272
FXUS63 KLSX 070809
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
309 AM CDT Mon Sep 7 2026

.KEY MESSAGES...

- After cooler and less humid conditions today, heat will
  briefly return Tuesday into Wednesday, but heat index are
  expected to be overall lower than with the previous heatwave.

- The next opportunity for showers and thunderstorms will come
  with a cold front Wednesday into early Thursday.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 307 AM CDT Mon Sep 7 2026

Low-level northeasterly to easterly flow will facilitate further
infiltration of a relatively cooler and drier airmass across the
entire CWA today. As a result, high temperatures will be in the mid-
80s to around 90 F with dewpoints lowering into the mid-50s to
around 60 F by this afternoon. Low temperatures tonight will largely
cool to between 60 and the mid-60s F, but sheltered and river valley
locations across southeastern/east-central MO and southwestern IL
could reach the upper 50s F with light/calm winds near an inverted
surface pressure ridge.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Issued at 307 AM CDT Mon Sep 7 2026

By Tuesday, the Omega Block in place across the CONUS will begin to
break down with an upper-level ridge over the Central Plains
shifting back eastward as flow becomes more progressive, inducing
low-level south/southwesterly WAA with 850-hPa temperatures
returning to between 21 and 23 C (95-99th climatological
percentile). This process will result in high temperatures in the
low to mid-90s F, but humidity/dewpoints will be much slower to
increase and keep heat index values closer to air temperatures.

A potent upper-level trough tracking across southern Canada will
drive a deepening surface cyclone toward Hudson Bay on Wednesday
with a trailing cold front reaching the Mid-Mississippi River
Valley. The timing of this front in model guidance continues to
trend ever-so-slightly earlier, bringing Wednesday`s temperatures
more into question. Showers and thunderstorms are also expected to
be ongoing along and moreso behind the front with the potential for
some clouds to spread into the warm sector. Since the general
consensus is for the front to reach the I-70 corridor by early
afternoon, the hottest temperatures mainly in the mid to upper 90s F
are forecast along and south. This trend is less supportive of
record-breaking high temperatures around 100 F, but increasing and
pooled dewpoints near the front still favors afternoon heat index
values of 100 to 105 F with the potential for a Heat Advisory (105+
F) needing to be weighed going forward.

Showers and thunderstorms will accompany the cold front as it slowly
advances southward through the CWA Wednesday into early Thursday,
supported by 80 to 90+ percent probabilities of measurable rainfall
in ensemble model guidance that are focused on Wednesday afternoon
into Thursday morning. Probabilities of 0.5" and 1" of rainfall also
continue to increase near the I-70 and I-44 corridors, reaching
around 60 and 20 percent, respectively. Deep-layer wind shear is
indicated to be 25 to 35 kt accompanied by 60 to 80 percent
probabilities of 1000+ J/kg MUCAPE, suggesting there is at least a
low threat of a few strong to severe thunderstorms Wednesday
afternoon and evening. However, a capping inversion across the warm
sector and strongest forcing and wind shear lagging behind the front
decreases confidence in thunderstorms being favorably surface based.
Showers and clouds will gradually end from north to south on
Thursday morning into midday with a cooler airmass promoting high
temperatures in the low to mid-80s F with upper 70s F not out of the
question across portions of southeastern MO/southwestern IL if
precipitation and thicker cloud cover can linger there into
afternoon.

Model guidance continues to disagree on the position and movement of
an amplifying upper-level ridge across the CONUS into the weekend,
split largely into two camps highlighted by cluster analyses: one
where the ridge migrates eastward over the Mid-Mississippi River
Valley with hot, well-above average temperatures returning; and one
where the ridge remains positioned to the west or southwest with
temperatures remaining slightly above average. There is at least
some potential for showers and thunderstorms in either solution as
one or more shortwave troughs navigate the ridge in the former
solution or northwesterly flow in the latter. However, ensemble
model members, although widely varying, have any periods of
precipitation being transient with most of the period being dry
rather than wet.

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1040 PM CDT Sun Sep 6 2026

VFR flight conditions and light easterly winds are expected to
continue to prevail through the end of the period. There`s still
a small chance (<30%) that valley fog will form over parts of
the eastern Ozarks. If fog does develop, it could locally reduce
the visibility to 1SM or less.

&&

.CLIMATE...
Issued at 258 AM CDT Sun Sep 6 2026

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

September 8:
KSTL: 101/1947...77/1939
KCOU: 103/1947...75/1922
KUIN: 102/1947...79/1922

September 9:
KSTL: 101/1925...79/1925
KCOU:  99/1925...76/1947
KUIN: 100/2013...75/1912

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.

&&

$$

SHORT TERM...Pfahler
LONG TERM...Pfahler
AVIATION...Carney
CLIMATE...Pfahler