780
FXUS63 KLSX 280835
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
335 AM CDT Mon Sep 28 2026

.KEY MESSAGES...

- Temperatures will warm a few degrees today, and then back
  above normal into the 80s Tuesday and Wednesday.

- Rain is expected to overspread the area late Wednesday night
  into Thursday, and continue through Friday. 1-3 inches of rain
  is possible across the area, with the higher end of this range
  most likely across central and northeast Missouri into west
  central Illinois.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 317 AM CDT Mon Sep 28 2026

This morning`s upper air analysis shows a high amplitude pattern
with a ridge over the central U.S. and lows to either side over both
coasts.  Models show the pattern shifting slowly eastward over the
next couple of days, with the ridge axis moving from the Great
Plains and Upper Midwest into the Mid Mississippi Valley and Great
Lakes by Tuesday afternoon.  High pressure over the Midwest at the
surface will also drift east through the Ohio Valley into the Mid-
Atlantic as the pattern shifts.  Low level flow will turn to the
east today, and then to the south Tuesday as the high moves.
Increasingly southerly flow and rising heights will allow
temperatures to rise a few degrees today and Tuesday.  Forecast
soundings are also hinting at less cloud cover across Missouri and
Illinois than over the past couple of days which will help push
temperatures up as well.  Deterministic guidance pushes 850mb
temperatures to 12C-14C today and to around 16C Tuesday.  Think NBM
mid 70s highs look good today given these factors, particularly
since it looks like clouds will stick around for at least part of the
morning.  Low to mid 80s are reasonable for Tuesday with the more
persistent southerly flow and warmer 850mb temps.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Issued at 317 AM CDT Mon Sep 28 2026

The warm up continues Wednesday as the upper ridge continues
drifting east into the Ohio Valley.  This brings deep southwest flow
to the Mid Mississippi Valley, with 850mb temps rising to 18C-20C
according to the deterministic GFS and ECMWF.  This agrees well with
850mb temps forecast by the LREF which is only showing around 1
degree difference between the 25th and 75th percentile. 18C-20C at
850mb would mix down to 88F-91F at the surface Wednesday afternoon,
which is very close to the NBM max T forecast.  Cooler temperatures in
the low 80s are showing up across northern Missouri and west central
Illinois, but this can be explained by more extensive cloud cover
ahead of the next upstream trough.

The trough will pick up the remnants of hurricane Polo on Tuesday as
it drifts across the Desert Southwest, and latest guidance has it
beginning to impact our forecast area Wednesday night into Thursday
morning.  The incorporation of tropical moisture from Polo brings
the possibility of heavy rain to the area as the trough drifts
through the Plains into the Mississippi Valley on Thursday and
Friday.  LREF mean precipitable water values rise to around 2 inches
and forecast soundings show warm cloud depths in excess of 12kft
Thursday into Friday.  With this in mind, there will be a threat for
locally heavy rain late Wednesday night through Friday night.
However, QPF guidance has trended lower over the past couple of runs
as the trough and surface front are more progressive through the Mid
Mississippi Valley.  LREF mean QPF totals through Saturday are in
the 1.25 to 1.75 inch range, and even the 90th percentile tops out
at about 3.30 inches over a period of 48-56 hours.  That much rain
over a 2+ day period shouldn`t cause too many problems, but some
localized flooding will be possible in flood-prone locations if
there`s training or we ultimately get more rain than expected.
Temperatures cool off for the end of the week into the weekend,
first due to clouds and precipitation on Thursday and Friday.
Current guidance then pushes the cold front through the forecast
area Friday night which will bring a cooler and drier airmass into
the Midwest for the weekend.  Temperature IQRs in the LREF are
around 8 degrees Saturday and Sunday, but even the 75th percentile
high temperatures are a degree or two below normal.  This leads to
high confidence in cooler weather at the end of the forecast period.

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1035 PM CDT Sun Sep 27 2026

Dry and VFR flight conditions will likely persist through the TAF
period, since it is still uncertain if and where sufficient clearing
will take place for patchy fog development overnight into Monday
morning. Light winds will vary from northerly to easterly.

&&

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

&&

$$

SHORT TERM...Carney
LONG TERM...Carney
AVIATION...Pfahler