804 
FXUS63 KLSX 270632
AFDLSX

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

.KEY MESSAGES...

- Widespread rain is forecast beginning late Tuesday and lasting
  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