194
FXUS63 KLSX 040833
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
333 AM CDT Fri Sep 4 2026

.KEY MESSAGES...

- A late summer heatwave will continue through Saturday before
  temperatures cool slightly Sunday into Labor Day.

- Another period of heat and near-record breaking temperatures
  is forecast Tuesday into Wednesday, but it is uncertain how
  long the heat persists thereafter.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 327 AM CDT Fri Sep 4 2026

Little change in airmass will occur today and Saturday as 850-hPa
temperatures remain steady around 21 to 23 C (~99th climatological
percentile) with high temperatures in the mid-90s F to around 100 F,
approaching daily records, and low temperatures in the 70s F, around
80 F in the immediate St. Louis metro area. Daily peak heat index
values are forecast to remain around 100 to 110 F, especially before
dewpoints lower during each mid/late afternoon as boundary layer
mixing becomes deepest. Mainly dry conditions are also favored as an
anomalous upper-level ridge remains overhead. That being said, HREF
probabilities of measurable rainfall are around 10 to 20 percent
today and Saturday suggesting a few isolated showers or
thunderstorms during peak heating (afternoon and evening). For
today, southeastern MO/southwestern IL are most favored with a
weaker capping inversion and model guidance signals for weak surface
convergence, and for Saturday, south-central/southwestern IL is most
favored as a backdoor cold front arrives. Showers/thunderstorms are
anticipated to be localized and brief enough to have little impact
on temperatures.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Issued at 327 AM CDT Fri Sep 4 2026

The upper-level ridge will retrograde westward late Saturday,
accompanied by backdoor cold frontal passage through the CWA
Saturday evening into night as a surface high pressure center builds
across the Great Lakes. Behind the front, northeasterly, low-level
CAA will be weak but persistent from Sunday into Monday. Although
850-hPa temperatures will cool in response, they are never really
projected to fall below the 90th percentile in most ensemble model
guidance. Therefore, high temperatures will be cooler, but still
around 90 to the mid-90s F (10 to 15 F above average). The heat
index forecast for Sunday is much more nuanced than preceding days
with a corridor of higher pooled dewpoints giving way to a much
drier airmass filtering southwestward into the CWA through the day.
Sunday`s ensemble model probabilities of 100+ F heat index values
are highest (40 to 60 percent) in central and southeastern MO where
CAA will be weakest and dry air arrival latest, but the
aforementioned nuances preclude extension/issuance of another heat
headline for Sunday at this time. A more widespread, confident
respite from the heat will be on Monday (Labor Day) with 80 to near
100 percent probabilities of BELOW 100 F heat index values as
dewpoints lower into the upper 50s to mid-60s F.

Tuesday through mid-next week, the upper-level ridge will migrate
back eastward over the Mid-Mississippi River Valley, further
amplifying to near record-breaking 500-hPa heights. This evolution
will establish a similar pattern to the once currently in place with
another round of heat Tuesday and Wednesday. In this pattern, high
temperatures would again be in the mid-90s to around 100 F, but the
arrival or approach of a cold front on Wednesday decreases
confidence in exact values, especially in northeastern MO/west-
central IL.

On Wednesday through Thursday, global model guidance is beginning to
better align on upper-level trough propagating from the Northern
Plains through the Great Lakes, which forces the aforementioned
front into the Mid-Mississippi River Valley late Wednesday and
flattens the ridge. Subsequently, it is unclear if the front fully
passes through the CWA before it wavers with upper-level flow at
least briefly becoming quasi-zonal or west-northwesterly as the
ridge potentially retrogrades westward once again. The position of
this front will be crucial in determining temperatures, with heat
favored to persist to the south/southwest and cooler temperatures,
periodic showers/thunderstorms and clouds to the north/
northeast. Unsurprisingly, NBM interquartile temperature ranges
span 10 F by that time, albeit still entirely above average for
mid-September.

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1147 PM CDT Thu Sep 3 2026

Dry and VFR flight conditions will persist through the TAF period
with light winds, mainly southwesterly during the day Friday.

&&

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

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

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...Pfahler
LONG TERM...Pfahler
AVIATION...Pfahler
CLIMATE...Pfahler