942
FXUS63 KLSX 310433
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1133 PM CDT Sun Aug 30 2026

.KEY MESSAGES...

- A prolonged regional heatwave expected Monday through at least
  Friday, with near-record warmth at times. Heat impacts will
  be compounded by the long duration of the event, and there is
  a heightened potential for more significant impacts within the
  St. Louis metro area and southwest Illinois.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 231 PM CDT Sun Aug 30 2026

Once again, the focus of this forecast period continues to be the
arrival of a prolonged late-season heatwave, which begins today and
will continue through the end of the work week. While the worst of
the heat doesn`t arrive until Tuesday, temperatures have already
climbed into the low 90s as of 1 PM, and are likely to reach the mid
to even upper 90s by the hottest part of the afternoon. However,
dewpoints are only in the low 60s in most areas, which is keeping
heat index values within a degree or two of actual temperatures, and
generally mitigating the impact of temperatures that are roughly 10
degrees above the seasonal average. Breezy southwest winds are also
likely mitigating heat impacts outdoors slightly, although it is
still advised to avoid spending too much time in direct sunlight if
you are sensitive to heat. Meanwhile, a few showers and
thunderstorms have continuously moved along the MO/IA border through
the day, but have weakened considerably and are not expected to
impact our area.

Overnight, the relatively low dewpoints should allow for some
efficient cooling in wind-protected, low-lying river valleys,
particularly in the Ozarks. However, this relief will be less
pronounced in parts of central/northeast Missouri where light winds
may persist through the night, and there is less terrain to generate
efficient cool air drainage. Here, low temperatures may remain in
the low to mid 70s, although this will be watched closely tonight as
a litmus test for low temperature trends the rest of the week.

For tomorrow, the upper level ridge will continue to build and move
directly overhead, with 500 mb geopotential heights reaching near
5940 m (99th percentile), and 850 mb temperatures nearing 24 degrees
C (also 99th percentile). This will boost afternoon temperatures
into the mid to upper 90s, perhaps touching 100 in a few spots, and
begin our multi-day stretch of near-record temperatures. As has been
discussed in previous discussions, we remain skeptical of the
quality of moisture return achievable by this southwesterly low
level flow pattern, and suspect that NBM dewpoint values are being
artificially inflated at least slightly by a summertime bias
correction that no longer applies. In any case, peak moisture values
are not expected to arrive until mid-week, and tomorrow should once
again see heat index values only a few degrees warmer than the
actual temperatures (roughly 98 to 105). Given that this is day one
of a multi-day heatwave and most areas are likely to achieve a heat
index near or slightly above 100 degrees, Monday will be included
within our new heat headlines (more on those in the next section),
even though it is expected to be the "mildest" day of the work week
for most areas.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Issued at 231 PM CDT Sun Aug 30 2026

Between Tuesday and Thursday, the upper level ridge will remain in
place across the region, and as a result, heat will persist and even
slightly increase during this timeframe. Confidence is quite high
that we will see a continuation of upper 90s to near 100 degree
actual temperatures each day during this window, and likely Friday
as well for a majority of the area. Not only will afternoon
temperatures remain within this range for several days, but
overnight lows are also expected to slowly increase through the week
thanks to warming ground/pavement temperatures. We still suspect
that Ozark (and other) sheltered river valleys that experience cool
air drainage will see some effective overnight heat relief, and that
overnight lows are being over-estimated by a few degrees in these
areas. However, this may not be the case in places that will not
have this benefit, and particularly urban areas like the St. Louis
metro. Meanwhile, as alluded to in the short term discussion,
current NBM forecast dewpoints in the upper 60s to low 70s are
likely a few degrees too high, due to the suspected influence of the
model`s bias correction. If this is the case, afternoon heat index
values are also being slightly overestimated.

Still, at a minimum, we are now very confident that almost all of
the area will meet the 4-day, 100 degree heat index threshold
required for a Heat Advisory. There is also a reasonable potential
(50%-70%) for more significant heat impacts within the St. Louis
Metro area and southwest Illinois, where heat index values may
exceed 105 degrees for 4 days, and overnight lows may only drop to
near 80 degrees.

As such, area-wide heat headlines have been issued from Monday
through Friday, and are distributed as follows:

- FOR ST. LOUIS METRO AREAS and parts of SOUTHWEST IL, an EXTREME
HEAT WATCH has been issued, due to the potential for more
significant local heat impacts.

- For all other locations in the NWS St. Louis forecast area, a Heat
Advisory has been issued.

By Friday, confidence in the coverage of impactful heat decreases
slightly, as the upper ridge may begin to shift westward and open
the door for a weak cold front and more northwesterly flow aloft.
Confidence is relatively low that this will provide widespread
heat relief as there remains relatively high model spread and
run-to-run variability, so we have opted to keep the Advisory
in place through Friday evening. Not only this, but even if this
slight pattern shift does provide some modest heat relief, our
area remains favored for the continuation of above average
temperatures through the weekend. Finally, this transition also
allows for the return of modest precipitation chances (20-30%)
Friday onward, particularly in northeast MO and IL that could be
more exposed to northwest-flow shortwaves.

&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1132 PM CDT Sun Aug 30 2026

Confidence is high in dry and VFR flight conditions through the
entire TAF period at all local terminals.

&&

.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...Heat Advisory from noon Monday to 8 PM CDT Friday for MOZ018-
     019-026-027-034>036-041-042-047>052-059>065-072>075-084-
     085-099.
IL...Heat Advisory from noon Monday to 8 PM CDT Friday for
     ILZ058>060-064-065-069-070-074-079-095>102.

&&

$$

SHORT TERM...19
LONG TERM...19
AVIATION...Elmore
CLIMATE...Elmore