390 
FXUS63 KLSX 151941
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
241 PM CDT Tue Sep 15 2026

.KEY MESSAGES...

- A late season heat wave with peak heat index values of
  100-105F+ will continue through at least Friday along/south 
  of I-70.

- Widespread showers and thunderstorms are forecast tonight,
  mainly across northeast Missouri and west-central Illinois
  where locally heavy rainfall is possible. A few may also be
  strong to severe with damaging winds the main threat through
  this evening.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 240 PM CDT Tue Sep 15 2026

Regional radar imagery shows an outflow boundary from morning 
convection oriented from southwest to northeast across central 
Missouri into west-central Illinois. This boundary is moving 
southeast, and extrapolation would put it near a Jefferson City 
to Carlinville line around 2100 UTC. This area may be where 
storms initiate later this afternoon, though CAMs also suggest 
additional development within the broad warm sector to the 
southeast too. Given building cumulus on GOES-19 visible 
satellite imagery, this has merit and have isolated thunderstorm
wording to the southeast, with scattered thunderstorms more 
likely near the aforementioned outflow boundary. Additional 
storms are also forecast to develop late this afternoon into 
early this evening along/behind the synoptic cold front. This 
boundary is currently located near a KMCI>>KIRK line and is also
slowly moving to the southeast. There is better shear near the 
actual cold front (30+ knots), and this is where the risk for 
severe weather is primarily located this evening (northeast 
Missouri into west-central Illinois). Damaging winds gusts are 
the primary threat in multicellular clusters/mini bowing 
segments, though marginally severe hail may be possible in any 
transient supercells. Further to the southeast this 
afternoon/early evening, much weaker effective shear (<20 knots)
strongly suggests pulse convection. Most storms will long last 
long as rain-cooled downdrafts quickly extinguish parent 
updrafts. However, the latest SPC mesoanalysis does show steep 
low-level lapse rates and high DCAPE values, so a microburst or 
two with gusts as high as 60 mph are possible.

The coverage of showers and thunderstorms should increase late this 
evening into the early overnight hours as the low-level jet 
strengthens. Widespread showers and thunderstorms are forecast 
across northeast Missouri and west-central Illinois north of the 
frontal boundary through early to mid morning on Wednesday. While 
the severe threat should diminish by late this evening, there is a 
threat for locally heavy rainfall given anomalous precipitable water 
values (~1.9", >95th percentile of climatology), deep warm cloud 
depths, and the possibility of training. The signal for the heaviest 
rainfall (2-4+") is across northwest Missouri, which is also where 
the strongest low-level moisture convergence is forecast overnight. 
However, some streaks of 2-3+" may occur in our area given the 
chance of training in a favorable environment for efficient rainfall 
processes.

&&

.LONG TERM /WEDNESDAY THROUGH TUESDAY/...
Issued at 240 PM CDT Tue Sep 15 2026

(Wednesday - Monday)

There is still some uncertainty with the exact location of the front 
on Wednesday morning, but the most likely location is near the I-
44/I-55 corridors in Missouri and Illinois respectively. Widespread 
showers and thunderstorms should continue to percolate during the 
morning hours along/poleward of this boundary. A gradual decrease in 
coverage is expected as the low-level jet and associated moisture 
convergence slowly wanes with time. There will be a pretty sharp 
temperature difference across the front Wednesday afternoon. North 
of the boundary across parts of northeast Missouri and west-central 
Illinois, low clouds and lingering rain will really hamper 
temperatures. Highs may not get much higher than the mid 70s to near 
80 degrees. To the southeast on the warm side of the front, 
widespread mid 90s are forecast though this may be a bit too warm if 
outflow and/or clouds linger. Regardless, dewpoints pooling near the 
front should result in 100F+ heat index values within the advisory 
area on Wednesday so the heat advisory was extended in time all the 
way through Friday. The chances for outflow or thicker clouds are 
less across that area later this week as the front moves back to the 
north as a warm front. There is now pretty high confidence that near-
record highs will continue at least through Saturday. Once the 
boundary does retreat northward, dewpoints should come down each day 
Thursday through Saturday. In other words, a bit of a transition to 
a "drier" heat. Heat index values on Saturday may not even be much, 
if at all, above the ambient air temperature. That is the reason the 
heat advisory was not extended all the way through Saturday. If 
dewpoints are a bit higher than forecast, more widespread ~100F heat 
index values would occur so it is possible that another day could be 
required. 

In the more bad news if you don't like heat department, there has 
been a pretty substantial change for late this weekend into early 
next week. The mid/upper level trough on both the GEFS/EPS has 
trended more cutoff and slower as it moves across the northern 
Plains/Upper Midwest. That means a shift toward a slower cold 
frontal passage. Sunday looks most likely to be a continuation of 
this very late season heat wave with another day topping out in the 
90s for most locations. This more prolonged period of anomalous 
warmth should equate to one of the warmest first 20 days of 
September at all 3 of our climate stations, and probably THE warmest 
at KSTL. Monday is the day with the most uncertainty now, and high 
temperatures will depend on the exact timing of the front. If the 
front is delayed further, you can likely book another day of 
anomalous warmth with highs in the 90s. If the front does make it 
through, highs closer to normal (70s/80s) would be expected. 


(Monday Night - Next Tuesday)

There is still a strong signal that the cold front will finally make 
it through by Tuesday, though the closed mid/upper level low and 
overall "blockiness" of the pattern does shred at least some minor 
doubts. There still remains differences on the strength of the air 
mass behind the front early/mid next week, with the EPS still quite 
a bit cooler than the GEFS. A safe bet is for temperatures to 
finally get back closer to normal, though whether it ends up a bit 
below or slightly above normal is still uncertain.

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1211 PM CDT Tue Sep 15 2026

A cold front will slowly move southeast this afternoon/tonight
and stall out across the area, likely near the St. Louis metro.
This front and points northward is where the best chances of
showers and thunderstorms are tonight into Wednesday morning.
KUIN should be most heavily impacted, with more persistent
showers and on/off thunderstorms. Also behind the front, look
for low MVFR ceilings. The best chances for these low ceilings
are also at KUIN, followed by KCOU and then KJEF. The metro
sites may tend to stay just far enough southeast. However, I did
add a SCT015 group to account for this possibility.

&&

.CLIMATE...
Issued at 231 PM CDT Mon Sep 14 2026

Daily record high temperatures may be threatened for both St. Louis 
(KSTL) and Columbia (KCOU) this week. NOTE: Records at Quincy (KUIN) 
are not shown below due to cooler temperatures forecast:

          9/15        9/16        9/17        9/18        9/19
KSTL   101/1939     98/1954     97/1931     97/1925     96/1955     
KCOU   101/1893     99/1954     98/1895     98/1893     98/2022

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT Friday for MOZ047>049-059>065-
     072>075-084-085-099.
IL...Heat Advisory until 8 PM CDT Friday for ILZ064-069-070-074-079-
     100>102.

&&

$$

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