092
FXUS63 KLSX 301039
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
539 AM CDT Sun Aug 30 2026

.KEY MESSAGES...

- Record-threatening and potentially dangerous heat is expected
  through this week.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 317 AM CDT Sun Aug 30 2026

Upper-level ridging is building into the Midwest per morning water
vapor imagery as an expansive area of surface high pressure sits over
the Southeast. As the upper-level ridge continues to build into the
region today, 850 mb temperatures will warm to around 22 C. Deep
mixing is expected given mostly clear skies, favoring surface
temperatures in the mid to upper 90s. Dew points largely in the low
to mid 60s will lead to little to no heat index, with southwesterly
winds around 10 mph helping to lessen the feel of the heat.

With the large-scale pattern holding and temperatures only bottoming
out in the low to mid 70s early Monday morning, widespread upper 90s
are expected for highs during the afternoon. The NBM has dew points
trending upward on Monday by a couple of degrees; however, there are
a few factors why confidence in dew points increasing is low. First,
the bias-corrected guidance the NBM is based off of is likely being
influenced by the typical summer humidity of the last couple of
months. Next, surface flow will be southwesterly off the Ozark
Plateau leading to adiabatic warming that will give temperatures a
slight boost and decrease dew points. Therefore, dew points tomorrow
have been decreased slightly, with resulting afternoon heat index
values hovering around 100 degrees.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 317 AM CDT Sun Aug 30 2026

The axis of the upper-level trough will still be oriented along the
Mississippi River at the start of the extended period on Tuesday,
with the surface high holding strong over the Southeast. With this
pattern holding through at least Thursday, the early week heat will
only continue. One variable that may fluctuate is dew points, as
continuous southerly flow with uninhibited Gulf access will gradually
advect greater moisture into the area. Assuming high daily high
temperatures top out in the upper 90s to around 100 degrees, this
would lead to an uptick in heat index values. While this is the
current NBM solution, an increase in humidity would lead to a couple-
degree decrease in temperatures. Additionally, there is a strong
consensus among guidance for surface flow to be out of the
southwest daily, which would lower dew points and boost temperatures
thanks to downsloping off the Ozarks. Therefore, confidence remains
low to medium in heat index magnitude as we get later into the week.

Confidence decreases in our local weather Friday into the weekend,
as guidance notably diverges on the phasing of the upper-level
ridge. While there is some consensus on the ridge amplifying, where
exactly this occurs across the CONUS varies among deterministic
guidance and ensemble clusters. If the ridge axis remains generally
over the Midwest, then we will see a continuation of the heat into
the weekend. If the ridge amplifies over the Plains as some guidance
suggests, then the CWA will be beneath northwesterly flow and
temperatures will decrease. The current forecast splits the
differences with warm by gradually decreasing temperatures through
the weekend.

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 536 AM CDT Sun Aug 30 2026

Confidence is high in conditions remaining dry and VFR through
the TAF period at all local terminals as winds prevail out of
the south-southwest.

&&

.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