144
FXUS63 KLSX 012314
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
614 PM CDT Tue Sep 1 2026

.KEY MESSAGES...

- The ongoing late summer heat wave is forecast to continue into
  the holiday weekend. A heat advisory is now in effect through
  Saturday across the entire area.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 227 PM CDT Tue Sep 1 2026

The center of the midlevel anticyclone is nearly right over us,
somewhere near the Mississippi-Ohio River confluence early this
afternoon. There remains very little/no change in the ambient
air mass, with very anomalous aforementioned ridging aloft
(>99th percentile of climatology) and 850-hPa temperatures
(>97th percentile). One subtle change that does look likely on
Wednesday is that there is a better signal for diurnal cumulus
compared to the past couple of days. These clouds should not be
too impactful, but may serve as a "lid" on temperatures and
possibly keep highs 1-2F cooler in some spots than today. That
will be imperceptible though to almost anyone, and we still
expect peak heat index values in the 100-105 degree range for
most. Persistence is also a good forecast for lows tonight and
Wednesday night with temperatures only dropping back into the
low to mid 70s for most locations. Readings may not get below 80
degrees within the urban heat island of metropolitan St. Louis.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 227 PM CDT Tue Sep 1 2026

(Thursday - Saturday)

The overall synoptic pattern remains largely locked in place through
the end of the week into the weekend, with an omega block across
North America. The mid-Mississippi Valley will remain near the
anomalous mid/upper level ridging during this time period, moving
eastward on Thursday before shifting back to the west Friday into
Saturday. There remains very high confidence in widespread highs
each day in the 95-100F range, with peak heat index values of 100-
105F. The heat advisory was extended in time areawide, and now goes
through Saturday. Any backdoor front should hold off until Saturday
night, so barring any convection spilling into Illinois, Saturday
will not be any different sensibly than previous days. I will note
that the deterministic ECMWF does show a complex of storms moving
south along/east of the Mississippi River on Saturday, but there is
little support amongst LREF membership. Only ~20% of members show
any measurable precipitation ending at 1800 UTC Saturday.

An upgrade to an extreme heat warning was also considered from St.
Louis metro southeast into southwest Illinois due to the potential
for longevity (4+ days of 105+F heat index) criteria. However, each
day is questionable for the 105+, and the time of year (early
September) is also a factor. The sun angle is lower (not as intense)
and is analogous to early/mid April. Days are shorter and people
have also acclimatized to summerlike heat over the past few months.
Given the uncertainty mentioned above on achieving duration criteria
along with the extenuating factors listed above, elected to keep the
advisory in those areas, but we will continue to evaluate moving
forward. Of note, there has not been a heat warning issued anywhere
in our area within the month of September dating back to at least
2005.


(Saturday Night - Next Tuesday)

There have been some subtle, but potentially impactful, changes for
the rest of Labor Day weekend. Both the EPS/GEFS have trended
slightly more westward with the mid/upper level ridge, which allows
for the backdoor cold front to move a bit more into our area most
likely. In addition, there has been a bit better of a signal for
late Sunday of potential thunderstorms moving south out of the Upper
Midwest into our region. LREF probabilities Sunday afternoon are in
the 30-40% for measurable rainfall, or nearly double that of
Saturday. The timing of this possible round of showers and
thunderstorms though may be a bit too late to impact temperatures
too much, but it does look likely that at least an increase in cloud
cover should occur. Highs are most likely to drop into the low to
mid 90s, though the spread is higher compared to earlier days. The
inter-quartile range of the NBM jumps into the 5-8F range, and is
the difference between at least a temporary end to the 100+ heat
index values compared to a continuation of the heat wave.

Ensemble guidance then suggests that the anomalously ridging will
shift back eastward into the mid-Mississippi Valley, with 500-hPa
heights approaching the 99th percentile on the NAEFS once again.
So, even if there is a (brief) break from the heat, it likely will
not last. There remains no coherent signal for a breakdown of the
omega block across North America with the mid/upper level ridging
oscillating across the center of the CONUS through next work
week. At some (inevitable) point the calendar will win out and
the heat will break down as the days get shorter, but when
exactly this will occur remains an open question.

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 614 PM CDT Tue Sep 1 2026

Dry and VFR flight conditions will persist with only diurnal VFR
cumulus at times. Light winds are expected become southerly around
sunset, then gradually veer to southwesterly by mid-Wednesday
morning.

&&

.CLIMATE...
Issued at 1118 AM CDT Mon Aug 31 2026

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

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

September 4
KSTL: 104/1954...81/1925
KCOU: 103/1913...73/1947
KUIN: 105/1913...75/1947

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT Saturday for MOZ018-019-026-027-
     034>036-041-042-047>052-059>065-072>075-084-085-099.
     Air Quality Alert from 9 AM to 8 PM CDT Wednesday for
     MOZ061>065.
IL...Heat Advisory until 8 PM CDT Saturday for ILZ058>060-064-065-
     069-070-074-079-095>102.
     Air Quality Alert from 9 AM to 8 PM CDT Wednesday for
     ILZ100>102.

&&

$$

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