389
FXUS63 KLSX 092325
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
625 PM CDT Sun Aug 9 2026

.KEY MESSAGES...

- Dangerous heat and humidity will continue across the area at
  least through Wednesday. An Extreme Heat Warning and Heat
  Advisories therefore remain in effect.

- The chance for rain increases late Wednesday into the end of
  the week. Therefore cooler temperatures are forecast, however
  there are some indications that above normal temperatures could
  linger beyond Wednesday.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 251 PM CDT Sun Aug 9 2026

Low level south-southwest flow will continue through the short term.
This will keep a hot and humid airmass locked into place over the
Mid Mississippi Valley.  Models have been consistently showing 22-
25C temps in the low level thermal ridge which mixes down to 95-100F
at the surface which continues to agree well with NBM max
temperature forecasts.  Dew point temperatures are in the mid 70s
across the area this afternoon, and there`s no reason for Tds to be
any lower Monday and Tuesday.  This will result in 100-110 degree
heat index readings both afternoons, with the higher end of those
readings generally along and east of the Mississippi River in
southwest Illinois where highest humidity tends to pool due to
transpiration from agriculture.  With this in mind, The Heat
Advisories and Extreme Heat Warning look to be in good shape
and no changes are needed at this time.

Short range guidance shows a modest low level jet developing over
Missouri and western Illinois tonight.  Some models also produce
precip, although most are dry.  While confidence isn`t particularly
high, there`s enough of a signal to include some mention of precip
tonight, so slight chance PoPs across parts of northeast Missouri
into west central and south central Illinois seemed appropriate. Any
precip that does occur should dissipate after sunrise as the low
level jet weakens.  The jet strengthens again Monday night, however
guidance shows the resulting moisture convergence to the east-
northeast of our forecast area.  Have therefore kept Monday
night`s forecast dry.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Issued at 251 PM CDT Sun Aug 9 2026

Ensemble guidance continues to show a chance for rain Wednesday as a
short wave trough moves from the Great Plains into the Mid
Mississippi Valley.  The GFS shows a couple of QPF bullseyes
between 1.5 and 1.75 inches of rain Wednesday afternoon, but the
ECMWF is much more conservative.  A lot of afternoon convection
could keep heat index values well below advisory/warning criteria,
however forecast soundings for Wednesday afternoon show a
substantial low-level capping inversion in the pre-storm
environment.  I`m therefore skeptical about convective initiation.
Therefore think the heat headlines are still good for Wednesday...and
could in fact be needed into the end of the week.

The latest medium range guidance is now shifting the western upper
level ridge east into the Mississippi Valley Thursday into Friday.
This is a change from yesterday`s runs where the ridge retreated to
the west and a long wave trough dug into the Great Lakes and
southeastern U.S.  The change means that the back door cold front
which was supposed to move through the area in the Thursday/Friday
time frame only makes it to central/northern Illinois.  This would
have profound impacts on the temperature forecast for the rest of
the week into the weekend, and the rainfall forecast for the period
as well.  The latest NBM still shows a significant temperature drop
on Thursday from the low to mid 90s on Wednesday to the low to mid
80s for most of the area for Thursday through Sunday.  The LREF mean
high temperatures also follow this pattern.  In contrast, the 12Z
GFS extended MOS (MEX) high temperatures at KSTL for Thursday
through Sunday are 95, 98, 98, and 94 respectively.  EC MOS is
similar at 97, 96, 96, and 89.  Digging a little deeper in the LREF,
the max T probabilities are shifted to favor temperatures below 90
degrees both Thursday and Friday.  The histogram is more even for
the weekend, but still slightly favors highs AOB 90 degrees.  With
this in mind, will continue to keep the cooler and wetter forecast
for the end of the week.

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 624 PM CDT Sun Aug 9 2026

Dry and VFR flight conditions are most likely through the TAF
period, aside from a low (20 percent) chance of showers and
thunderstorms late tonight into Monday morning north of I-70 into
northeastern MO/west-central IL. However, confidence in any impacts
at TAF sites is far too low for inclusion at this point. As a 40-kt
southwesterly low-level jet (LLJ) encroaches the region overnight,
LLWS is expected into Monday morning before surface winds strengthen
and veer to southwesterly and the LLJ weakens. Winds will gust 15 to
20 kt at times Monday afternoon.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT Wednesday for MOZ018-019-026-027-
     034>036-042-052-072>075-084-085-099.
     Extreme Heat Warning until 8 PM CDT Wednesday for MOZ041-
     047>051-059>065.
IL...Heat Advisory until 8 PM CDT Wednesday for ILZ058>060-064-065-
     070-095>099.
     Extreme Heat Warning until 8 PM CDT Wednesday for ILZ069-074-
     079-100>102.

&&

$$

SHORT TERM...Carney
LONG TERM...Carney
AVIATION...Pfahler