River Flood Warning 110, NWS St. Louis, MO

Area Forecast Discussion
National Weather Service St Louis MO
648 AM CDT Sun Sep 20 2026

.KEY MESSAGES...

- A cold front begins slowly moving through the region today
  bringing cooler temperatures the rest of the week.

- Thunderstorms along the front tonight bring the best chance
  for rain all week.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 315 AM CDT Sun Sep 20 2026

Showers and thunderstorms are currently moving northeast through
northern MO and southeastern IA along a cold front and along the
nose of the low-level jet. As they progress, the convection in
northern MO is moving into a region with less favorable
conditions and a weakening low-level jet to continue to support
their development. As a result, these storms don't pose a
threat to our area aside from a stray shower in our northern
counties during the early morning hours.

An upper level ridge, which has been the driver behind the heat
and humidity during the last several days, is beginning to be
suppressed by an upper level trough as it moves east. This
trough will push the cold front into our area starting today.
Temperatures today will still be hot and humid to the south of
the front as it begins its slow push through the region. This
will lower high temperatures into the 70s and 80s in northern
MO, while southeastern MO will remain well into the 90s. Thanks
to slightly more moderate humidity values in the region,
widespread 100+ heat index values are not expected. Mondays
high temperatures are slated to be even lower as the front makes
its trek south through the region. However, there still remains
some uncertainty in the exact high temperatures for Monday.
Latest NBM guidances interquartile range for the high
temperatures is 6-10 degrees across the region due to the
uncertainty in the fronts exact position and speed. That being
said, behind the front we can expect high temperatures in the
70s and 80s with some areas in northern MO experiencing upper
60s for high temperatures.

Showers and thunderstorms are expected along and behind the cold
front tonight into Monday morning. While the confidence is low
on the exact timing and location of this convection, HREF
guidance shows there is a 80-100% chance across the region of
measurable rainfall during this period, and 30-70% chance of
seeing 0.25 inches of rain. These showers and thunderstorms will
be multi-cellular with the strongest thunderstorms possibly
producing heavy localized rainfall and gusty winds in regions
where CAPE is sufficient (~1500-2000 J/kg). Due to the limited
nature of the severe threat, the severe potential is not being
messaged for this event.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 315 AM CDT Sun Sep 20 2026

The end of the relentless heat is in sight. Behind the front
temperatures are expected to be much cooler. For Tuesday and
Wednesday, NBM guidance shows that high temperatures will be in
the low to mid 70s, with the northern portions of the area
possibly staying in the upper 60s for their high temperatures
(60-80% chance). Low temperatures will also dip into the 50s.
This air mass will continue to dominate for the rest of the week
with a gradual warming through the 70s, reaching the low 80s by
next weekend.

The GFS and ECMWF both show a mid-level ridge building over the
Great Lakes and into the southeastern Canada region with a
center of low pressure over the Ohio Valley by mid week. The
ridge should remain fairly stationary over southeastern Canada,
while the low pressure center will slowly move to the east coast
of the CONUS. This pattern will allow for continued westerly and
northwesterly flow to help moderate our temperatures into next
weekend and limit the moisture transported into the region,
preventing another return of the heat and humidity this week.

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 647 AM CDT Sun Sep 20 2026

A cold front is moving south through the area bringing chances
of showers and thunderstorms. UIN, JEF, and COU will be impacted
by some showers this morning, but the best chances for
thunderstorms for all terminals will be tonight. There lies some
uncertainty with the exact coverage and timing of the
convection, so TEMPOs and PROB30s were included for the time
periods with the highest confidence. VFR conditions are expected
ahead of the front, but ceilings are expected to drop quickly to
IFR after it passes the terminals.

&&

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

&&

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