Chester, Illinois Weather Forecast Discussion

696
FXUS63 KLSX 171047
AFDLSX
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
547 AM CDT Wed Sep 17 2025
.KEY MESSAGES...
- Relief from the heat will begin over parts of the area beginning
  on Friday.
- There are multiple chances of thunderstorms starting on Thursday
  afternoon through early next week. The best chance (30-60%) will
  be across Missouri on Thursday night and Friday.
&&
.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 331 AM CDT Wed Sep 17 2025
The latest surface analysis depicts a ridge extending from the Great
Lakes through Missouri and Illinois into the central Plains with a
stationary front stretching from the upper Midwest into the
central High Plains. The water vapor imagery is showing the
persistent omega block with the associated upper ridge mirroring
the surface ridge. The ridging through the deep layer of the
atmosphere is providing light winds and clear skies over the area
early this morning which may allow for some river fog to develop,
particularly across parts of central Missouri where visibilities
dropped as low as 1/4SM early yesterday morning at Jefferson City.
Otherwise, the upper ridge will provide subsidence and
consequently a second day of capping seen on the RAP/NAM soundings
that should suppress shower and thunderstorm development again
today. The CAMS are dry through tonight keeping convective
development this afternoon just to the west of the CWA border.
It will be yet another very warm day with a warm start and mixing up
into the 800-750mb range allowing for highs in the low to mid 90s.
With dewpoints in the 50s, heat index readings are not expected to
be higher than the temperatures today.  Lows tonight will drop back
into the upper 50s and low 60s.
Britt
&&
.LONG TERM...  (Thursday Night through Tuesday)
Issued at 331 AM CDT Wed Sep 17 2025
There is decent agreement in the model guidance that the upper ridge
will begin to move off to the east Thursday into Friday, though not
as quickly as 24 hours ago.  This will slow the progression of the
aforementioned Rockies low moving into Missouri, and any associated
precipitation moving into the CWA.  It still looks like enough of
the cap will break down over a small part central Missouri on
Thursday for showers and thunderstorms to develop.  The
NAM/GFS/ECMWF is then showing reasonable agreement moving the upper
low over central Plains into the Missouri Thursday night into
Friday.  This will cause the chance of showers and thunderstorms to
spread east across the area with the best chance still on Thursday
night and Friday, mainly to the west of the Mississippi River (40-
60%). Thereafter, there is increasing spread in the individual
models with the timing and depth of additional lows moving in from
upstream later in the weekend and early next week.  The LREF is
showing a mean trough over the Midwest during this timeframe with 30-
50% of its members producing precipitation Saturday into Monday
before the chances taper off beginning on Tuesday.  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%.
Thursday still will have highs in the low to mid 90s, but with a
slower progression of the trough, highs on Friday have increased
into the lower 90s over the southeast half of the CWA with the rest
of the CWA staying in the 80s.  Highs are expected to be in the 80s
the rest of the forecast period. The NBM IQR does reflect the noted
increase in the forecast spread in the models as mentioned above,
with it being only 2-4 degrees Thursday through Friday, but then
increasing to 7-9 degrees Saturday through Tuesday.
Britt
&&
.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 545 AM CDT Wed Sep 17 2025
Mainly dry and VFR conditions are expected through the period
with light winds. The only exception will be river fog at JEF and
SUS through 13Z that may cause MVFR/possibly IFR visibilities.
Britt
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&
$$
WFO LSX

NWS LSX Office Area Forecast Discussion