570
FXUS63 KLSX 032252
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
552 PM CDT Thu Sep 3 2026

.KEY MESSAGES...

- A late summer heat wave will continue areawide through
  Saturday.

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

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 236 PM CDT Thu Sep 3 2026

Surface temperatures and heat index values are nearly identical
to yesterday`s values as of 1900 UTC, though maybe a degree or
so lower in many locations. Diurnal cumulus though has
developed ~1 hour earlier and more to the northeast compared to
yesterday, so these clouds may have a slight (1-2F at most)
impact across southeast and east-central Missouri as well as
southwest Illinois. That being said, most areas will still see
peak heat index values in the 100-105F range, with some spots of
105F+. Speaking of the cumulus field, a few brief rain showers
are possible, but the vast majority of the area will stay
completely dry. The coverage of any shower activity looks to be
too low to mention explicitly via a slight chance (15+%) PoP.

Persistence, persistence, persistence remains the rule through
Saturday. Any change to the ambient air mass is negligible, at best.
Lows each night are forecast to be in the 70s, with the exception of
near 80 degree minimums within the urban heat island of
metropolitan St. Louis. Highs both Friday and Saturday are
expected to be in the mid to upper 90s, with a few locations
topping out near the century mark. Peak heat index values of
100-105+F are expected as well near or just after midday before
some mixing out of dewpoints occurs.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Issued at 236 PM CDT Thu Sep 3 2026

(Saturday Night - Labor Day)

Subtle changes have been noted for the latter part of the Labor Day
holiday weekend forecast. Notably, the surface high pressure across
the Great Lakes has trended a bit stronger on ensemble guidance,
helping to push the backdoor cold front more to the west. It appears
more likely that much of (if not all) the area should see some low-
level cold air advection behind this front as winds turn out of the
east/northeast. Now high temperatures will not exactly be "cool" by
any means, but should drop on average a few degrees each day from
Saturday through Labor Day. Highs more in the low to
mid 90s are forecast. Dewpoint (relative humidity) values are
always harder to forecast, but there is a pretty strong signal
for a fairly significant push of dry air to advect in from the
Great Lakes. Probabilities for dewpoints to be below 65 degrees
Labor Day afternoon are above 60% areawide, with along/east of
the Mississippi River even having pretty good chances for sub
60 degree values (over 40%). So, not only are temperatures
cooling, but lower humidity is also likely to move into the
region by Labor Day. If this holds true, we may at least get a
brief break from heat index values near 100 degrees across the
entire area for Labor Day, and many locations beginning Sunday
behind the backdoor front.


(Tuesday - Next Thursday)

There remains high confidence in a return to near-record breaking
temperatures by Wednesday as mid/upper level ridging moves back over
the mid-Mississippi Valley. The strength of the 500-hPa level ridge
is likely to be record breaking for early September. In
addition, the 850-hPa thermal ridge is also expected to bisect
the CWA, with temperatures at that level well above +20C (>99th
percentile). This is also 6 days out, with mostly hot and dry
conditions so soils should continue to dry out. In theory, that
may yield our hottest temperatures of the entire stretch. The
latest NBM has widespread upper 90s with some locations around
the century mark, which looks pretty representative. If
anything, it may end up a touch warmer than currently forecast.

Uncertainty increases sharply Wednesday night into next Thursday due
to the possibility of a cold frontal passage. Two of the four
clusters of the 500-hPa height pattern from the LREF (37% of
members) show the anomalous ridging holding strong while the other
two clusters (63% of members) show more zonal flow with a stronger
trough as well crossing the Great Lakes. The former pattern would
mean a continuation of near-record breaking heat while the later
would send the cold front through the area and bring much cooler
(closer to normal) temperatures. The inter-quartile range of the NBM
shows this spread well, with about 10-15 degrees between the
25th/75th percentiles, and nearly 15-25 degrees from the 10th
to the 90th percentile.

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 548 PM CDT Thu Sep 3 2026

Somewhat variable/light (5kt) southerly surface winds are forecast
to veer southwest by Friday morning with dry and VFR conditions
expected for the entire forecast period.

&&

.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...Gosselin
LONG TERM...Gosselin
AVIATION...Peine
CLIMATE...Britt