913 
FXUS63 KLSX 071122
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
622 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 /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 622 AM CDT Mon Sep 7 2026

Dry and VFR flight conditions will continue through the TAF period 
along with light easterly winds becoming more southeasterly by 
Tuesday morning.

&&

.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...Pfahler
CLIMATE...Pfahler