094
FXUS63 KLSX 130905
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
405 AM CDT Sun Sep 13 2026

.KEY MESSAGES...

- Unseasonably hot weather is expected this week with heat index readings
  at or above 100 degrees in some locations Tuesday into
  Thursday.

- There are chances (30-70%) of showers and thunderstorms each
  day that are mainly confined to parts of northeast Missouri
  and west- central Illinois near a stalled front.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 403 AM CDT Sun Sep 13 2026

The latest surface analysis showed that the surface front was
located across the southern CWA with light northerly winds to the
north of it.  The light northerly flow is bringing slightly drier
air into the region so there are no fog reports so far.  We may
still see localized fog closer to front or in river valleys in the
next few hours.

Showers and thunderstorms were occurring along the MO/KS border
on the nose of a low level jet. I do not expect this activity
to affect our area. There is a shortwave trough seen digging in
water vapor imagery into the north central CONUS that will move
through the Upper Midwest today which may be enough to kick off
a few showers and thunderstorms across northeast Missouri and
west central Illinois this afternoon and early evening. Farther
south, RAP soundings are showing some weak instability in the
vicinity of the cold front this afternoon warranting keeping 20%
PoPs over parts of central into southeast Missouri. There will
likely be a brief lull this evening before we see additional
showers and thunderstorms develop over western Missouri that
move northeast across parts of central/northeast Missouri into
west central Illinois late tonight into Monday morning.

There will be decent contrast in highs today with the northerly
winds and the front over the southern part of the CWA.  Highs will
range from 70s over north to near 90 over the south.  Highs will be
mainly in the 80s on Monday with the lower 90s over central
Missouri.  With dewpoints in the 70s, heat index readings over
central Missouri will be close to 100 degrees on Monday afternoon.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 403 AM CDT Sun Sep 13 2026

The LREF continues to show an unseasonably strong upper high
building over southeastern CONUS setting up southwesterly flow
across Missouri and Illinois Tuesday into Thursday. This will
bring 850mb temperatures of 20-22C Tuesday into Thursday (98th
percentile) which supports the NBM having high temperatures in
the 90s over most of the area. With this said, there will be a
stalled front in the area beneath the upper flow, quite possibly
across the northern CWA with the NBM 25th/75th percentile
spread at UIN from Tuesday-Thursday being 8-12 degrees compared
to STL which is only 3-4 degrees. This spread increases at STL
late in the week as the upper ridge weakens and the upper flow
becomes zonal which increases the chance the front moves south.

With highs in the 90s Tuesday into Thursday and dewpoints back in
the 70s, heat index readings of 100-105+ are forecast mainly along
and south of I-70 Tuesday into Thursday.  The chance for dangerous
heat is expected to drop off by end of the week as the front moves
south bringing clouds and a chance of rain.

The LREF has 40-70% of it members producing rain at UIN Tuesday into
Wednesday with WPC highlighting the northern part of the CWA in a
slight risk for excessive rainfall.  Can`t rule out some locally
heavy rainfall in this part of the CWA during this time frame.  The
LREF also shows 30-50% of members producing rain at STL Thursday
into Saturday as the front potentially moves southward.

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 1056 PM CDT Sat Sep 12 2026

VFR conditions with light winds are expected to continue through the
TAF period. Confidence has increased that showers and thunderstorms
will stay far enough west that JEF and COU should not be
impacted.

&&

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

&&

$$

SHORT TERM...Britt
LONG TERM...Britt
AVIATION...Asel/Kimble