708
FXUS63 KLSX 210013
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
713 PM CDT Sun Sep 20 2026

.KEY MESSAGES...

- A cold front will move through the region tonight
  bringing showers and thunderstorms to the area.

- Starting Tuesday, seasonably cool temperatures and
  dry conditions are forecast for the remainder of the
  work week.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 112 PM CDT Sun Sep 20 2026

Today: A cold front is currently located just west of Columbia,
MO and just south of the IA/ MO border. The cold front has been
very slowly moving south and east this afternoon with no
precipitation. This is likely due to a weak mid-level wave over
eastern IL with MO in weak mid-level subsidence. Expect high
temperatures 5 to 10 degrees colder today compared to yesterday
(Saturday).

Tonight: A weak mid-level wave over western Missouri will
approach from the west with baggy weak surface low pressure
moving northeast across southeastern Missouri. This will pull
the surface cold front south towards I-70. This wave interacting
with a surface cold front will likely (60 - 70%) initiate
multiple showers and thunderstorms across the region. There does
appear to be a anafrontal component as the dry conveyor belt
rapidly approaches from the west and PVA rides over the surface
front. The threat for severe weather looks very low given the
CAPE is elevated and skinny. The slightly higher threat does
appear to be for heavy rainfall. Most areas will observe 0.25"
to 0.50", with 30% chance of observing 1" - 3". WPC does have
the St. Louis Metro in a slight risk to account for this, but
flooding impacts should be limited. Precipitation will likely
start around 7 PM and not come a complete end until 7 AM Monday
morning. As the rain ends, there is a brief window for patchy
fog to develop. The highest threat area will be across central
Missouri which is where showers and thunderstorms will come to
an end before sunrise while also being sufficiently on the cool
side of the boundary.

Monday: The surface cold front will clear the region with
showers and thunderstorms coming to an end in the morning.
Behind the front, widespread cloud cover is forecast with high
temperatures only in the upper 60s (north) to mid 80s (far
south). We can`t also rule out some patchy drizzle along and
north of I-70 as warm cloud depth saturation is approaching 1.5
to 2 km in places. There is weak CAA on the backside of the
cold front which should help limit the drizzle potential, but
this level of low level saturation with dry mid-levels is hard
to ignore.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Issued at 112 PM CDT Sun Sep 20 2026

Seasonably cool temperatures and dry weather is forecast through
most of the long term, with global models and ensembles in
relatively good agreement.

Tuesday morning, a weak closed low (mentioned above) will be
located over Wisconsin and heading east. Simultaneously, mid-
level ridging over Ontario will help to slow the eastward
progression of this wave. At 250 mb, the core of the
ageostrophic convergence will be near central Missouri which
means sinking air, no precipitation, and seasonably cool
temperatures are forecast. Another mid-level wave will then dive
south out of Quebec constructively amplifying with the wave
from the west. This low will cutoff from the main flow with a
pseudo Rex Block forming. This will allow the dry and cool
temperatures to persist through Friday. The latest run of the
NBM and LREF is advertising an IQR of only a few degrees which
matches the high confidence of the WPC Cluster Analysis.
Currently, Wednesday and Thursday morning appears to be the
coolest with lows in the upper 40s to lower 50s.

Saturday into Sunday, model guidance shows a mid-level wave
approaching from the west but confidence does decrease this far
in the extended. Some moderation in temperatures are forecast,
but even this far out the NBM and LREF only has IQRs of 5 to 10
degrees.

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 635 PM CDT Sun Sep 20 2026

There will be an increasing chance (30-60%) for showers and a
few thunderstorms at the COU/JEF and the St. Louis area
terminals between 03-11Z and at UIN between 07-10Z. The heavier
showers and thunderstorms will have the potential to reduce
ceilings and visibilities to MVFR and IFR conditions. Then
ceilings are expected to fall below 1000 feet at UIN around 02Z,
COU/JEF around 04Z, and the St. Louis area terminals around 11Z
and remain IFR the rest of the period. Winds will stay out of
the north to northeast at 10 knots or less.

&&

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

&&

$$

SHORT TERM...BAH
LONG TERM...BAH
AVIATION...Britt