066
FXUS63 KLSX 152311
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
611 PM CDT Wed Jul 15 2026

.KEY MESSAGES...

- Typical warm and humid weather will continue through the
  period. Most days will see heat index values between 95 and
  100 with some spots possibly reaching 105 early this weekend
  into early next week.

- There will be a chance for scattered afternoon and early
  evening thunderstorms, into the weekend.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 317 PM CDT Wed Jul 15 2026

Warm and humid weather will continue through the short term as a
strong upper level high moves from the Midwest into the Ohio Valley
over the next 24-36 hours.  Weak disturbances are rotating north-
northwest into the Mid Mississippi Valley around an upper level low
over Texas this afternoon.  This is producing scattered convection
across the eastern Ozarks.  Expect this convection to continue
through the afternoon, and slowly weaken and dissipate after 00Z as
daytime heating wanes.  CAPE values this afternoon are running in
the 1500-2000 J/Kg range, but deep-layear shear is only around 20-
25kts so organized severe storms are not expected.  However,
precipitable water values are around 2 inches so storms will be
efficient rain-makers so locally heavy rain is possible.

Convection is expected to redevelop and will likely spread farther
to the north and east on Thursday as the upper level high shifts
east.  Instability Thursday afternoon will be similar to today,
though shear will be weaker.  Therefore the chance for severe storms
Thursday remains low.  Pwats remain high though, so locally heavy
rain remains a threat.  Temperatures Thursday afternoon should be
very similar to today`s, in the upper 80s to low 90s with heat index
values ranging from 95 to around 100.  This will be the first day in
a string with maximum heat index values in the 100-105 range,
primarily in the St. Louis Metro area, but also in parts of southwest
and south central Illinois and parts of central Missouri.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Issued at 317 PM CDT Wed Jul 15 2026

The long wave upper level ridge currently over the Great Plains
Friday into Saturday as a trough deepens over the eastern U.S.  This
results in northwest flow aloft over the Mississippi Valley
beginning late Friday, and the pattern persists into next week.
Unfortunately the latest model runs fail to translate this northwest
flow into a real frontal passage that will bring cooler weather like
they did yesterday.  A short wave trough moves across the Midwest
Sunday into Monday that pushes a cold front into Illinois, perhaps
as far west as the Mississippi River.  However the upper pattern
begins attenuating, and the short wave never really digs farther
south than the Ohio or Tennessee Valley, so the cooler airmass
behind the front never really reaches into our area.  This means
temperatures will remain warm, and humidity will stay relatively
high as well.  Heat index values continue near 100 into next week,
with the possibility of reaching 105+ on Saturday and perhaps Monday
in a few spots.  Therefore a Heat Advisory may be needed heading
into this weekend, although confidence is low that the heat index
will reach 100 degrees on 4 consecutive days, or 105 on any given
day due to afternoon clouds and possibly thunderstorms.  Afternoon
CAPE values generally continue in the 2000-2500 J/Kg range, though
shear remains relatively weak so organized severe storms do not look
likely at this time.

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 607 PM CDT Wed Jul 15 2026

There is only a low chance for impacts at the local terminals
through the forecast period. First, we will be monitoring for
fog development tonight, with the strongest signal for this
threat remaining well south of the local terminals. However, the
usual trouble spots of KSUS and KCPS have a low chance of seeing
at least brief impacts. Additionally, scattered showers and
thunderstorms are expected tomorrow afternoon. Confidence is low in
direct impacts from these at the local terminals at the moment.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.

&&

$$

SHORT TERM...Carney
LONG TERM...Carney
AVIATION...Elmore