Chesterfield, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

981
FXUS63 KLSX 122331
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
631 PM CDT Sat Sep 12 2026

.KEY MESSAGES...

- Above normal temperatures will continue for the next several
  days with the hottest conditions forecast on Tuesday and
  Wednesday.

- Chances (30-60%) of showers and thunderstorms each day will be
  mainly confined to parts of northeast Missouri and west-
  central Illinois near a stalled front.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 219 PM CDT Sat Sep 12 2026

A weak cold front continues to move southeast, now located near
a KVIH>>K3LF line as of 1900 UTC. There has not been much, if
any, convective activity along this front due to strong capping.
This boundary will clear the southeastern CWA close to
midnight, with light northwest winds in its wake. Low-level
moisture convergence increases after midnight, mainly across
eastern Kansas. Widespread showers and thunderstorms are
expected on the nose of this jet, with activity moving
east/southeast with time. That means that most, if not all, of
these showers/storms should stay to our west/southwest. However,
parts of central Missouri may be on the far northeastern
periphery and this is where low (20-30+%) chances of showers and
perhaps a few thunderstorms exists late tonight into early
Sunday morning.

Forecast uncertainty increases heading into Sunday afternoon, with
deterministic models (including CAMs) showing a wide range of
solutions. There does look to be two separate areas of somewhat more
focused forcing: 1) northeast Missouri/west central Illinois where
weak low-level moisture convergence and low/mid level frontogenesis
resides and 2) far southern Missouri near the surface front and
downstream of a midlevel shortwave. Nothing looks too widespread (or
significant) in either zone, but this is where the highest (30-50%)
PoPs reside. Activity should wane Sunday evening with loss of
daytime heating, before the low-level jet kicks up again near the
Missouri-Kansas border. Similar to tonight, much (if not all) of
this activity Sunday night should stay to our west but may clip far
western and/or northern section of the CWA.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Issued at 219 PM CDT Sat Sep 12 2026

(Monday - Thursday Night)

The quasi-stationary front is expected to quickly lift north of the
area on Monday as cyclogenesis commences across the central Plains.
Very anomalous mid/upper level ridging is forecast to amplify across
the mid south early/mid week. In fact, 500-hPa heights of 594+ dm
would be record-breaking for this time of year. In addition, 850-hPa
temperatures climb back above +20C (>99th percentile). The latest
EPS Extreme Forecast Index (EFI) is also picking up on this heat,
showing EFI values of 0.8-0.9 for high temperatures Tuesday and
Wednesday, with even a slight Shift of Tails (SOT). This air mass
does seem capable of threatening record highs (upper 90s to around
100 degrees) Tuesday and Wednesday, though it will depend on cloud
cover and rain chances. Speaking of which, a cold front is likely to
stall out near or just north of our CWA on Tuesday. This front may
serve as the focus for showers and thunderstorms each day, with any
widespread activity creating debris clouds which ultimately could
impact temperatures over much of the area. Therefore, widespread mid
90s are not a given, though confidence does increase the farther
south you head within the region.

A likely scenario may be for each successive round of convection to
help help augment the boundary and help ooze it southward. This
points to a gradual "cool" down heading toward late week into next
weekend. Temperatures each day get more uncertain due to the high
variability in the location of the front, and the fact that clouds
and showers/storms near the boundary may enhance the temperature
gradient from north to south.

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 630 PM CDT Sat Sep 12 2026

VFR conditions are expected to prevail through the TAF period
with calm to light winds out of the northwest. There is a chance
for showers and thunderstorms Sunday afternoon for central MO,
but there is uncertainty if the convection will extend east
enough to impact COU and JEF terminals. As a result rain was
left out of their TAFs, but we will monitor for any changes.

&&

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

&&

$$

SHORT TERM...Gosselin
LONG TERM...Gosselin
AVIATION...Asel/Kimble