303
FXUS63 KLSX 291119
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
619 AM CDT Tue Sep 29 2026

.KEY MESSAGES...

- Temperatures will remain above normal today through Thursday.

- Widespread 1-3 inches of rain is expected as a slow moving
  cold front drifts through the area Wednesday night through
  Friday. The higher end of this range of rainfall is expected
  across northeast Missouri and west central Illinois.

&&

.SHORT TERM /THROUGH WEDNESDAY/...
Issued at 307 AM CDT Tue Sep 29 2026

A surface high centered over the Great Lakes Region will continue to
produce dry weather for the Mid Mississippi Valley today.  Winds
will turn to the south this morning and warm southerly flow will
continue through Wednesday ahead of the next storm system.  The warm
southerly low-level flow combined with a strong long wave ridge over
the area will produce above normal temperatures in the low to mid
80s today, and the mid to upper 80s on Wednesday for most of the
area.  Latest guidance shows the upstream low pressure system moving
into the Plains on Wednesday morning, and the associated warm/q-
stationary front extends from the surface low over western Oklahoma
northeast through Kansas into southern/central Iowa.  Southerly
850mb flow in the deterministic GFS and ECMWF generates weak to
moderate low level moisture convergence in the vicinity of the
front Wednesday.  This produces a few hundredths of an inch of QPF
in far northeast Missouri into west central Illinois Wednesday
morning.  The combination of some light precip in the morning and
more extensive cloud cover closer to the front will keep
temperatures several degrees cooler in northeast Missouri and west
central Illinois on Wednesday.  The deterministic temperature
forecast from the NBM has highs around 80, however the max temp IQRs
are around 7-8 degrees, and the 25th percentile stays in the 70s.
Therefore highs in the mid to upper 70s are certainly possible in
northeast Missouri and west central Illinois Wednesday afternoon if
the cooler solutions come to pass.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Issued at 307 AM CDT Tue Sep 29 2026

The trough drifts east through the central Plains Wednesday night
through Thursday night and drags a cool high pressure system out of
southern Canada into the northern Plains and Upper Midwest.  This
pushes the surface front slowly southeast out of Iowa and through
Missouri and Illinois by 00Z Saturday.  The trough will still have
residual tropical moisture from the remnants of hurricane Polo
wrapped into it, and locally heavy rain is still a possibility.
However, model trends continue to favor northwest Missouri for the
heaviest rain.  LREF mean precipitable water values drop as the
front moves from northwest into central and northeast Missouri
Thursday, and continue to drop slowly through Friday.  The latest
storm-total QPF for Wednesday night through Friday night from the
NBM ranges from about 2.5 inches in northeast Missouri to a little
over an inch in south central Illinois.  Flash flood guidance is
lowest where QPF is highest in northeast Missouri, but it`s still
1.75+ inches in 1 hour.  Forecast soundings are still supportive of
heavy rain with warm cloud depths in excess of 12kft and a deep
moist layer aloft.  However there occasionally indications of dry
air entrainment which would likely eat into rainfall totals, and the
front remains progressive through our area which lowers the chance
for training.  Bottom line is that occasional heavy rain is possible
given the thermodynamics and remnant tropical moisture, confidence
that there will be enough rain to cause flooding is still low at
this time.

Temperatures in the rain on Thursday in parts of central and
northeast Missouri as well as west central Illinois will drop back
to near or below normal in the mid to upper 70s.  The remainder of
the area will see another warm day in the mid to upper 80s well
south of the front and the more persistent rain and cloud cover.
Cool high pressure builds across the Midwest and Great Lakes Friday
into Saturday north of the front.  It also looks like there will be
lingering clouds and rain, mostly along and south of I-44 in
Missouri and I-64 in Illinois both Friday and Saturday. Guidance
temperatures drop back to near and below normal between the
lingering precipitation and cooler airmass moving into the region.
Another short wave aloft moves across the Upper Midwest and Great
Lakes late in the weekend which brings a reinforcing shot of cool
air down into the Mississippi Valley late Sunday into Monday.  This
keeps temperatures several degrees below normal int the upper 60s to
around 70 Sunday and Monday.  Could even see temperatures falling
below 50 over a good portion of the forecast area on Monday morning
as the reinforcing high settles across the Midwest.

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 617 AM CDT Tue Sep 29 2026

VFR flight conditions and light southerly flow is expected to
prevail through the period. A few showers and possibly a
thunderstorm are possible across northeast Missouri into west
central Illinois late tonight into Wednesday morning. Ceilings
are expected to remain above 3000 feet, but a stronger shower or
storm may briefly reduce the visibility to 3SM or less.

&&

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

&&

$$

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