001
FXUS63 KLSX 181853
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
153 PM CDT Fri Sep 18 2026

.KEY MESSAGES...

- Near record warm temperatures will continue through Saturday,
  but heat relief will slowly spread south Sunday and Monday
  behind a cold front. Below to near-average temperatures can be
  expected Tuesday-Thursday.

- A few showers and thunderstorms are expected this afternoon
  (20-30%), but more substantial rain chances (60-80%) arrive
  Sunday and Monday as a cold front moves through the area.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 150 PM CDT Fri Sep 18 2026

HEAT: Once again, near-record maximum temperatures are expected in
most areas this afternoon thanks to a persistent and anomalously
strong upper level ridge. At St. Louis yesterday, record high
maximum (99) and minimum (79) temperatures were observed, and with
little change to speak of in the overall pattern between then and
now, we will likely get very close to those values again today. The
12Z sounding at WFO Springfield this morning recorded an 850 mb
temperature of 21 degrees C, which ties the daily record according
to SPC`s sounding climatology. Meanwhile, dewpoints also remain very
high this morning relative to the time of year, with values
generally in the low to mid 70s. Meanwhile, a weak cold front that
settled into the area overnight is now retreating north, although
areas north of the Highway 36 corridor are only now emerging from a
blanket of low clouds. Already by noon today, temperatures have
climbed into the 90s south of the stalled front, with heat index
values already near 100 degrees and likely to top out near 105 once
again. Further north where morning clouds shaved a few hours off of
the morning sun, temperatures and heat index values are likely to be
around 5 to 10 degrees cooler. Persistent humidity will also keep
overnight temperatures rather warm, with minimums settling once
again into the low-mid 70s (or slightly warmer in the STL metro).

For tomorrow, southwesterly low level flow is expected to develop,
which may add a degree or two to observed temperatures thanks to
downsloping, but also erode some of the lingering surface moisture.
The net result will be heat index values similar today (if not a few
degrees lower) even actual though temperatures may approach 100
degrees in some places. In any case, no changes were made to the
Heat Advisory, as we feel it represents the expected conditions
well.

THUNDERSTORMS: Several pulse-type thunderstorms developed yesterday
afternoon across east-central/southeast Missouri and southern
Illinois, which were focused along lingering outflow boundaries.
While most of these storms were ordinary pulse-type storms, one
storm in particular managed to produce quarter sized hail and a
fairly substantial downburst, and today`s environment appears to be
very similar. Steep low level lapse rates substantial downdraft CAPE
(1200 J/kg) have once again developed, with rather high Microburst
Composite values as well (4-8). Meanwhile, shear remains very weak,
although today features slightly higher mean winds (~15-20kt) that
should give storms just a bit more forward motion. Still, pulse-type
storms are the most likely storm mode, with only a few that manage
to produce gusty downburst winds and small hail. We can`t rule out a
stray severe wind gust, either (60 mph). Convection has been slower
to develop by about an hour or so compared to yesterday, but we are
seeing a few showers across the Ozarks as of 1:15 pm. These are
expected to slowly expand and strengthen over the next several
hours, and highest confidence in thunderstorms exists in these
areas. It is less clear whether or not additional storms will
develop along or north of the I-70 corridor, where cumulus
development has been much slower. Still, a stray thunderstorm may be
observed as far north as Highway 36/I-72. Overall, the window for
the most substantial storms is favored between 3 and 7 PM. While a
few showers will be possible early in the day in southern IL
tomorrow, these should quickly exit to the east and the remainder of
the area will stay dry.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
Issued at 150 PM CDT Fri Sep 18 2026

Overnight Saturday, a strengthening low level jet is expected to
drive nocturnal thunderstorm development just to our northwest, and
these storms may skirt our portion of northeast Missouri as they
move slowly southeast through Sunday morning. It is unclear how far
into our area these storms will make it before diminishing, but in
any case their outflow should help nudge the stalled cold front
slowly southward during the day Sunday. As this boundary slowly
moves south between Sunday morning and Monday night, additional
rounds of showers and storms are expected to develop, although the
timing and placement of each round is very uncertain.
Precipitation probabilities during this window remain rather
high (60-80%) in most areas and appear to be slightly favored
Sunday night/Monday morning, but confidence in the specific
timing is very low. While it is too soon to pinpoint timing and
placement, most areas are favored (60+%) to see at least some
rain during this two day window, with a reasonable chance
(40-70%) to see a beneficial soaking (1/2 inch or more).

Meanwhile, temperatures behind the advancing front continue to trend
cooler and cooler in latest ensemble guidance, and it is becoming
more and more clear that significant heat relief is on the way. This
relief is first expected in our far north Sunday, although the
remainder of the area is expected to remain in the 90s for one more
day (albeit with lower humidity). By Monday, this front is expected
to sink well into Missouri, and latest trends continue to increase
its speed. Still, forecast deterministic temperatures continue to
exhibit a 20-25 degree range across the area Monday, and ensemble
temperature spreads remain high due to timing uncertainty of the
front. By Tuesday though these uncertainties disappear as the front
is expected to clear well to the south, with latest ensemble mean
temperatures only in the 60s and 70s area-wide along with much
narrower ensemble spreads.

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1145 AM CDT Fri Sep 18 2026

VFR conditions are expected to prevail through the 18Z TAF
period, but with a few possible exceptions. First, widely
scattered thunderstorms are expected roughly along the I-70
corridor again today, with an emphasis on eastern MO, southwest
IL. Coverage will be limited, but enough to justify a mention in
the TAF this afternoon at all St. Louis area terminals
(STL/COU/CPS). This activity may produce heavy rain and reduced
visibilities, along with erratic downburst winds and small hail.
Confidence in direct impacts to local terminals is low.
Overnight, confidence is low that low stratus will once again
reach UIN like it did the previous night, so it has been omitted
from the TAF for the time being.

BRC

&&

.CLIMATE...
Issued at 254 AM CDT Fri Sep 18 2026

Near record heat and very warm overnight lows are expected the
next couple of days.

Record High Temperatures:

September 18:
KSTL: 97/1925

September 19:
KSTL: 96/1955

Record High Minimum Temperatures:

September 19:
KSTL: 76/1925
KCOU: 75/2022

September 20:
KSTL: 75/2024
KCOU: 73/2018

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT this evening for MOZ041-050>052.
     Heat Advisory until 8 PM CDT Saturday for MOZ047>049-059>065-
     072>075-084-085-099.
     Air Quality Alert until 8 PM CDT this evening for MOZ061>065.
IL...Heat Advisory until 8 PM CDT this evening for ILZ058>060-065-
     098-099.
     Heat Advisory until 8 PM CDT Saturday for ILZ064-069-070-074-
     079-100>102.
     Air Quality Alert until 8 PM CDT this evening for ILZ100>102.

&&

$$

SHORT TERM...BRC
LONG TERM...BRC
AVIATION...BRC
CLIMATE...Kimble