Area Forecast Discussion

860
FXUS63 KLSX 161044
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
544 AM CDT Tue Sep 16 2025

.KEY MESSAGES...

- The hot weather is expected through Thursday with relief expected
  over the weekend.

- There is a chance of thunderstorms starting on Thursday
  afternoon that will last through the weekend, with the best
  chance (30-70%) across much of the area Thursday night into
  Friday.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 334 AM CDT Tue Sep 16 2025

Missouri and Illinois has dry weather this morning compared to early
yesterday as there is a lack of low level convergence and the upper
level ridge has shifted to the west.  With this trend, today looks
to be dry as mid level subsidence sets in over the area, and the RAP
is also showing what weak low level convergence there is has shifted
west of the CWA.  The RAP is also showing a strong cap over the
area, and the CAMS is keeping convective development to our west
supporting the dry forecast.

Low level conditions will be similar to yesterday with mixing up to
800mb.  Expect highs to climb back into the low to mid 90s with lows
tonight in the low to mid 60s

Britt

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 334 AM CDT Tue Sep 16 2025

The omega block that the upper ridge is part of will continue to
stay in place through Wednesday night.  This will keep subsidence
over the area which will suppress shower and thunderstorm
development over the CWA.  This will begin to change by Thursday
when the GFS/ECMWF are in good agreement that the ridge will move
off to the east which will allow the mid-level cap to break down
over the western CWA by afternoon allowing scattered showers and
thunderstorms to develop.  We will then move into an unsettled
weather pattern late this week through the weekend as a series of
upper troughs move through the area starting with upper low over the
Plains associated with the omega block. Model guidance is showing
this low opening up as it moves across Missouri and Illinois on
Thursday night and Friday. Additional mid-level troughs are seen in
the deterministic guidance moving across the Midwest each day from
Saturday through Monday with each model showing slight differences
in timing. The LREF is showing 40-70% of its members producing rain
on Thursday night and Friday with the initial trough, with 30-50%
of members producing precipitation with the troughs Saturday into
Monday. The greatest rainfall amounts still look to stay across
central and northeast Missouri where the LREF has a chance of
seeing >1" of rainfall around 60%.

Highs are still on track to stay in the low-mid 90s through Thursday
before we see a break in the heat once the upper ridge moves off to
the east.  Then the 850mb temperatures will drop off from 20C to 15C
by the weekend as the the mid level trough moves into the area.  The
added clouds and rain chances will also help lower highs into the
upper 70s and 80s.

Britt

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 538 AM CDT Tue Sep 16 2025

Have included fog through 13-14Z at JEF/SUS for IFR/MVFR
visibilities. Thereafter, dry and VFR conditions are expected the
rest of period with light winds.

Britt

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion