Sunday into Labor
  Day before another period of near-record breaking heat returns
  by the middle of next week.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 244 AM CDT Thu Sep 3 2026

The Heat Advisory has been upgraded to an Extreme Heat Warning for
the entire area with this forecast.  Heat indexes yesterday
afternoon were at or above 105 degrees over much of the area
including central Missouri.  These same areas are expected to see at
or above 105 degrees for four straight days (through Saturday) which
increases the dangerous nature of this current heatwave.

Persistence will be the best tool through Saturday as the HREF is
showing an upper ridge building across the central CONUS.
Temperatures at 850mb are forecast to stay around 22C with deep
mixing up to 700mb and southwesterly winds keeping highs in the
middle 90s to near 100 degrees.  Even with the deep mixing,
dewpoints yesterday afternoon only dropped into the middle 60s to
around 70 degrees which didn`t stop heat indexes from climbing above
105 degrees in many areas.  The soil temperatures have gone up
slightly here at the NWS office over the past week which may also
help boost temperatures, and a weak "cold" front will be moving in
from the east by late Saturday which may slightly elevate dewpoints.
Have kept the NBM temperatures and used a short term blend for
dewpoints to account for the mixing which produces heat indexes
between 102 and 110 degrees during the afternoon and early evening
hours.

There were some brief showers yesterday over central Missouri as the
atmosphere became uncapped, but they did not last long under the
upper ridge.  We may see similar activity today and Friday, but the
area will remain mainly dry with the ridge.  There will be a
slightly better chance on Saturday as the ridge shifts west and the
both the ECMWF/GFS hint at a MCS moving southeast across the Midwest
Friday night into Saturday, though it is farther east than 24 hours
ago. This eastward trend is also reflected in the LREF at it now
only has 15-25% of its members producing rain at St. Louis on
Saturday.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
Issued at 244 AM CDT Thu Sep 3 2026

Model guidance remains in good agreement with a weak cold front
moving through the area Saturday night. The NBM 25th/75th percentile
temperature forecast at St. Louis for both Saturday and Sunday is
still only showing 3-5 degree spread. Most areas will remain dry as
only 15-20% of the LREF members are producing rain on Saturday night
and Sunday.  Temperatures will still be unseasonably hot Sunday and
Monday with highs in the lower to middle 90s.  Dewpoints will be
slower to drop with the easterly winds and readings mainly in the
70s on Sunday versus 60s on Monday.  This will result in heat
indexes on Sunday over parts of the area still climbing over 100
degrees which may require an extension to the going headlines.
Temperatures will climb back into the middle 90s by the middle
of next week as the upper high builds back into the area. Highs
will be close to records again over parts of the area. The upper
high will also keep the area dry through the middle of next
week.

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 446 AM CDT Thu Sep 3 2026

Mainly dry and VFR conditions are expected through the period.
Light winds will turn out of the southwest less than 10 knots
this morning.

&&

.CLIMATE...
Issued at 1220 PM CDT Wed Sep 02 2026

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

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

September 4
KSTL: 104/1954...81/1925
KCOU: 103/1913...73/1947
KUIN: 105/1913...75/1947

September 5
KSTL: 103/1954...80/1899
KCOU: 103/1899...75/1983
KUIN: 100/1925...76/1912

&&

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

&&

$$

SHORT TERM...Britt
LONG TERM...Britt
AVIATION...BSH
CLIMATE...Britt