Severe Thunderstorm Warning 139, NWS St. Louis, MO

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
641 PM CDT Mon May 4 2026

.KEY MESSAGES...

- Showers and thunderstorms are expected tonight. Some storms may
  be severe with 1 to 1 1/2 inch hail this evening, primarily a.
  long and north of I-70. There may also be damaging winds,
  however this is not as likely as large hail.

- Periods of rain will continue on Tuesday as a cold front moves
  through the area.

- Temperatures will be below normal Tuesday through Thursday, but
  will warm back up for Friday into the weekend.

&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 255 PM CDT Mon May 4 2026

A cold front which is currently stretching from northeast Kansas
into northwest Missouri and southwest/central Iowa will move
slowly southeast through Missouri and Illinois over the next 24-36
hours. Latest RAP and GFS show as much as 2000-2500 J/Kg of
MUCAPE across parts of central and northeast Missouri late this
afternoon into early this evening with around 30kts of deep-layer
shear. However, they also show a good deal of CINH capping the
boundary layer. The CINH will likely lead to only sparse
development of storms this evening, at least initially. There is
enough CAPE and shear to support severe storms, but any storms
that do form are not expected to be rooted in the boundary layer,
so large hail will be the primary threat. Damaging wind gusts are
less likely, though still possible if a particularly strong storm
dumps its core and produces a microburst. A modest low level jet
will develop ahead of the front closer to midnight which produces
moderate moisture convergence across the area. This will lead to
an increase in thunderstorm coverage overnight, but instability
will be decreasing rapidly by that time. Even though there will be
more storms after midnight it is unlikely they will be severe.

The cold front continues moving slowly southeast through the area on
Tuesday with moderate to occasionally strong low level moisture
convergence ahead of an 850mb low over the eastern Plains.  This
will lead to widespread across most of the area.  Clouds and rain
will keep temperatures cool with little chance for the atmosphere to
become unstable through the day.  While there will likely be some
rumbles of thunder through the day, severe storms are unlikely due
to lack of instability.  Rain should taper off from west to east
through the evening.

Carney

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 255 PM CDT Mon May 4 2026

The upper level trough axis digs into the Great Plains on Wednesday
which forces the cold front farther south into the lower Mississippi
Valley.  A cool high pressure system builds into Missouri and
Illinois, however the approaching trough produces moderate to strong
mid level frontogenesis over Missouri and Illinois.  The GFS and to
a lesser extent the NAM print out some light QPF due to the
frontogenesis, but forecast soundings show a dry layer near the
surface associated with the approaching surface high.  Think chance
PoPs remain appropriate in this situation.  Northerly flow, with
clouds and potential showers will likely make Wednesday the coolest
day of the next 7 with highs struggling into the mid to upper 50s.
Temperatures warm back into the mid 60s, and then to near normal in
the 70s for Friday through Sunday as the long wave trough moves east
and the upper level flow becomes more zonal.  There will be a brief
chance for rain Friday as a weak short wave moves across the Mid
Mississippi Valle, but the next substantial chance for rain holds
off until Sunday ahead of the next upstream cold front.  The LREF is
showing a good deal of uncertainty with regards to the amplitude of
the trough which is driving this cold front.  This translates into
low confidence in the position of the front, and wide variations in
the temperature forecast. The interquartile range of high
temperatures is 10+ degrees on Sunday, and around 8 degrees on
Monday behind the cold front.  With that in mind, it does look
cooler on Monday with around a 10 degree drop in mean temperatures,
and deterministic guidance cools from the mid to upper 70s on Sunday
in most locations to the upper 60s and low 70s.

Carney

&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 638 PM CDT Mon May 4 2026

Chances of showers and thunderstorms are the main concern through
tonight ahead of an approaching cold front. The best chances are
in the overnight hours, mainly at the central Missouri and metro
St. Louis terminals. IFR visibilities are possible in any storm
that directly impacts a terminal. Winds will swing to the
north/northeast behind the front, with lingering showers
continuing much of the day on/off for central MO/metro sites along
with low MVFR ceilings.

Gosselin

&&

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

WFO LSX

Hazardous Weather Outlook
National Weather Service St Louis MO
355 PM CDT Mon May 4 2026

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-052100-
Greene IL-Macoupin IL-Montgomery IL-Bond IL-Fayette IL-Clinton IL-
Marion IL-Washington IL-Randolph IL-Adams IL-Brown IL-Pike IL-
Calhoun IL-Jersey IL-Madison IL-Saint Clair IL-Monroe IL-Knox MO-
Lewis MO-Shelby MO-Marion MO-Monroe MO-Ralls MO-Pike MO-Boone MO-
Audrain MO-Moniteau MO-Cole MO-Osage MO-Callaway MO-Montgomery MO-
Lincoln MO-Gasconade MO-Warren MO-Saint Charles MO-Franklin MO-
Saint Louis MO-Saint Louis City MO-Jefferson MO-Crawford MO-
Washington MO-Saint Francois MO-Sainte Genevieve MO-Iron MO-
Madison MO-Reynolds MO-
355 PM CDT Mon May 4 2026

This Hazardous Weather Outlook is for portions of eastern and
central Missouri as well as west central and southwest Illinois.

.DAYS ONE THROUGH SEVEN...This afternoon through Sunday.

More detailed information can be accessed at:
https://www.weather.gov/media/lsx/DssPacket.pdf

The Area Forecast Discussion is available at:
https://forecast.weather.gov/
product.php?issuedby=LSX&product=AFD&site=lsx

.KEY MESSAGES...

- Showers and thunderstorms are expected tonight. Some storms may
  be severe with 1 to 1 1/2 inch hail this evening, primarily a.
  long and north of I-70. There may also be damaging winds,
  however this is not as likely as large hail.

- Periods of rain will continue on Tuesday as a cold front moves
  through the area.

- Temperatures will be below normal Tuesday through Thursday, but
  will warm back up for Friday into the weekend.

NOTE: Starting May 29, 2026, this outlook product will be
discontinued. Please see the above products for more detailed
forecast information.

WFO LSX

Preliminary Local Storm Report
National Weather Service St Louis MO
643 PM CDT Mon May 4 2026

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

0638 PM     Hail             Litchfield              39.18N 89.66W
05/04/2026  E0.25 Inch       Montgomery         IL   Amateur Radio

&&

JPG

Preliminary Local Storm Report
National Weather Service St Louis MO
644 PM CDT Mon May 4 2026

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

0644 PM     Hail             New Douglas             38.97N 89.67W
05/04/2026  E1.50 Inch       Madison            IL   Trained Spotter

&&

JPG

Preliminary Local Storm Report
National Weather Service St Louis MO
646 PM CDT Mon May 4 2026

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

0631 PM     Hail             1 SW Butler             39.19N 89.54W
05/04/2026  E0.75 Inch       Montgomery         IL   Public

            Report from mPING: Dime (0.75 in.).

&&

JPG

Preliminary Local Storm Report
National Weather Service St Louis MO
649 PM CDT Mon May 4 2026

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

0633 PM     Hail             2 E Worden              38.94N 89.80W
05/04/2026  E1.75 Inch       Madison            IL   Public

&&

JPG

Preliminary Local Storm Report
National Weather Service St Louis MO
650 PM CDT Mon May 4 2026

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

0639 PM     Hail             Livingston              38.97N 89.76W
05/04/2026  E2.00 Inch       Madison            IL   Public

&&

JPG

Preliminary Local Storm Report
National Weather Service St Louis MO
652 PM CDT Mon May 4 2026

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

0616 PM     Hail             Dorsey                  38.97N 90.00W
05/04/2026  E0.75 Inch       Madison            IL   Public

            Twitter report.

&&

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