258
FXUS63 KLSX 051002
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
502 AM CDT Sat Sep 5 2026

.KEY MESSAGES...

- The late summer heatwave will continue today followed by a
  break in the heat with relatively cooler temperatures Sunday
  and especially Monday.

- Aside from a few isolated showers and thunderstorms today,
  predominantly dry conditions will persist through Tuesday

- Another period of heat is expected Tuesday into Wednesday,
  but indications are that this period will be much shorter.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 231 AM CDT Sat Sep 5 2026

The upper-level ridge that has been in place over the Mid-
Mississippi River Valley for the last week is finally retrograding
westward in response to troughing across the eastern CONUS. This
process will facilitate southwestward advancement of a backdoor cold
front through the CWA between late this afternoon and overnight
tonight. Ahead/along of this front, 850-hPa temperatures will remain
around 21 to 23 C (95-99th climatological percentile) favoring
similar high temperatures to the last several days in the mid-90s to
around 100 F. Due to moisture pooling, dewpoints today are also
expected to be slightly higher and may "mix out" less than previous
days with higher peak heat index values of around 105 to 110 F.
Weak, broad mid-level height falls associated with the retrograding
ridge combined with afternoon/evening heating and instability will
provide a better opportunity for isolated showers and thunderstorms
today, mainly across southeastern MO with some orographic assistance
and southwestern IL with the backdoor cold front. A stray microburst
with gusty winds cannot be ruled out given inverted-V forecast
sounding signatures, high DCAPE, and 1500 to 2500 J/kg MLCAPE, but
modest wind shear will significantly limit the intensity and
longevity of thunderstorms. Any showers and thunderstorms will wane
with the loss of daytime heating.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Issued at 231 AM CDT Sat Sep 5 2026

Model guidance is aligning on a quicker passage of the backdoor cold
front, fully occurring by Sunday. The highest heat index values on
Sunday in the CWA will be across central and southeastern MO where
higher dewpoints lag behind the front, but probabilities of 100+ F
heat index have been decreasing with each successive run in global
ensemble models. This factor decreases confidence in issuance of
additional heat headlines at this time, if at all. The rest of the
CWA Sunday and CWA-wide Monday (Labor Day) will experience a break
from the heat as low-level CAA and infiltrating dry air result in
high temperatures in the mid-80s to low-90s F and dewpoints lowering
into the 60s F, even 50s F along/east of the Mississippi River on
Monday afternoon. Although these temperatures will be cooler
comparatively they are still above average for early/mid-September.
A high pressure center will also extend into the area, enforcing dry
conditions aside from a stray shower or thunderstorm clipping
northeastern/central MO on Sunday afternoon/evening.

Tuesday through Wednesday, the upper-level ridge will shift eastward
again with the high pressure center quickly retreating and low-level
southwesterly flow/WAA raising 850-hPa temperatures back to similar
values that we are currently seeing today. This airmass has the
potential to again yield high temperatures in the mid-90s to around
100 F, but the longevity of these temperatures is becoming more in
question as the growing consensus in model guidance is for a cold
front to be advancing southward into the CWA on Wednesday.
Moisture/dewpoints will be slower to increase on Tuesday, keeping
heat index values from rising much above air temperatures. As for
Wednesday, dewpoints should be higher with moisture pooling near the
front, but actual temperatures are more uncertain with the incoming
front and potential for associated clouds and showers/thunderstorms.
Therefore, it is not yet clear if additional heat headlines will be
needed.

Late Wednesday through the remainder of the next work week, a series
of upper-level troughs will act to flatten the ridge with flow over
the Mid-Mississippi River Valley becoming quasi-zonal. Wednesday`s
cold front is likely to subsequently waver across the region or make
only gradual southward progression in this pattern. This front will
act to separate the unseasonable heat to the south and a more
seasonable airmass to the north with waves of showers and
thunderstorms tracking along the front as probabilities of
measurable rainfall vary between 30 and 60 percent Wednesday onward
in ensemble model guidance. However, the many variables impacting
temperatures during this period inflates the NBM interquartile range
to around 10 F, but the distribution is largely skewed to the 80s to
around 90 F at least Thursday and Friday.

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 501 AM CDT Sat Sep 5 2026

Dry and VFR flight conditions are expected to continue through the
TAF period. However, there is a low (20 percent) chance of isolated
showers and thunderstorms this afternoon and evening mainly in
southeastern MO/southwestern IL. As such, confidence is low that any
TAF sites will be impacted. A weak cold front will also advance
southwestward through the area this evening veering northwesterly
winds to northeasterly or causing them to become variable.

&&

.CLIMATE...
Issued at 231 AM CDT Sat Sep 5 2026

Record Highs...Record Max Lows (value/year):

September 5
KSTL: 103/1954...80/1899
KCOU: 103/1899...75/1983
KUIN: 100/1925...76/1912

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...Extreme Heat Warning until 8 PM CDT this evening for MOZ018-
     019-026-027-034>036-041-042-047>052-059>065-072>075-084-
     085-099.
IL...Extreme Heat Warning until 8 PM CDT this evening for
     ILZ058>060-064-065-069-070-074-079-095>102.

&&

$$

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