465
FXUS63 KLSX 291046
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
546 AM CDT Sat Aug 29 2026

.KEY MESSAGES...

- Abnormal heat is expected through the upcoming week, and there
  is a 30% chance for highs reaching 100 degrees through the
  week.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 305 AM CDT Sat Aug 29 2026

Water vapor imagery shows upper-level ridging over the Great Plains,
with a shortwave and associated convection moving into Minnesota
and Iowa. Surface observations reveal that the area of high pressure
at the surface responsible for our seasonable weather the last
couple of days has moved well eastward of the CWA, with low-level
flow becoming increasingly southerly.

Through the day today, the ridge will build into the Midwest,
pairing with the low-level southerly flow to bring warm air into the
region. The aforementioned shortwave is expected to pass to the
northeast of the CWA, and while the associated convection will not
impact the CWA, cloud cover will. This cloud cover will be greatest
over our Illinois counties. Given this cloud cover and warm air
advection ramping up west to east, portions of central Missouri will
see afternoon temperatures reach the low to mid 90s, while on the
opposite side of the CWA across south-central Illinois, temperatures
in the mid to upper 80s can be expected.

Tomorrow, the upper-level ridge builds further into the Midwest; its
axis being oriented roughly along the Mississippi River. This will
allow the low-level thermal ridge to nose into the Middle
Mississippi Valley, with 850 mb temperatures toping out in the low
20s degrees C. Mostly clear to partly sunny skies will allow for
deep mixing, favoring afternoon temperatures in the mid to upper 90s
for most locations. Despite these air temperatures, heat index
values for most locations will stay beneath 100 degrees thanks to
dew points in the low to mid 60s.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 305 AM CDT Sat Aug 29 2026

Confidence remains high that upper-level ridging will drive our
local weather through the upcoming work week. Guidance consensus
remains steady in that the axis of this ridge will be roughly
centered along the Mississippi Valley, keeping an area of high
pressure at the surface centered over the Southeast. The positioning
of both of these features will keep deep south-southwesterly flow
over the Middle Mississippi Valley from Monday through at least
Friday. During that time, 850mb temperatures will hover in the low
to mid 20s degrees C, climatologically favoring surface temperatures
in the mid to upper 90s degrees F. Confidence in this heat is high
given the aforementioned continued consensus among guidance and that
surface flow will at times be southwesterly. This flow will allow
for adiabatic warming off of the Ozark Plateau, providing a boost to
temperatures and a drop in dew points particularly for locations
across central and southeastern Missouri. Highs reaching 100 degrees
can`t be ruled out, with the NBM continuing to have roughly a 30%
chance of this happening - particularly within the St. Louis metro
area. Highs of this magnitude will come close to records, with
overnight lows having even a greater potential for breaking max low
records. Please see the Climate section for more details.

Even with this degree of heat, confidence remains low in the need
for heat headlines. The most likely scenario is that at least
portions of the area see a Heat Advisory thanks to 4 or more days of
100+ degree heat index values. While there are many locations that
currently reach those criteria in our current forecast, this hinges
on dew points not trending any lower than they currently are. The
NBM and many guidance sources typically do not handle local
temperatures and dew points well on days with southwesterly surface
flow in place. Given this and that bias-corrected guidance is being
influenced by the higher humidity we`ve experienced the last couple
of months, it may not be until we experience a day or two of this
pattern before we are able to confidently say how humid the air
mass will be the rest of the week.

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 543 AM CDT Sat Aug 29 2026

Steam fog will continue to be a threat at river valley sites
KJEF, KSUS, and KCPS only through very early in the period. Once
the fog dissipates, dry and VFR flight conditions are expected
through the remainder of the period among southerly winds.

&&

.CLIMATE...
Issued at 305 AM CDT Sat Aug 29 2026

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

August 31
KSTL: 103/2013...79/2013
KCOU: 103/1913...78/1947
KUIN: 100/2011...76/1924

September 1
KSTL: 104/2011...82/2011
KCOU: 102/1913...76/1913
KUIN: 104/2011...78/2011

September 2
KSTL: 101/2011...80/2011
KCOU: 100/1913...77/1913
KUIN: 103/1913...80/1913

September 3
KSTL: 101/2011...80/1913
KCOU: 103/1939...75/1939
KUIN: 103/1913...78/1913

&&

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

&&

$$

SHORT TERM...Elmore
LONG TERM...Elmore
AVIATION...Elmore
CLIMATE...Elmore