943
FXUS63 KLSX 052116 RRA
AFDLSX

Area Forecast Discussion...Retransmission
National Weather Service St Louis MO
252 PM CDT Sat Sep 5 2026

.KEY MESSAGES...

- Cooler (and less humid) air will move into the region for the
  rest of the holiday weekend before a (brief) return to heat
  midweek.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 251 PM CDT Sat Sep 5 2026

As expected, heat index values are running a few degrees on
average higher than the past several days, with most locations
already above 105F. Most sites are likely nearing their peak,
but may continue to tick up a bit more as temperatures climb.

GOES-19 satellite imagery has shown pockets of agitated cumulus, and
even some isolated shower/thunderstorm activity over the past
1-2+ hours. Most of this activity should focus across southeast
Missouri, but a stray shower/storm may be as far north as
northeast Missouri/west central Illinois this afternoon. SPC
mesoanalysis shows moderate instability (1000-1500+ J/kg MLCAPE)
and very weak shear. Therefore, most of these storms should be
weak and brief in nature as individual updrafts are choked off
by their own downdrafts. One storm in Lincoln county did have a
weak MARC signature followed by a weak microburst (winds 30-40
mph), so cannot rule out a stronger storm or two, but this
should be the exception and not the rule. Nocturnal
stabilization should help dissipate things, though there is
about a 10-15% chance roughly along the backdoor cold front as
it moves through overnight tonight.

Speaking of this front, latest surface analysis is showing it
located across southeast Iowa into eastern Illinois. This boundary
will make it through the area late tonight into early Sunday
morning. Weak low-level cold air advection will commence behind it,
though surface dewpoints will still lag the front by approximately
12 hours. Therefore, it will be another warm and humid day for parts
of the area, mainly in central/southeast Missouri. Peak heat index
values of near or just above 100 degrees are forecast for portions
of these areas, and have a heat advisory until 7 PM Sunday evening.
Widespread lower dewpoints (relative humidity) will move in areawide
Sunday night into Labor Day. Highs in the upper 80s to low 90s are
forecast. Dewpoints in the upper 50s to low 60s are also expected,
so heat index values will not be any higher than the ambient air
temperature. While those temperatures are still 5-10 degrees above
normal, it will feel 20+ degrees cooler when comparing to today`s
heat index values.

&&

.LONG TERM /SUNDAY THROUGH SATURDAY/...
Issued at 251 PM CDT Sat Sep 5 2026

(Tuesday - Wednesday)

Model guidance shifts the anomalous mid/upper back toward the mid-
Mississippi Valley for midweek. Temperatures will warm back up each
day through Wednesday as a result, though relative humidity values
Tuesday will still be low. The spread on the timing of the midweek
cold front has lowered, and now looks like late Wednesday night into
early Thursday. The hottest day will be ahead of this front, with
highs Wednesday back in the upper 90s to near 100 degrees, again
threatening daily records. Heat index values may approach 105
degrees as well in some parts of the area, though the confidence in
how widespread these values will be is low at this time range.


(Wednesday Night - Next Saturday)

The best chances (40-60%) of showers and thunderstorms is associated
with the frontal boundary, though some weak low/mid level
frontogenesis lags the surface front, which may prolong the threat.
These rain chances also are likely to come up as the differences in
the frontal timing decrease in the coming days. Chances for
measurable rain on the LREF over a 24-hour period ending Thursday
evening are 60-80%. There is skepticism on widespread significant
rainfall at this time, but any rain would be a welcome sight given
the dryness over the past 2+ weeks. Probabilities for 0.25"+ of rain
are only in the 25-40% range along/north of I-70 for reference.

Behind the cold front, highs in the mid to upper 80s are expected
for Thursday and Friday (Thursday potentially being aided by cloud
cover and rain). While still about 5 degrees above normal, this may
be the first time below 90 degrees for many areas.

Uncertainty in the evolution of the pattern across the CONUS
increases heading toward next weekend. While there is still going to
be an omega block, where the trough-ridge-trough pattern sets up is
the question mark. Three of the four clusters of the 500-hPa height
pattern from the LREF have our area beneath zonal or northwest flow
aloft (i.e., the center of the ridge is to our south or southwest).
This in theory could yield either another cold frontal passage
sometime or a "ridge-runner" shortwave and rain chances. One cluster
has the amplified ridge right over us, which would mean hotter and
dry weather. This cluster though only comprises 20% of the LREF
membership, so this scenario is not favored at this time.

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1210 PM CDT Sat Sep 5 2026

Isolated showers, and perhaps a few rumbles of thunder, are
possible this afternoon, mainly in southeast Missouri. A weak
backdoor cold front will move through from northwest to
southeast tonight/early Sunday morning, with northwest winds
veering to the northeast behind the boundary.

&&

.CLIMATE...
Issued at 231 AM CDT Sat Sep 5 2026

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

September 5
KSTL: 103/1954...80/1899
KCOU: 103/1899...75/1983
KUIN: 100/1925...76/1912

September 8:
KSTL: 101/1947...77/1939
KCOU: 103/1947...75/1922
KUIN: 102/1947...79/1922

September 9:
KSTL: 101/1925...79/1925
KCOU:  99/1925...76/1947
KUIN: 100/2013...75/1912

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Extreme Heat Warning until 8 PM CDT this evening for MOZ018-
     019-026-027-034>036-041-042-047>052-059>065-072>075-084-
     085-099.
     Heat Advisory from 8 PM this evening to 7 PM CDT Sunday for
     MOZ041-047>050-059-072-099.
IL...Extreme Heat Warning until 8 PM CDT this evening for
     ILZ058>060-064-065-069-070-074-079-095>102.

&&

$$

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