Severe Thunderstorm Warning 250, NWS St. Louis, MO

Area Forecast Discussion
National Weather Service St Louis MO
711 PM CDT Wed Sep 9 2026

.KEY MESSAGES...

- Periods of showers and scattered thunderstorms are expected across
  the area through Thursday.

- Thunderstorms through early this evening will be capable of
  isolated damaging wind gusts mainly across southeastern
  Missouri.

- Locally heavy rainfall is expected tonight through Thursday
  morning. There is a low chance of flooding, mainly across
  southeastern Missouri.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 323 PM CDT Wed Sep 9 2026

A shortwave is digging into the Northern Plains along the western
periphery of upper-level troughing per water vapor imagery this
afternoon. At the surface, a cold front is moving slowly through the
Midwest, and while it is north of the CWA currently, an outflow
boundary is serving as an effective front across southeastern
Missouri and southwestern Illinois. Along and south of this boundary
resides upwards of 2,500 J/kg of MLCAPE, which has lead to scattered
showers and thunderstorms. Deep-layer shear here is fairly weak -
around 20 kts or less - leading to single or multicellular
thunderstorms with shorter lifespans. As storms collapse, high LCLs
and inverted V soundings in this area will enhance downdrafts to
lead to a isolated threat of damaging downbursts through the early
evening. Confidence is low in storms becoming strong enough to
produce this threat, as a majority of model soundings show
subsidence around 500 mb that will tend to keep updrafts subdued.

Further north across the CWA, an area of low to mid-level
convergence and pooling moisture is contributing to the elongated
region of stratiform rainfall with embedded convective elements seen
in regional radar. This convergence will slowly move southward
through this evening as the aforementioned shortwave approaches the
CWA and a low-level jet ramps up. The result will be convection
enhancing and becoming more prevalent into the overnight hours. The
orientation of the forcing and low-level jet favors training showers
and thunderstorms roughly along the I-70 corridor through Thursday
morning. The HREF and REFS have been consistent with a footprint of
greater rainfall totals along this corridor reaching 2-3" with
isolated pockets of around 4". Given our recent lack of rainfall,
soils and local waterways are expected to readily absorb this
rainfall. However, there is a low chance that if locations in the
Ozarks see 4" or greater over the course of a couple of hours,
flooding could occur.

The shortwave moves overhead tomorrow with the effective boundary
moving south of the area. Therefore, rain chances will taper off
from northwest to southeast through the day as the post-frontal
airmass settles into the Midwest. Most locations tomorrow will see
temperatures top out around 80 - the coolest highs we've seen so far
this month.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Issued at 323 PM CDT Wed Sep 9 2026

For Friday through Sunday, guidance consensus is that weak troughing
will gradually diminish over the region as upper-level ridging
builds into the Midwest. While a cold front associated with a system
in southern Canada will swing south-southeastward toward the CWA,
the building ridge is expected to weaken the front and its
progression on Saturday. This front will also meet with a warm front
lifting northeastward associated with the building ridge. Both
features will provide a medium chance (up to 50%) for showers and
isolated thunderstorms mainly on Saturday.

The building ridge and warm front mark the start of another round of
anomalous heat for the CWA. Low-level southwesterly flow will advect
warm air into the region starting Sunday and particularly Monday.
Then, global ensemble members are tightly clustered around highs in
the low to mid-90s across the CWA, leading to high confidence that
this will be the warmest day of the period.

Guidance diverges on the phasing of the ridge for Tuesday and
beyond, leading to high uncertainty in our local weather,
specifically temperatures. If the ridge holds strongly over the
southern CONUS, then daily highs in the 90s will continue through
the middle of next week. However, if the ridge weakens, we could see
daily highs around 80 degrees. The current NBM and forecast attempts
to split the difference with a front stalling across our CWA,
leading to the warmer solution mainly for areas along and south of I-
70, with the cooler solution favored for areas north of I-70. While
this is also a reasonable outcome, confidence in any solution at
this point is low, and the forecast for this portion of the period
will change in the coming days.

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 708 PM CDT Wed Sep 9 2026

Current conditions are VFR at all terminals. Overnight the
coverage of showers and thunderstorms will increase, creating
prevailing periods of MVFR for each terminal. There remains some
uncertainty on the exact timing of the convection, so the TAFs
include a time window when our confidence is the highest that
the terminals could be impacted creating periods of IFR
conditions. Some guidance indicates that IFR conditions could
extend south into the St. Louis metro, but our confidence is
low. To indicate the possibility of the lowered ceilings, a low
scattered cloud deck was included for the St. Louis metro
terminals. Due to the low-level moisture associated with the
rainfall, confidence is high that ceilings will remain lower
through tomorrow morning keeping terminals in MVFR. Conditions
are expected to improve and return to VFR conditions by the end
of this TAF period.

&&

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

&&

SHORT TERM...Elmore
LONG TERM...Elmore
AVIATION...Asel/BSH

Preliminary Local Storm Report
National Weather Service St Louis MO
905 PM CDT Wed Sep 9 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0904 PM     Tstm Wnd Gst     4 NE Ashland            38.82N 92.22W
09/09/2026  M58 MPH          Boone              MO   ASOS/AWOS

            Gust measured by KCOU ASOS.

&&

PW