Flash Flood Warning 16, NWS St. Louis, MO

056 
WGUS73 KLSX 190118
FFSLSX

Flash Flood Statement
National Weather Service St Louis MO
818 PM CDT Mon May 18 2026

ILC027-051-121-190128-
/O.EXP.KLSX.FF.W.0016.000000T0000Z-260519T0115Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Clinton IL-Fayette IL-Marion IL-
818 PM CDT Mon May 18 2026

...FLASH FLOOD WARNING HAS EXPIRED FOR EASTERN CLINTON, SOUTHERN
FAYETTE AND MARION COUNTIES IN SOUTH CENTRAL ILLINOIS...

The heavy rain has ended. Flooding continues to impact various roads
throughout the county. A flood advisory has replaced the flash flood
warning. Please continue to heed remaining road closures.

A Flood Watch remains in effect until 100 PM CDT Tuesday for a
portion of south central Illinois.

&&

LAT...LON 3863 8914 3892 8914 3891 8869 3884 8869
      3877 8870 3848 8870 3848 8914

$$

Maples

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
126 PM CDT Mon May 18 2026

.KEY MESSAGES...

- Widespread showers and thunderstorms are expected throughout the
  day with heavy rainfall in some locations potentially leading to
  flash flooding.

- A few strong to severe thunderstorms are possible this afternoon
  and evening with the main hazards being hail and damaging wind
  gusts.

- Additional thunderstorms are forecast late tonight and early
  Tuesday morning with another round possible along a cold front
  Tuesday afternoon/evening.

&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 402 AM CDT Mon May 18 2026

A complex of thunderstorms, that as of 09z, is currently positioned
near Quincy MO to Warrensburg MO. This activity initiated across
KS/NE on Sunday evening and has continued to push southeastward
early this morning, with arrival in central/northeast MO expect
around 09-10z. Within the last few hours, this complex has become
outflow dominant, which has led to a gradual weakening trend from
the severe wind gusts it was producing across northwestern MO around
midnight. Upstream observations indicate that the gust front has
been achieving winds gusts in the 30-50mph range. As a result, the
main severe hazard tied to these storms as they move into the area
are expected to be scattered wind gusts up to 50mph, along with
frequent lightning and heavy rain.

In fact, one of the main concerns regarding the short term is now
the threat for heavy rainfall and flash flooding, mainly across
central/south-central MO and points west. CAMs are in good consensus
that the southeastward progress of this thunderstorm complex will
continue to slow as it makes its way into the area this morning.
Additionally, there are signs that backbuilding of thunderstorms may
occur as the remnant outflow boundary/line of convection stall in a
west-east orientation, with strong low-level southerly warm/moist
advection overrunning this stationary boundary. Analysis of forecast
soundings unveils  a plentiful amount of uniform CAPE (around 1,500
J/kg) throughout the column, along with warm cloud temperatures with
freezing levels around 12-14kft AGL. In addition, PWATs are
currently in the 1.4-1.5" range, with the latest HREF indicating
values increasing to 1.7-1.9" today, which is near the 99th
percentile. Lastly, the HREF LPMM precipitation reveals the
potential for a west-east oriented swath of 2-4" with localized
pockets of 4-6" possible. Given all of the variables mentioned
above, the risk of heavy rainfall has prompted us to issue a Flash
Flood Watch for central/south-central MO and portions of east-
central MO that is in effect from 12z today through this evening.

Regarding the severe weather potential throughout the day, the
threat remains for an isolated thunderstorm becoming strong/severe
with hazards being hail along with damaging winds. This potential
appears to be lower due to the lingering showers and thunderstorms
across a large portion of the area, inhibiting solar insolation, and
thus, robust destabilization this afternoon. Regardless, given the
warm/moist summerlike airmass in place and the continuation of
steepened mid-level lapse rates, maintaining sufficient instability,
the threat for an isolated severe storm remains. A few locations may
be able to clear by this afternoon, allowing the environment to
recover and destabilize, and potentially the threat for strong to
severe thunderstorms to increase in localized areas. With
uncertainty regarding the evolution/coverage of lingering
showers/storms, confidence is low regarding the extent of severe
weather that may be realized across the area this afternoon and
evening.

By tonight, the large-scale pattern features a broad mid/upper level
trough to the west, with an embedded shortwave ejecting
northeastward into the Great Plains. This mid-level feature triggers
lee-side surface cyclogenesis across the Central Plains/Midwest near
a quasi-stationary baroclinic zone. Convection is modeled by high
resolution guidance to initiate along this boundary in KS/NE, with
quick upscale growth ensuing with eastward progression. The surface
low is modeled to lift northeast into the Midwest on Tuesday, which
begins to slowly shift the quasi-stationary baroclinic zone
southeastward as a cold front. However, this convection is progged
by guidance to outrun this slowly advancing cold front out west, and
gradually weaken as it approaches the area. This weakening trend
appears reasonable since the best deep-layer shear remains displaced
to the north and west of the area and forcing for ascent will be
limited without convection tied to the boundary. Regardless, this
threat for severe weather late Monday night into early Tuesday
morning will include the potential for large hail and damaging wind
gusts. Additionally, if surges or bowing segments occur, then
severe outflow gusts or embedded circulations leading to brief
tornadoes may also be possible.

Guidance is in fairly good agreement that the cold front will
approach northeast MO/west-central IL Tuesday morning with it
exiting to the southeast late Tuesday evening. Locations that remain
pre-frontal for a majority of the day on Tuesday will have the
greatest severe weather threat. The big question that remains is,
how quickly will the atmosphere recover from the early Tuesday
morning convection by the time the front is working its way through
the area. As of now, it appears the severe weather threat will be
greatest from east-central MO and points southeastward, as these
locations will have the best chance to destabilize in the early
afternoon on before the arrival of the front. Regardless,
instability and mid-level flow are still forecast to be sufficient
enough to support organized storms with potential for large hail,
damaging winds, and tornadoes. Any lingering outflow boundaries from
morning convection may serve as areas of focus for thunderstorm
development in the afternoon ahead of the cold front.

Peine

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 402 AM CDT Mon May 18 2026

Low-level cold air advection within the post-frontal airmass will
lead to much cooler conditions for Wednesday, with highs forecast to
be around 10-15 degrees below average. Quasi-zonal flow aloft
encompasses the region through the end of the week, which should
allow for a period of dry conditions until Friday. By the end of the
week, the general trend of LREF temperature IQRs is a gradual warmup
to temperatures right around climatological normals.

Peine

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 101 PM CDT Mon May 18 2026

A complex of showers and thunderstorms continues across central
Missouri this afternoon. This round of thunderstorms has persisted
since the overnight hours with almost continuous thunder at the
TAF sites (except KUIN) since 12z. The line of thunderstorms has
stalled just south of KCPS this afternoon with a shortwave wave
currently near the Arkansas/ Missouri border. The shortwave is
causing additional thunderstorm development just north of the
boundary (or close to Columbia, MO). This means that all
terminals will likely (>90% chance) observe thunderstorms through
the remainder of the afternoon into the early evening.

As the shortwave moves northeast of the region, showers and
thunderstorms will begin to finally move east of the terminals
with thunder temporarily ending from west to east. This break will
be brief though. Another line of showers and thunderstorms will
approach the terminals from the northwest Tuesday morning between
4 and 7 AM. This line of storms will likely weaken as they
progress southeast, but the question is how quickly will they
weaken. As of current, the thinking is the line of thunderstorms
will hold together long enough to make it to KUIN, KCOU, and KJEF.
It is unclear if there will be thunder at the St. Louis terminals.
For now have only added SHRA (no thunder) for KSTL, KSUS, and
KCPS.

A cold front will then cross the terminals Tuesday afternoon with
MVFR/ IFR cigs behind.

BAH

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch until 7 PM CDT this evening for Boone MO-Callaway MO-
     Cole MO-Crawford MO-Franklin MO-Gasconade MO-Jefferson MO-
     Lincoln MO-Moniteau MO-Montgomery MO-Osage MO-Saint Charles
     MO-Saint Louis City MO-Saint Louis MO-Warren MO-Washington
     MO.

IL...Flood Watch until 7 PM CDT this evening for Bond IL-Clinton IL-
     Fayette IL-Madison IL-Marion IL-Monroe IL-Saint Clair IL-
     Washington IL.

&&

WFO LSX

Hazardous Weather Outlook
National Weather Service St Louis MO
600 AM CDT Mon May 18 2026

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-191100-
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-
600 AM CDT Mon May 18 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...Today 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...

- Widespread showers and thunderstorms are expected throughout the
  day with heavy rainfall in some locations potentially leading to
  flash flooding.

- A few strong to severe thunderstorms are possible this afternoon
  and evening with the main hazards being hail and damaging wind
  gusts.

- Additional thunderstorms are forecast late tonight and early
  Tuesday morning with another round possible along a cold front
  Tuesday afternoon/evening.

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
910 AM CDT Mon May 18 2026

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

0738 AM     Tstm Wnd Dmg     4 N Warrenton           38.88N 91.13W
05/18/2026                   Warren             MO   Emergency Mngr

            Relayed from social media: Tree blocking
            Highway 47 at Big Creek north of Warrenton.
            Time estimated from radar.

&&

MCF

Preliminary Local Storm Report
National Weather Service St Louis MO
1109 AM CDT Mon May 18 2026

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

1105 AM     Hail             1 NNE Oakville          38.46N 90.31W
05/18/2026  M0.70 Inch       St. Louis          MO   Public

            NWS employee reported dime-size hail.

&&

MCF

Preliminary Local Storm Report
National Weather Service St Louis MO
1203 PM CDT Mon May 18 2026

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

0907 AM     Tstm Wnd Dmg     Litchfield              39.18N 89.66W
05/18/2026                   Montgomery         IL   911 Call Center

            Multiple reports of large tree limbs down
            around Litchfield.

&&

Pfahler