558 
FXUS63 KLSX 270301
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1001 PM CDT Sat Sep 26 2026

.KEY MESSAGES...

- Confidence continues to increase in widespread rainfall
  arriving Tuesday through mid-week, with the highest amounts
  (as high as 5.00 inches locally) confined across northern and
  north- central Missouri.

- Temperatures gradually warm through the next several days
  before cooling again, all staying roughly around late-
  September normals.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 231 PM CDT Sat Sep 26 2026

Mid-level water vapor imagery from GOES-East nicely depicts 
diffluent 500mb flow across the central CONUS that is providing 
enough ascent coincident with ample moisture to force showers 
and mid-level stratus over the region. No instability of note 
exists, precluding a threat for deep convection or 
thunderstorms. The main effect of the rain and clouds is to 
keep temperatures in check, with most locations struggling to 
warm past the upper 60s/low 70s again today. Forcing and 
moisture availability both become far less impressive this 
evening, which will end the light rain from west to east. 

We remain seasonably-cool and largely dry under the influence of 
easterly low-level flow from the ridge centered across the Great 
Lakes. As a modest low-level jet amplifies tonight over eastern 
Kansas and far western Missouri, additional showers and weak 
thunderstorms are expected to redevelop and meander across western 
and central Missouri overnight. With the LLJ waning and convection 
moving further from the initiation source, any rain will struggle to 
maintain itself east of a line roughly from Knox County through 
Osage County. Sunday appears largely dry and tranquil after the rain 
dissipates, with winds gradually turning more southeasterly 
resulting a warmup of a few degrees compared to 24 hours prior. The 
only potential for showers exists across the northern Ozarks, where 
topography may aid in some weak lift of a humid airmass. Any threat 
for rain ends with sunset, at which point yet another seasonably-
cool night is in store.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Issued at 231 PM CDT Sat Sep 26 2026

We begin the work week much like we ended it: seasonable (mid/upper 
70s for highs) and largely dry. The surface ridge stalled over the 
Great Lakes drops further south and west by then, shunted in this 
direction both by impulses in the longwave trough across the western 
CONUS/southern Canada and by the slight retrograde of the nor'easter 
ongoing off the Northeast U.S. coastline. The longwave trough will 
also serve to assist in the transport of tropical moisture from what 
remains of T.C. Odalys and T.C. Polo in the eastern Pacific (see NHC 
bulletins for more information). Precipitable water values are 
consistently forecast to approach climatological maxima and couple 
with deep southwest diffluence, two important large-scale factors 
needed for an increased heavy rain threat. 

The pattern currently modeled in a preponderance of ensemble and 
deterministic guidance closely resembles those of predecessor rain 
events (PREs) that allow for extremely efficient rainfall. 
Resultantly, rain chances will be on the rise across northern 
Missouri and west-central Illinois by late Tuesday and become more 
widespread by Wednesday. This general area, particularly northern 
and north-central Missouri, have the best chances locally for heavy 
rain and flooding (both flash flood and river) due to a combination 
of relatively moist soil (per NASA SPoRT analysis) and quality of 
atmospheric moisture. Global-scale guidance is currently bearish on 
the probabilities of high rainfall totals (for example, only a 30-
40% chance of 2.00" of rain at most per the NBM), partially due to 
its coarse spatial resolution and partially owing to uncertainty in 
where the heaviest rain falls. That said, model soundings leave 
little doubt in efficient warm rain processes that could push multi-
day localized totals as high as 5.00 inches. Currently the 
experimental WPC Excessive Rainfall Outlook highlights northern 
Missouri in a 25-40% chance area (translating to the new "Enhanced 
Risk" in this product). 

Further into the week, rain becomes more widespread (particularly 
late Wednesday onward) as that remnant moisture lingers over the Mid-
Mississippi Valley. By this point, while rain chances are fairly 
high (60-80%) areawide, rainfall totals and the threat of flooding 
impacts becomes more uncertain. Some guidance begins to suggest the 
quality of this moisture becomes more modified now that is several 
days and over 1000 miles removed from its source region. If this 
comes to fruition, while still more than sufficient for rain to 
occur, the efficiency of that rain would lessen and impacts would be 
more limited. Still, some deterministic guidance and ensemble 
members point to that moisture staying very much in place with 
little modification. That scenario can't be discounted, as PREs have 
a tendency to overperform and remain prolific longer than models 
suggest. This threat will need to be monitored, however one factor 
decidedly in our favor is the far drier conditions across the 
southern two-thirds of Missouri and southern Illinois. Regardless of 
which scenario prevails, even the more parched parts of the region 
south of I-70 can expect at leas 0.50 inches of rain (per the NBM). 
A quasi-zonal pattern looks poised to settle over the CONUS, opening 
our region to renewed rain chances through next weekend while 
keeping temperatures in check as we start October.

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 1000 PM CDT Sat Sep 26 2026

Showers will increase in coverage overnight mainly across 
central/northeastern MO into west-central IL including KCOU, KJEF, 
and KUIN with uncertainty in how far east they will reach before 
waning through mid-Sunday morning. Most showers will be light and 
unimpactful, but a few brief, heavier showers could result in MVFR 
flight conditions at KCOU and KJEF. Otherwise, VFR flight conditions 
will prevail with light winds.

&&

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

&&

$$

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