299
FXUS63 KLSX 301946
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
246 PM CDT Wed Sep 30 2026

.KEY MESSAGES...

- Widespread rain will overtake the region early tomorrow
  morning and last through Friday morning. One to two inches of
  rain are expected where rain is heaviest with locally heavier amounts
  possible.

- There is a medium chance of minor flooding on the Mississippi
  River and northeast Missouri tributaries next week due to
  upstream heavy rain.

- Near record highs will continue through tomorrow. A cold front
  will then drop temperatures into the 70s for highs and 40s and
  50s for low for the foreseeable future.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 242 PM CDT Wed Sep 30 2026

This week`s much anticipated rainmaker has nearly arrived.
Currently, a mid-level closed low is swirling over the Four Corners
states and flinging vorticity lobes and tropical moisture across the
country and into the Mid-Mississippi Valley. At the surface, a quasi-
stationary front extends northward through the Central Plains
providing a source of forcing for the mid-level lift and moisture in
place. Portions of northwestern Missouri have already reaped the
benefits of this setup, with MRMS estimates of 0.25" to 0.50" as of
noon. Our CWA will have to wait until early tomorrow morning to see
any appreciable rainfall. It`s only then that a shortwave will push
the stationary front and cause it to progress southeast.

This progression is one of the reasons why we don`t expect impactful
accumulations across the area. Although we will have a deep warm
cloud layer of ~13k feet and PWATs above the 99.5th percentile of
climatology, MUCAPE values are very slim (up to 250 J/kg) and short-
lived, and soundings show signs of subsidence at times through the
day tomorrow. This doesn`t exactly bode well for high rainfall
rates, which is the second reason we don`t expect impactful
accumulations. HREF 1 hr QPF reflects this well, with rates only
reaching 0.25" to 0.50" per hour. On the unsaturated soil we
currently have, this will not be enough to support flash
flooding. Overall, the forecast remains similar to the last
issuance. Amounts of 1" to 2" are expected in northeast Missouri
and west- central Illinois. Notably, models have been hinting
at a second corridor of heavier rain extending from
central/south- central Missouri into east-central Missouri. For
the reasons listed above, 1" to 2" are also expected within the
corridor if it comes to be realized. Locally higher amounts are
possible areawide.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Issued at 242 PM CDT Wed Sep 30 2026

The rain is expected to shut off by Friday afternoon as a cold front
exits the area, but we may still be feeling this system`s impacts
next week. Upstream rain traversing the Mississippi River is
forecast to make it into the CWA late this weekend and to Chester by
next weekend. This has the potential to cause minor flooding at
multiple points, but exactly where and when the points crest remain
to be seen.

Otherwise, cooler and drier air will drive into the region behind
the cold front. High temperatures between Thursday and Friday could
drop as much as 15-20 degrees for most of the area east of I-55 in
Illinois and south of I-44 (~90 degrees to ~70 degrees). Low
temperatures will feel the shift, too, albeit slightly more
gradually. Expect widespread lows in the 40s and 50s by Friday
night. The change of pace is expected to last, too. The NBM IQR
shows highs comfortably in the 70s through early next week. Even its
75th percentile stays in the 70s for the majority of the period.
Rain will be all but a ghost as well, as the LREF 24 probs of a
trace of rain peak at 15% through next Thursday.

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1218 PM CDT Wed Sep 30 2026

VFR and mostly dry conditions are forecast through tonight. Very
early Thursday morning, marginal LLWS is possible areawide. Did
not add mention to the TAFs based on it only being speed driven
and not especially potent. A shield of rain is forecast to
overtake the region from northwest to southeast around sunrise
ahead of a cold front. Ceilings are forecast to drop to IFR
shortly after rain arrives and visibilities will likely drop to
MVFR. In any stronger cells, IFR conditions will be possible.

&&

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

&&

$$

SHORT TERM...Jaja
LONG TERM...Jaja
AVIATION...Jaja