587
FXUS63 KLSX 150019
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
719 PM CDT Mon Sep 14 2026

.KEY MESSAGES...

- Unseasonably hot and humid weather is expected this week with heat
  index values reaching 105F+ on Tuesday mainly south of I-70.

- Scattered showers and thunderstorms are forecast on Tuesday. A
  few may be strong to marginally severe with gusty winds the
  main threat.

&&

.SHORT TERM /THROUGH WEDNESDAY/...
Issued at 228 PM CDT Mon Sep 14 2026

A surface warm front is marching northeastward, and is nearing
central/southeast Missouri as of 1800 UTC. This boundary should
clear the CWA overnight tonight into Tuesday morning from west
to east. Warmer and more humid air moves in behind this front,
with temperatures currently in the 90s in southwest Missouri
with dewpoints in the upper 60s. Widespread low 70s for
dewpoints are not too far behind into southeast Kansas and
northeast Oklahoma. That is the air mass heading our way on
Tuesday. In other words, pure summer and nothing that is
remotely typical for the middle of September is imminent
beginning Tuesday. This incoming air mass remains quite
extraordinary in many ways, including: 850-hPa temperatures
above +20C (>99th percentile of climatology), record-breaking
500-hPa heights/ridge, and near-record dewpoints. High
temperatures may also threaten daily records at St. Louis (KSTL)
and Columbia (KCOU) Tuesday afternoon as readings top out in
the mid to upper 90s southeast of a slow-moving cold front.
Please refer to the CLIMATE section below for more details on
daily high temperature records Tuesday through Saturday. These
temperatures combined with aforementioned dewpoints in the low
70s means widespread 100-105F+ degree peak heat index values. A
heat advisory was issued in areas where widespread magnitude
(105F+) criteria is likely to be reached on Tuesday, which is
for areas mainly south of the Missouri River and south of I-70
in Illinois. NOTE: This is the latest heat advisory from our
office dating back to at least 2004, and likely quite a bit
longer. As was discussed yesterday, it is incredibly rare for
105F+ heat index values this late in the year. For St. Louis,
(since 1930) this would only be the third day with a peak heat
index at least of 105 degrees on or after 9/15 (105F on
9/17/1931 and 106F on 9/19/1972).

By mid morning on Tuesday, the cold front is expected to enter
portions of northeast Missouri and west-central Illinois. Scattered
showers and a few thunderstorms are forecast along/behind this
boundary as it slowly progresses to the southeast with time. A
weakening low-level jet and associated moisture convergence suggest
diminishing coverage and overall organization late morning toward
midday. The most likely scenario is for an outflow boundary to push
southeastward toward the I-44/I-70 corridors in Missouri and
Illinois respectively by early-mid afternoon. This boundary in most
CAMs serves the focus for scattered showers and thunderstorms
through the evening hours. The higher shear lags behind the cold
front, so mainly pulse convection with a low threat for damaging
winds (via downbursts most likely) is expected. The joint
probability for at least 1500 J/kg of SBCAPE and >25 knots of deep-
layer shear only peaks out 35-45% on the HREF, and that is right
near the front itself. Lower probabilities exist to the southeast
where there seems to be a stronger signal for convective initiation.

The coverage of showers and thunderstorms should increase Tuesday
night as moisture convergence strengthens on the nose of a ~25 knot
southwesterly low-level jet. The best chances (60-80%) of showers
and thunderstorms will be along/poleward of the effective front. At
this time, that boundary is likely to be somewhere across southeast
Missouri into southwest Illinois, so the highest PoPs are located
along/north of I-70. These rain chances will continue at least
through Wednesday morning, and really lessens the confidence in
specific temperature and heat index values. The low-level jet also
does not weaken much into the early afternoon, so scattered showers
and storms may continue. If this occurs, temperatures and heat index
values may be at least several degrees too high in the current
forecast. There is a pretty good chance that Wednesday is actually
the "coolest" day through Saturday. Even lingering mid/high level
clouds would probably be enough to keep heat index values below 100
degrees. That is the reason the heat advisory only includes Tuesday
at this time.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Issued at 228 PM CDT Mon Sep 14 2026

(Wednesday Night - Saturday Night)

The main source of uncertainty is how the quasi-stationary front
evolves through the end of the week. Deterministic guidance has
shifted slightly toward an overall placement more poleward than
yesterday, with the front shifting northward Wednesday night into
Thursday. I am a bit skeptical of this scenario given a lack of
leeside cyclogenesis to the west and the possibility of convection
helping push outflow farther south. That being said, there are
rising mid/upper level height rises, so there may be some merit to a
shift back northward. The quasi-stationary front should remain near
northeast Missouri and west-central Illinois through Thursday night.
There is a better signal for it to push northward (and north of our
area entirely) Friday into Friday night as a surface low moves
northeast across the north-central Plains. South of the boundary,
near-record heat and humidity is expected to continue with highs
well into the 90s forecast through Saturday. The most uncertainty
remains across northeast Missouri and west-central Illinois
regarding temperatures due to their expected close proximity to the
boundary and higher chances of showers and thunderstorms (as well as
cloud debris). This uncertainty is illustrated well looking at the
inter-quartile range (IQR) differences for high temperatures
Thursday-Saturday from the NBM. At KUIN, the IQR spread is 5-8+
degrees while the spread is only about 2-4F at KFAM well south of
the boundary.


(Sunday - Next Monday)

There may be light at the end of the proverbial tunnel looking ahead
to early next week. While there is not surprisingly a lot of spread
in the specifics, all four clusters from the LREF of the 500-hPa
height pattern show significant weakening of the mid/upper level
ridge. Flow aloft across the mid-Mississippi Valley transitions to
more zonal, with an increasing signal for a cold frontal passage
sometime Sunday/Sunday night. The specifics with the air mass behind
this boundary are not yet known, but it does appear likely that high
and low temperatures should get at least much closer to normal.
After ~4 straight hot and humid days for some locations, near normal
would be quite the change. While there is still a lot of spread,
even the 75th percentile from the NBM for high temperatures next
Monday are in the 80s so it should be at least 10 degrees cooler. If
the cooler EPS solution verifies, widespread 70s for highs would be
possible (closer to the 25th percentile of the NBM).

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 705 PM CDT Mon Sep 14 2026

VFR conditions are expected to continue through at least tonight. A
southwesterly low level jet overnight will cause some LLWS at UIN.
There is a northward moving warm front that in combination with an
outflow boundary that will cause winds to veer to the southwest
tonight through Tuesday. The outflow boundary will settle near STL,
SUS, and CPS providing the highest chance of showers and
thunderstorms near those terminals during the afternoon and evening.
There remains some uncertainty in the exact timing and coverage of
these showers and thunderstorms among model guidance, but
development of some kind is currently expected and possibly
continuing beyond the TAF period too. The strongest convection will
be capable of gusty winds as well of period of MVFR or IFR flight
conditions.

&&

.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 from 11 AM to 7 PM CDT Tuesday for MOZ047>049-
     059>065-072>075-084-085-099.
IL...Heat Advisory from 11 AM to 7 PM CDT Tuesday for ILZ064-069-
     070-074-079-100>102.

&&

$$

SHORT TERM...Gosselin
LONG TERM...Gosselin
AVIATION...Asel/Pfahler
CLIMATE...Gosselin