512
FXUS63 KLSX 021915
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
215 PM CDT Wed Sep 2 2026

.KEY MESSAGES...

- The ongoing late summer heat is expected to continue through
  at least Saturday.

- Slightly "cooler" temperatures are forecast Sunday into Labor
  Day before another period of near-record breaking heat
  returns for midweek.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 155 PM CDT Wed Sep 2 2026

Observations into early this afternoon are running every so
slightly warmer for both temperatures and heat index values
compared to yesterday. Dewpoints should mix down several degrees
though by late afternoon, which means most areas are right
around their peak heat index for the day (if they have not hit
it already). Most stations are in the 100-105F range, but at
least a few will top out just above 105F.

The overall pattern remains locked in place thanks to an omega block
across the North American continent. Therefore, persistence
continues to be a very good forecast for the next couple of days.
Southerly low-level flow may advect ever so slightly "cooler"
850 hPa temperatures (~0.5C) into the area on Thursday. This
along with more widespread diurnal cumulus suggests highs may be
about a degree on average cooler than this afternoon. Highs in
the mid to upper 90s are forecast both Thursday and Friday
afternoons along with peak heat index values of 100-105F with
spots of 105+. Low temperatures each night should be in the 70s,
with near 80 degree readings within the urban heat island of
metropolitan St. Louis.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 155 PM CDT Wed Sep 2 2026

(Saturday - Labor Day)

Ensemble guidance remains in very good agreement that the mid/upper
level ridge will retrograde into the Plains early this holiday
weekend. This should help allow a backdoor "cold" front to move
toward the mid-Mississippi Valley, but how far southwest it reaches
remains a question. Regardless, I do expect at least slight cooling
as guidance shifts the 850-hPa thermal ridge northwestward and away
from the area. Now, the air mass in place remains quite hot for
early September, just not as hot. Temperatures should cool
everywhere each day through Labor Day, with low to mid 90s
becoming more common. Peak heat index values also should cool
slowly, though they will be offset somewhat by higher dewpoints
pooling near the frontal boundary. Central, southeast, and east-
central Missouri have the best chance of 100-105+ heat index
values both Saturday and Sunday. By Labor Day, most of the area
should be around or just below the century mark for heat index
values. The exact high temperatures though each day, unlike this
work week, are a bit more uncertain due to the chances of
showers and thunderstorms. Deterministic guidance on/off has
shown a signal for a potential MCS along/east of the Mississippi
River Saturday and Sunday. Given the retrogression of the
mid/upper level ridge expected, this does seem plausible. This
signal is still weak/nebulous however, with only 20-40% of LREF
members showing measurable rain both Saturday and Sunday.

Regarding heat headlines, no changes were made with this forecast
package. There remains a possibility that the St. Louis metro and
parts of southwest Illinois may hit duration (4+ days of 105F+)
criteria for an extreme heat warning. However, at this point
confidence is still not high enough that this will be widespread.
Other extenuating factors mentioned yesterday remain, including the
weaker sun intensity of early September and that people are well
acclimated to summertime temperatures. Impact data from prior heat
waves, including heat illnesses and hospitalizations, have also
shown a consistent downturn with each subsequent heat event we
have had this summer.


(Tuesday - Next Wednesday)

Clusters of the 500-hPa height pattern from the LREF show three of
the four with the anomalous mid/upper level ridge moving back
over our area for the middle of next week. Those three clusters
comprise over 90% of the total membership of the LREF. This
means that there is increasing confidence in another warming
trend, with highs in the mid to upper 90s increasingly likely.
The strength of that air mass may be quite comparable to the one
currently responsible for bringing us near-record temperatures.
Heights at 500 hPa from the NAEFS approach the 99th percentile
of climatology, with 850-hPa temperatures well above the 95th
percentile. Conceptually, another week of mostly very hot and
dry conditions may be capable of yielding even hotter high
temperatures than this week as soils dry out further and
evapotranspiration rates decrease. This could portend to deeper
mixing and lower dewpoints in the afternoon, so it may be
difficult to get 105+ heat index values, even if temperatures
are similar or a hair warmer. We will be deeper into the month
of September though, so it does become more difficult to hit
100 degrees as we lose approximately 15 minutes of daylight each
week heading toward the Autumnal equinox. In fact, St. Louis
(Columbia) has only achieved this feat 8 (9) times 9/8 or later.
This air mass though may be strong enough to do it.

Looking further out, there is still no definitive end to what should
be an incredibly long stretch of anomalously hot temperatures. This
is likely to be one of the hottest, if not the hottest, starts to
September on record at all 3 of our climate stations (KSTL, KCOU,
KUIN). Medium to long range ensemble guidance suggests some
weakening of the mid/upper level ridge as it may tend to shift
more toward the southeast. This in theory may allow a cold front
to move closer to the region in zonal flow aloft, but mean
850-hPa temperature anomalies on both the GEFS/EPS stay in the
+3 to +5C range. That points to more above normal temperatures
into the middle of September, but probably at least not quite as
anomalous.

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 155 PM CDT Wed Sep 2 2026

Dry/VFR conditions are forecast with west/southwest winds
backing this evening to the south and then veering to the
west/southwest Thursday morning.

&&

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

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

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

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...Heat Advisory until 8 PM CDT Saturday for MOZ018-019-026-027-
     034>036-041-042-047>052-059>065-072>075-084-085-099.
     Air Quality Alert until 8 PM CDT this evening for MOZ061>065.
IL...Heat Advisory until 8 PM CDT Saturday for ILZ058>060-064-065-
     069-070-074-079-095>102.
     Air Quality Alert until 8 PM CDT this evening for ILZ100>102.

&&

$$

SHORT TERM...Gosselin
LONG TERM...Gosselin
AVIATION...Gosselin
CLIMATE...Gosselin