780 
FXUS63 KLSX 080931
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
431 AM CDT Tue Sep 8 2026

.KEY MESSAGES...

- Heat returns today, with afternoon heat index values around
  105 F in the Heat Advisory for central MO. Humidity will 
  increase on Wednesday, but a cold front may limit how hot 
  temperatures become and if heat index values are impactful.

- Showers and thunderstorms are forecast Wednesday into
  Thursday, most widespread Wednesday evening into Thursday
  morning. Gusty winds, small hail, and locally heavy rainfall
  are possible.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 327 AM CDT Tue Sep 8 2026

An anomalous upper-level ridge, featuring near record-breaking 500-
hPa heights, will become positioned across the Mid-Mississippi River 
Valley today. As a result, low-level flow will become increasingly 
southerly with 850-hPa temperatures reaching 21 to 22 C (~95th 
climatological percentile), supporting high temperatures mainly in 
the low to mid-90s F with considerable insolation. Humidity/ 
dewpoints will be slow to increase for much of the CWA, remaining in 
the upper 50s and 60s F and keeping heat index values close to air 
temperatures. The exception will be across central MO, where a 
corridor of higher dewpoints nearing 70 F and high temperatures 
moreso in the mid/upper 90s F is forecast to yield several hours of 
heat index values around 105 F this afternoon. Therefore, a Heat 
Advisory has been issued for that area.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 327 AM CDT Tue Sep 8 2026

On Wednesday, a cold front trailing a deepening surface cyclone near 
Hudson Bay will reach the Mid-Mississippi River Valley. Model 
guidance still varies on how quickly this front advances southward, 
but there is a growing consensus that it will reach along or just 
south of the I-70 corridor around midday. Showers and thunderstorms 
are anticipated to be ongoing mainly behind the front Wednesday 
morning before a relative minimum in coverage around midday into 
afternoon during a lapse in stronger large-scale forcing/ascent. 
South of the front, 850-hPa temperatures will remain anomalously 
warm, but the position of the front and extent of cloud cover 
decrease confidence in high temperatures, which could reasonably 
range from the low to upper 90s F, per the NBM interquartile range. 
Confidence is high that dewpoints will increase on Wednesday with 
pooled moisture in the vicinity of the front, but it is still 
uncertain at this point if 105+ F heat index values materialize. 
Although still possibly needed, a Heat Advisory has not been issued 
for this reason.

Showers and thunderstorms will increase in coverage again sometime 
Wednesday evening, eventually becoming widespread, as large-scale 
ascent increases with falling mid-level heights ahead of an 
approaching upper-level trough and arrival of an equatorward jet 
entrance region. With a capping inversion across the warm sector and 
strongest forcing, wind shear, and instability behind the front; it 
is uncertain if any severe, surface-based thunderstorms with 
damaging winds will develop. Instead, the most organized and 
strongest thunderstorms are favored to exist behind the front which 
may limit threats to gusty winds and small hail. Anomalous PW 
exceeding 2", deep warm cloud depths, and deep-layer flow 
paralleling the slow-moving front also supports heavier rainfall 
where thunderstorms with efficient, high rainfall rates train. 
Through Thursday, the NBM and global ensemble models indicate that 
most locations see 0.25 to 1" of rainfall, with a swath of higher 
amounts of 1 to 3" somewhere near the I-44 and I-70 corridors. Since 
antecedent soil conditions are very dry, 3-hour FFG is high and generally
3 to 4", supporting heavier rainfall being more beneficial than
producing flash flooding.

Showers and a few thunderstorms could linger well into the day 
Thursday until an upper-level trough axis arrives and shunts the 
front and precipitation more quickly southward. The highest 
probabilities of rainfall in ensemble model guidance are along and 
south of I-44/I-70 during afternoon, which is where high 
temperatures may struggle to reach above 80 F with post-frontal CAA 
and clouds as well. To the north, decreasing clouds will promote 
slightly warmer, but still much cooler high temperatures in the low-
80s F.

After Thursday, global model guidance is coming into better 
agreement on the upper-level wave pattern becoming quasi-zonal 
through the weekend and early next week with a front arriving and 
wavering across the broader Mid-Mississippi River Valley. This 
pattern results in an extended period of at least low PoPs that 
begins Friday evening. Ensemble model membership with measurable 
rainfall is highest in the Friday night-into-Saturday night 
timeframe, but these members also show most of this period into 
early next week being dry vs. wet. With this front and waves of 
clouds/precipitation, temperatures are not completely certain, but 
the NBM interquartile range has been over the last 24 to 48 hours 
closing in on above-average high temperatures mainly in the low-80s 
to low-90s F.

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 431 AM CDT Tue Sep 8 2026

Dry and VFR flight conditions are favored to continue through 
tonight at all terminals with light southeasterly winds veering to 
southerly today, backing some again this evening. A cold front begin 
moving southward into the region Wednesday morning, accompanied by 
winds veering to northwesterly and chances of showers and 
thunderstorms. Although KUIN will be impacted first, these impacts 
lie just beyond the TAF period.

&&

.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...Air Quality Alert from 10 AM this morning to 8 PM CDT this 
     evening for MOZ061>065.
     Heat Advisory from noon today to 8 PM CDT this evening for 
     MOZ041-047-048.
IL...Air Quality Alert from 10 AM this morning to 8 PM CDT this 
     evening for ILZ100>102.

&&

$$

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