802
FXUS63 KLSX 300442
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1142 PM CDT Sat Aug 29 2026

.KEY MESSAGES...

- A prolonged late season heatwave is expected Sunday through
  Thursday. While moderate humidity may mitigate heat impacts
  slightly, near-record warm actual temperatures (highs 96 to
  102 degrees / lows 75 to 80 degrees) are forecast for 3 to 5
  consecutive days.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 238 PM CDT Sat Aug 29 2026

While the beginning of a late season heatwave is now only a day
away, temperatures this afternoon remain very near or only slightly
above daily averages. Not only this, but dewpoints have mixed all
the way down to the low-mid 60s, leading to relatively comfortable
humidity levels. Meanwhile, persistent light southerly breezes will
continue through the afternoon and evening, with occasional gusts
approaching 15 to 20 mph as the boundary layer deepens over the next
few hours. Otherwise, a few showers and weak thunderstorms have
developed across southern Iowa, and while we can`t rule out a stray
shower near the MO/IA border very late this evening or again
tomorrow evening, precipitation probabilities are only roughly
10% in these areas. Overnight, some patchy fog may develop in
Ozark river valleys and other wind-protected areas, but this is
not expected to be widespread or particularly impactful.

For tomorrow, the long awaited heatwave begins in earnest as a
large, persistent upper level ridge builds overhead. Between the
anomalous 500 mb heights and 850 mb temperatures (both 98+
percentile), persistent downsloping south-southwesterly surface
winds, and relatively low humidity levels, actual surface
temperatures are expected to shoot well into the mid to even
upper 90s by mid afternoon. Fortunately, with the dewpoints only
projected to reach the low to mid 60s, heat index values are
not likely to exceed actual temperatures much, if at all, and
should remain below 100 degrees for the most part. The real heat
impacts are expected to compound over the course of the week,
as near-record temperatures may persist for several days. More
on that in the section below.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Issued at 238 PM CDT Sat Aug 29 2026

The main focus of this forecast period continues to be the potential
for near-record heat from Monday through Thursday, and possibly into
Friday as well. As discussed in the short term period, a stout upper
level ridge will build overhead Sunday and is expected to remain in
place for several days thereafter, with 98+ percentile 500 mb
geopotential heights and 850 mb temperatures, along with downsloping
south-southwest surface winds. While dewpoints should increase very
slightly over the course of the week, they should remain in the 60s
early in the week and perhaps only approach the low 70s. While this
isn`t a desert airmass by any stretch, this should be low enough to
keep humidity levels (and likewise heat index values) in check. Add
in some afternoon breezes thanks to deep mixing, and this should
help to mitigate the heat slightly on each individual day.

On the other hand though, confidence remains high that actual
temperatures will approach the 100 degree mark each day from Monday
through Thursday. While not unprecedented, this is relatively late
in the summer for such high temperatures, and daily records
generally range from 100 to 104 degrees during that span at all
of our local climate stations. Not only this, but current model
forecasts for low temperatures forecasts during this period are
also near record "warm minimum" values (mid 70s to near 80).
This might be the most surprising aspect of the forecast given
that humidity levels are not especially high, although
persistent southwest low level winds may be contributing to
this. Still, there is some slight skepticism in these values,
and wind-protected areas are likely to see a bit more overnight
relief than that. Still, ECMWF Extreme Forecast Index (EFI)
values exceed .95 for both the max and min temperatures during
this period with a non-trivial "Shift of Tails" (1-2), adding
confidence that we are looking at an anomalously warm, near
record temperature event once you stack it all up.

At this point, while actual temperatures are forecast to be near 100
degrees for several days, confidence is relatively low (30-50%) that
we will see 105+ degree heat index values for 4 or more
consecutive days, and even lower that we will reach 110 degree
heat index values on any individual day (Extreme Heat Warning
criteria). One area to watch closely, though, will be the urban
heat island of the St. Louis metro area, and strictly based on
NBM deterministic forecast values, we get very close (just a
couple of degrees short on Monday) to meeting the bare minimum
for a duration-driven Heat Warning over a small footprint.
Considering the relatively low humidity, uncertainty surrounding
the morning low temperatures, and increasing ensemble
temperature spreads later in the event, we will refrain from
hoisting an Extreme Heat Watch for the time being. It should be
noted, though, that it is very likely that lower-level Heat
Advisories will be needed in at least parts of the area as we
get a little bit closer to the event.

Otherwise, there appears to be very little chance for meaningful
precipitation through at least Wednesday, but we do start to see
ensemble probabilities creep up into the 20-30% range Thursday
through the end of the week as the ridge weakens and a weak front
sinks south. As noted previously, NBM/LREF temperature spreads
increase considerably during this period as a result, offering
some hope that the worst of the heat may subside slightly late
in the week even if we remain above average for several more
days.

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1140 PM CDT Sat Aug 29 2026

The only potential threat this period is fog at KSUS pre-
sunrise. Otherwise, dry and VFR flight conditions are expected
through the 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...None.
IL...None.

&&

$$

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