Flash Flood Warning 48, NWS St. Louis, MO

199 
WGUS73 KLSX 122053
FFSLSX

Flash Flood Statement
National Weather Service St Louis MO
353 PM CDT Tue Aug 12 2025

ILC119-163-MOC183-189-510-122330-
/O.CON.KLSX.FF.W.0048.000000T0000Z-250812T2330Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Madison IL-Saint Clair IL-Saint Charles MO-Saint Louis MO-Saint
Louis City MO-
353 PM CDT Tue Aug 12 2025

...FLASH FLOOD WARNING REMAINS IN EFFECT UNTIL 630 PM CDT THIS 
EVENING FOR SOUTHWESTERN MADISON AND NORTHWESTERN SAINT CLAIR 
COUNTIES IN SOUTHWESTERN ILLINOIS, EAST CENTRAL SAINT CHARLES AND 
EASTERN SAINT LOUIS COUNTIES IN EAST CENTRAL MISSOURI AND THE 
EASTERN SAINT LOUIS CITY IN EAST CENTRAL MISSOURI...

At 353 PM CDT, Doppler radar indicated thunderstorms were producing 
heavy rain in St. Louis City, MO. Between 2 and 3 inches of rain 
have fallen. Additional rainfall amounts up to 1 inch are possible 
in the warned area. Flash flooding is already occurring.

HAZARD...Flash flooding caused by thunderstorms.

SOURCE...Trained spotters reported.

IMPACT...Flash flooding of small creeks and streams, urban areas,
         highways, streets and underpasses as well as other poor
         drainage and low-lying areas.

Some locations that will experience flash flooding include...
  St. Louis, Granite City, Alton, East St. Louis, Edwardsville, 
  Cahokia, Glen Carbon, Bellefontaine Neighbors, Wood River, 
  Bethalto, Pontoon Beach, Centreville, Washington Park, Madison, 
  Riverview, Venice, Spanish Lake, Cahokia Parks Airport, East Alton 
  and Fairmont City.

This includes the following State Parks...
Scott Joplin Historic Site, Confluence State Park and Horseshoe Lake 
State Park.

Numerous instances of flooding due to heavy rain rates and poor 
drainage were reported along highways 70, 64, 55, and 44 in downtown 
St. Louis City.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Turn around, don't drown when encountering flooded roads. Most flood 
deaths occur in vehicles.

&&

LAT...LON 3857 9023 3859 9028 3893 9014 3889 8991
      3855 9015

FLASH FLOOD...OBSERVED

$$

19

855 
WGUS73 KLSX 122228
FFSLSX

Flash Flood Statement
National Weather Service St Louis MO
528 PM CDT Tue Aug 12 2025

ILC119-163-MOC183-189-510-122238-
/O.CAN.KLSX.FF.W.0048.000000T0000Z-250812T2330Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Madison IL-Saint Clair IL-Saint Charles MO-Saint Louis MO-Saint
Louis City MO-
528 PM CDT Tue Aug 12 2025

...FLASH FLOOD WARNING FOR SOUTHWESTERN MADISON AND NORTHWESTERN 
SAINT CLAIR COUNTIES IN SOUTHWESTERN ILLINOIS, EAST CENTRAL SAINT 
CHARLES AND EASTERN SAINT LOUIS COUNTIES IN EAST CENTRAL MISSOURI 
AND THE EASTERN SAINT LOUIS CITY IN EAST CENTRAL MISSOURI HAS BEEN 
REPLACED WITH A FLOOD ADVISORY...

Flood waters have largely receded, aside from a few areas with poor 
drainage. Please continue to heed remaining road closures.

&&

LAT...LON 3857 9023 3859 9028 3893 9014 3889 8991
      3855 9015

$$

19

Area Forecast Discussion
National Weather Service Saint Louis MO
228 PM CDT Tue Aug 12 2025

.KEY MESSAGES...

- Scattered storms capable of locally heavy rainfall and gusty
  winds are expected to continue across portions of the area
  through this evening.

- Potentially dangerous heat returns to the region this weekend
  into early next week.

&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 227 PM CDT Tue Aug 12 2025

Current water vapor imagery shows upper-level troughing digging into
the Midwest with several shortwaves traversing the flow within it.
One of these shortwaves is moving through northern Missouri as of
early this afternoon, and a cold front can be seen in surface
observations moving through northeastern Missouri. Ahead of the
front, an area of surface convergence along the I-44 Missouri/I-70
Illinois corridor has led to convection developing early this
afternoon - about an hour or two earlier than anticipated. If ACARS
soundings out of KSTL are accurate, models have been overdoing
instability by about 1,000 J/kg for SBCAPE. Between this and the
slightly earlier convective initiation, updrafts may not be able to
reach their full potential later this afternoon. Effective shear is
around 20 kts across the CWA currently, leading to the poor storm
organization we have seen so far. While PWAT is around 2" and an
inverted "V" signature is noted in several soundings, the slightly
lower instability and convective debris may inhibit updrafts from
reaching a height that, when they collapse, would lead to severe-
strength winds. Even so, several downburst signatures have been
noted around the St. Louis metro area so far this afternoon, and
confidence is high that at least isolated sub-severe downburst winds
(up to 50 mph) will occur under stronger storms that collapse. The
high PWAT and slower storm motion may also lead to isolated flooding
issues, but the shorter lifespan of storms is expected to mitigate a
more notable flood threat. With instability waning with the setting
sun and convection easily becoming outflow dominant, a gradual
weakening and decrease in coverage of convection is expected this
evening.

The front will gradually pass through the rest of the CWA this
evening and tonight, clearing it to the southeast by Wednesday
morning. In its wake, high pressure will spread into the Middle
Mississippi Valley, characterized by generally northerly winds and
temperatures running at to just below climatological normals. There
is a low probability (20%) that there will be enough boundary layer
moisture behind the front to lead to isolated to scattered
convection across far southeastern Missouri and southwestern
Illinois as a shortwave and the upper-level trough axis swing
overhead during the afternoon, but most locations will remain dry.

Elmore

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 227 PM CDT Tue Aug 12 2025

The main focus for impactful weather in the extended forecast period
is going to be the return of above normal temperatures. While
guidance consensus is that upper-level ridging will still be
gradually building into the Midwest on Thursday, the trend has been
for it to do so more slowly. As a result, the surface high
responsible for our more seasonable weather mid-week will hang
around a little longer and inhibit southerly flow from returning to
the CWA, with conditions on Thursday being similar to those forecast
on Wednesday. However, the upper-level ridge will notably amplify on
Friday, quickly pushing the surface high eastward and deepening
southerly flow over the Middle Mississippi Valley. Temperatures at
850 mb are expected to see at least a 5 degree C jump, with values
forecast to top out in the low 20s C. This climatologically favors
surface temperatures in the mid 90s, which matches well with
ensemble means on Friday. Guidance consensus is that the ridge will
only continue to build into the region over the weekend, with
potentially dangerous levels of heat and humidity occurring. NBM-
based probabilities for high temperatures of at least 90 degrees top
out at 80-100% for nearly the entire CWA Saturday and Sunday, and 60-
80% of at least 95 degrees for locations mainly along and south of I-
70 over the same period. With dew points from the NBM and ensemble
mean hovering in the low to mid 70s, heat index values of at least
100 degrees are expected, and there is growing concern for Heat
Advisory criteria or perhaps Extreme Heat Warning criteria for
portions of the area over the weekend.

How long this heat carries into next week is uncertain. Both
deterministic guidance and ensemble clusters show the upper-level
ridge retrograding westward as troughing develops over the eastern
CONUS. However, guidance differs on the timing of this and the
amplitude of the trough, leading to the spread among the global
ensemble IQR increasing to 10 degrees next Tuesday. If the ridge is
able to hold on over the region, we may at most see temperatures
only decrease by a degree or two through the early week period
compared to the weekend. However, if the ridge moves far enough
west, the CWA will be beneath northwesterly flow aloft, allowing for
cooler air and increasing rain/cloud chances across the region early
next week. A majority of guidance favors the more stubborn ridge,
and this is reflected in the current forecast.

Elmore

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1218 PM CDT Tue Aug 12 2025

The main concern for impacts this TAF period is scattered showers
and thunderstorms along a cold front sinking southward through the
region. This showers and thunderstorms have the greatest chance of
impacting KSUS, KSTL, and KCPS, though a brief period of impacts
can't be ruled out at KCOU and KJEF. With showers or storms that
make direct impact to terminals, heavy rainfall will reduce
visibilities, and storms in proximity to local terminals could
produce gusty, erratic winds. Behind the front tonight and
tomorrow, winds will be very light and mostly variable.

Elmore

&&

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

WFO LSX

Hazardous Weather Outlook
National Weather Service St Louis MO
236 PM CDT Tue Aug 12 2025

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-131945-
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-
236 PM CDT Tue Aug 12 2025

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

.DAY ONE...Tonight.

There is a chance for thunderstorms through early tonight. Some
storms may produce locally heavy rainfall and gusty winds.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday.

There is a low chance of thunderstorms for portions of the area
Wednesday, Thursday and Monday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

Elmore