461
FXUS63 KLSX 162309
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
609 PM CDT Thu Jul 16 2026

.KEY MESSAGES...

- Warm and humid July weather is expected to continue through
  the weekend into early next week.

- Widely scattered showers and thunderstorms are possible at
  times through early next week, mainly during the afternoon.
  Widespread rainfall across the area does not look likely at
  this time.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 247 PM CDT Thu Jul 16 2026

Latest water vapor imagery is showing a compact area of low pressure
moving north through western Missouri on the southwest side of an
upper level ridge over the Midwest.  This area of vorticity is
producing scattered showers and thunderstorms across Missouri and
Illinois this afternoon.  This convection is expected to continue
through the afternoon, and diminish during the evening as daytime
heating wanes.  While severe weather is not expected, the storms
have proven to be efficient rain-producers with some reports of 2+
inches in less than an hour.  Locally heavy rain and minor flooding
will continue to be a possibility with these storms through early
evening.

The vortmax turns to the east over Iowa tonight, and then begins
sliding southeast Friday night.  Low level flow turns to the west
late tonight in response and guidance is showing some weak low level
moisture convergence before 12Z resulting in light QPF, mainly along
and east of the Mississippi River.  Have bumped up late-night PoPs
to around 20 in case there`s isolated convection before sunrise.
Expect scattered storms to form again during the late morning/early
afternoon on Friday, much like today.  The highest coverage will
likely be along and east of the Mississippi River where low level
moisture convergence continues.  While it will be warm and humid
again on Friday, afternoon showers and storms will likely keep
temperatures/heat index below advisory criteria.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Issued at 247 PM CDT Thu Jul 16 2026

A short wave trough moves through the Upper Midwest into the Great
Lakes region on Saturday.  This wave amplifies the long wave
pattern, with a trough to our east and an upper ridge to the west.
This produces northwest flow over the Mid Mississippi Valley which
persists into next week.  The amplified pattern allows a cooler high
pressure system to drift into the Great Lakes Region by early Sunday
which pushes a weak cold front through Illinois to near the
Mississippi River by 12Z Sunday.  Heat index values ahead of the
front on Saturday will flirt with single-day 105 degree criteria in
a few spots, but widespread 105+ does not very likely at this time.
Also, afternoon thunderstorms ahead of the front could put the
kibosh on the heat index saturday afternoon.  With respect to those
thunderstorms, the GFS is forecasting CAPE values of 3500-4500 J/Kg
Saturday afternoon across parts of central and south central
Illinois.  While deep-layer shear is 20kts or less, that kind of
instability will be capable of producing isolated severe
thunderstorms.  The most likely mode would be organized clusters or
even some severe line segments with damaging winds as the primary
severe weather threat.  Sunday`s temperatures look cooler with the
front in the area and the potential for afternoon clouds and
convection.

The front drifts northeast Sunday night and Monday as the eastern
long wave trough attenuates, and low level flow turns back to the
west and southwest.  Guidance noses the low level thermal ridge into
the forecast area Monday afternoon with 850mb temperatures around
24C.  This mixes down to upper 90s surface temps which would produce
the hottest day of the next 7.  Therefore Monday would be the most
likely day for widespread heat index values of 105+.  Global
guidance shows another short wave trough digging through the Midwest
Tuesday and Tuesday night which pushes a strong cold front through
Missouri and Illinois.  While Tuesday continues to look warm ahead
of the front, it most likely won`t be as hot as Monday with the
front moving through during the afternoon.  Wednesday and Thursday
look substantially cooler with lower humidity behind the front with
highs only in the mid 80s.  Given all this information, it is useful
to note that heat index values for Friday through Tuesday will flirt
with the 100-104 range each day, mainly along and south of the I-70
corridor.  However, heat index values of 100+ are by no means
certain as afternoon clouds and convection would produce cooler
conditions.  Therefore confidence is low that any given location
will see 4 consecutive days of 100-104 heat index which is necessary
for a Heat Advisory with readings below 105.

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 603 PM CDT Thu Jul 16 2026

Widely scattered showers and thunderstorms across the region
are expected to weaken and diminish early in the period, with a
very low chance for impacts at the local terminals. The weather
system responsible for these showers and thunderstorms will lift
northward tomorrow, taking the chance for rain with it and
causing winds to become west-southwesterly.

&&

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

&&

$$

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