414
FXUS63 KLSX 300830
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
330 AM CDT Wed Sep 30 2026

.KEY MESSAGES...

- Widespread rain will overspread the area from late tonight
  through Thursday, and continue Thursday night. Rain is
  expected to end from north to south from early Friday morning
  through the afternoon. One to two inches of rain is expected,
  primarily across central and northeast Missouri and west
  central Illinois. Locally higher amounts are possible.

- Above normal temperatures will continue today and Thursday for
  most of the area. A cold front will move through the area by
  Friday morning, bringing below normal temperatures back to the
  area for the weekend into next week.

- Heavy rain upstream may cause flooding on the Mississippi and Missouri
  Rivers rivers as well as northeast Missouri tributaries this
  weekend into early next week.


&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 326 AM CDT Wed Sep 30 2026

A closed upper level low over Arizona will drift east-northeast
through Thursday.  This low is in the process of ingesting residual
tropical moisture from the remnants of Tropical Storm Polo, and it
will spread this moisture into the Plains and Mid Mississippi Valley
over the next couple of days.  The upper level system will force low
level cyclogenesis over the Great Plains as it moves east-northeast
today and tonight, resulting in frontogenesis over southeast
Nebraska and Iowa.  Guidance shows the front remaining nearly
stationary stretching from central Kansas through southeast Nebraska
into Iowa into tonight.  The front then starts drifting slowly to
the southeast on Thursday.  Initial showers this morning across
parts of central an northeast Missouri into west central Illinois
match up nicely with an area of weak low level moisture convergence
ahead of the approaching low.  This activity should weaken and shift
north through the day as forcing shifts farther north and west
closer to the front.  A few short-range models are redeveloping some
showers this afternoon in central Missouri.  Soundings show a pretty
substantial dry layer near the surface, but also an area of elevated
instability in a pocket of moisture convergence this afternoon. Have
therefore decided to a chance for showers this afternoon and early
evening in central Missouri.

Rain chances shift north and west tonight, mainly out of our
forecast area into northwest Missouri and Iowa closer to the front.
However, a northern stream short wave will dip into the Midwest late
tonight into Thursday which will begin pushing the front southeast.
Meanwhile the closed upper low over the Plains will be weakening and
will slowly be absorbed into the mean flow aloft.  The moisture from
the remnants of Polo will spread southeast Thursday ahead of the
front as the now open wave moves into the lower Missouri and Mid
Mississippi Valley Thursday into Thursday night.  Models are still
advertising a good bit of rain as the front passes and the trough
moves across the area.  However precipitable water amounts are
forecast to be highest today and tonight well west of our area
across northeast Kansas, northwest Missouri and into Iowa.  P-wat
values drop off Thursday as the front moves southeast, and the
synoptic-scale forcing weakens as the upper low opens up and gets
pulled into the mean flow.  With that in mind, storm total QPF
amounts have backed off a bit over our area, now ranging from around
1/2 inch in southwest Illinois to about 2 inches in northeast
Missouri with the majority of that falling Thursday and Thursday
night.  Forecast soundings remain supportive of heavy rain with tall
skinny CAPE profiles and warm cloud depths in excess of 12kft.  The
front still looks progressive though, so training should be minimal.
 This is backed up by storm total.  There are a few streaks of 2+
inch QPF Thursday night in the REFS LPMM, however if this occurs
over a 6-12hr period as depicted in the model the soils and streams
should handle it without widespread flooding issues.  Will therefore
continue to refrain from issuing any flood headlines at this time.

Today will likely be the warmest day of the next 7 despite a decent
amount of mid and high cloud cover.  A warm start to the morning in
the mid and upper 60s, deep mixing, and a warm southerly flow will
produce highs in the upper 80s to around 90 today, except in
northeast Missouri and west central Illinois where thicker and lower
clouds as well as a few showers will suppress temperatures. Cooler
temperatures will spread southeast as the front moves into the area
with more widespread rain on Thursday.  80s remain likely along and
east of I-44 in Missouri and I-55 in Illinois with 70s
filtering in farther west.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Issued at 326 AM CDT Wed Sep 30 2026

The latest GFS and ECMWF now push the surface front south of the
forecast area by 12Z Friday, although the mid-upper level trough
axis is hanging back over western Missouri.  Low level dry air lags
behind the surface front according to forecast soundings, and the
broad-scale synoptic lift provided by the trough continues to
produce light rain along and south of I-70 Friday morning.  Dry air
does eventually filter through the area by Friday afternoon/evening
as a broad, cool high pressure system builds into the Midwest and
western Great Lakes Region.  The remainder of the forecast through
Tuesday looks seasonably cool and dry.  Daytime temperatures drop
into the 60s and low 70s with overnight lows primarily in the 50s. A
reinforcing shot of cold air drops into the Mississippi Valley as a
second, stronger Canadian high moves into the Midwest Sunday night
and Monday.  Guidance shows this cold airmass producing some of the
coldest overnight lows of the season so far, with temperatures
bottoming out in the mid to upper 40s both Monday and Tuesday
morning.

Will continue to monitor the longer term river flood threat on the
Missouri and Mississippi Rivers, as well as the tributaries.
Ensemble hydrologic model output almost universally puts the IQR of
river stages above flood at many of our main-stem river points by
late this weekend or early next week, mainly due to rainfall well
upstream.  To be honest, I take this with a large grain of salt as
only a small amount of the forecast QPF has fallen at this point.
The vast majority of the rainfall that`s being forecast and fed into
the river models has yet to materialize, and there are many factors
which will affect the levels to which our rivers ultimately
rise. Still, there is potential for flooding should some of the
heavier rainfall forecasts be realized.

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1003 PM CDT Tue Sep 29 2026

Showers and perhaps a rumble of thunder will continue across
north- central/northeastern MO into west-central IL at times
overnight before lifting northward mid/late Wednesday morning.
KUIN is most likely to be impacted, but outside of a brief
heavier shower or two, rain will be light and permit VFR
visibilities. Otherwise, dry and VFR flight conditions are
expected, with the next opportunity for showers and
thunderstorms not arriving until beyond the TAF period. Light
south/southeasterly winds will become southwesterly during the
daytime Wednesday, before backing again to southeasterly.

&&

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

&&

$$

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