721
FXUS63 KLSX 270636
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
136 AM CDT Sun Sep 27 2026

.KEY MESSAGES...

- Widespread rain is forecast beginning late Tuesday and last
  through late this week. Northeast Missouri and west-central
  Illinois have the best chance at receiving higher end amounts.


- A gradual warm-up will last through the early week with most seeing
  highs in the 80s on Tuesday.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 131 AM CDT Sun Sep 27 2026

Shower activity will decrease in coverage today as the low-level jet
dies and our once ample moisture wanes. Clouds are expected to break
up overnight from east to west, leaving portions of south-central
Illinois under mostly clear skies for the majority of the night.
Given the dry air and light winds already in place, temperatures
will be allowed to plummet under the effect of radiational cooling.
Although low-50s are forecast, some may see their first drop into
the 40s for the season. To the west where clouds will linger lows
will barely dip into the 50s. A gentle warm-up is in store for most
of the area through early this week. Given a lack of warm air
advection, today`s hop into the mid-70s can be attributed to a lack
of rain and decreasing cloud cover. Tomorrow, insolation will be the
primary driver of the 1-3 degree increase in temperature
expected.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 131 AM CDT Sun Sep 27 2026

The ECMWF extreme forecast index and shift of tails show that models
have high confidence in an anomalous rainfall event impacting
Missouri later this week. With PWATs forecast near their
climatological maximum, a deep longwave trough ejecting plenty of
energy into the region, and slow moving fronts stationed nearby,
this isn`t hard to believe. Currently, ensembles and global models
place the axis of heaviest rainfall from Kansas northeast into Iowa
late Tuesday into Wednesday. This is where confidence is highest in
heavy rainfall. To the southeast, where we reside, is where
uncertainty in rainfall amounts grows. Although guidance is
consistent regarding the general pattern, there are plenty of
discrepancies model to model including location, amplitude, and
phasing of the longwave trough and therefore the footprint and
strength of low-level moisture advection. This isn`t considering the
potential for model under-dispersion of the tropical moisture
or the speed at which this moisture is used up before it reaches
our area. In short, there are multiple failure points
concerning this week`s heavy rain potential.

If it is fully realized, localized pockets of 4+" cannot be ruled
out in northeast Missouri and west-central Illinois as suggested by
the LREF 90th percentile of total precipitation ending Saturday.
Much of the soil moisture in these areas falls below the 5th
percentile of climatology, which is beneficial from a flooding
perspective. What also helps is that the rain will be falling over
multiple days, so the potential for rapidly rising water appears low
at this point. This system will continue to be closely watched as it
evolves over the coming days.

&&

.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...Jaja
LONG TERM...Jaja
AVIATION...Pfahler