587
FXUS63 KLSX 291956
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
256 PM CDT Tue Sep 29 2026

.KEY MESSAGES...

- Widespread rain is expected along a slow-moving cold front
Wednesday night through Friday. A swath of 2-3 inches of rain is
most likely across northern Missouri and west-central Illinois, with
decreasing values further south and east.

- There are increased chances for river flooding by next week along
the Missouri River, Mississippi River, and northeast Missouri
tributaries due to heavy rain in and upstream of these basins.

- Temperatures stay abnormally warm through Thursday before a return
of cooler, seasonable air later this week.

&&

.SHORT TERM /THROUGH WEDNESDAY/...
Issued at 255 PM CDT Tue Sep 29 2026

Tranquil weather remains fixed across the Mid-Mississippi Valley at
this hour, with seasonable temperatures and mostly-sunny skies
resulting from a departing surface high. Further west, an amplified
upper-level trough evident on GOES-East mid-level water vapor
imagery is drawing moisture from the deteriorating T.C. Polo in the
Gulf of California. This moisture is spreading highly-anomalous
precipitable water values (near climatological maxima) into the
Plains along a cold front stretching from the Desert Southwest to
Minnesota. This feature along the upper-level wave will gradually
approach us from the west, but before that mainly dry conditions are
forecast tonight. Temperatures tonight/tomorrow morning will likely
be 10-20 degrees warmer than today`s lows thanks to increased cloud
cover and strengthening southerly winds.

As the wave slowly ejects from the Desert Southwest, cyclogenesis
lee of the Rockies will allow for a west-east oriented moisture axis
along a developing stationary front to stretch into far northern
Missouri, southern Iowa, and west-central Illinois. There, abundant
cloud cover and intermittent showers/weak thunderstorms will keep
temperatures fairly cool. Further south, stout southerly winds and
lessened cloud cover will allow temperatures to jump into the upper
80s and approach 90 degrees in spots. Otherwise, much of the day
Wednesday looks dry with stalled/rising heights aloft really
suppressing any isentropic lift (and precipitation chances) in the
warm sector through Wednesday night. Further north of our region,
moderate to heavy rain will be underway amidst the copious moisture
along the boundary. Some of this may drift south into northeast
Missouri, but I expect most of the night to remain dry for our
forecast area.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
Issued at 255 PM CDT Tue Sep 29 2026

By early Thursday morning, impressive synoptic lift and some
transient but present isentropic ascent arrives in the central
Plains to further support continued moderate to heavy rain across
that expansive area. Locally, this heavier rain continues to be
focused across northern Missouri and west-central Illinois,
with increasing signals in the ensemble guidance for a sharper
gradient with southward extent. Probabilities for exceeding 2
inches of rain in this axis across northern Missouri continue to
increase according to the NBM and LREF, with localized
probability-matched mean output from the HREF and REFS beginning
to paint concerning localized amounts of 4-6 inches. Most of
this higher-end potential will likely miss us to the north
through Thursday, but a swath of up to 3 inches appears more
likely to clip northeastern Missouri by late Thursday. We will
need to monitor this potential, but as it stands the duration of
the rainfall likely precludes a flash flood threat. The current
Flood Watch in northwestern Missouri was not extended further
east.

By Thursday night, the wave aloft becomes weaker and more
progressive. As a result, all available guidance begins to shift the
waves of rain south and east. That said, as the wave weakens, the
synoptic support that maintained a favorable heavy rain environment
becomes more muted. Moisture transport from the more tropical
airmass further south and west also weakens, but ambient
precipitable water values remain quite high until the front clears
the region. What this spells for those closer to St. Louis and areas
south/east is decreasing widespread rainfall amounts with scattered
instances of higher totals where slightly deeper convection can
occur. While high-end potential evident in cluster analyses of the
LREF does still highlight a secondary corridor of heavier rainfall
along I-44 into the St. Louis metropolitan area, most guidance
favors a quicker solution with lower amounts. The front largely
clears the region by late Friday, with some low threat for weak
anafrontal showers lingering into Saturday. By that point, a
reinforcing wave looks likely to not only force the front further
south, but also usher in cooler, drier air for the weekend and early
next week.

The increasing potential for minor to moderate river flooding on the
Mississippi and Missouri Rivers and their tributaries becomes a more
concrete concern 4-7 days following this rainfall despite the
heaviest rain falling to our north. Some output from the Hydrologic
Ensemble Forecast System suggests low (15-30%) chances for major
flooding in some spots on the mainstems north and west of St. Louis
in a reasonable worst-case scenario. This worst-case would come to
fruition only if the front were slower and synoptic forcing/lift
were stronger, adding more rain to the system upstream. No River
Flood Warnings are in effect for our region, but close attention is
being paid to the threat as it materializes in the coming days.

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1207 PM CDT Tue Sep 29 2026

Dry and VFR flight conditions are forecast through the valid TAF
period. Intermittent LLWS may impact our central Missouri
terminals tonight but appears more like turbulence than shear.
Rain chances increase at KUIN and the central MO terminals after
the TAF period, extending areawide by late-week, which will have
some impact on flight conditions.

&&

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

&&

$$

SHORT TERM...MRB
LONG TERM...MRB
AVIATION...MRB