River Flood Warning 37, NWS St. Louis, MO

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
646 AM CDT Tue May 20 2025

.KEY MESSAGES...

- There is a chance for one more round of severe thunderstorms
  this afternoon into early evening primarily across parts of
  southeast and east central Missouri as well as southwest and
  south central Illinois.

- Cooler and much less active weather is expected for the
  remainder of the forecast into early next week.

&&

.SHORT TERM...  (Through Late Tonight)
Issued at 305 AM CDT Tue May 20 2025

The long wave trough over the Great Plains is becoming negatively
tilted and will lift into the Upper Midwest today. The surface
low will move from eastern Kansas/Nebraska into Iowa by this
afternoon, and the associated cold front will sweep through
Missouri by late afternoon/early evening, and then through
Illinois by Midnight. Short range guidance builds instability back
up ahead of the front today, with MLCAPE as high as 2000-2200
J/Kg and 0-6km shear of 40- 50kts across east central and
southeast Missouri into southwest and south central Illinois.
Forecast soundings show a weak cap that erodes in some models but
stays firm in others. Convergence along the front still doesn't
look that strong, but several short range models develop
convection ahead of the front during the mid-late afternoon
primarily in Illinois. Hodographs do not show much curvature, but
the strength of the shear and instability suggest supercells will
be the initial convective mode. Straight hodographs will encourage
splits, so depending on the coverage there could be mergers and
upscale growth, however that may not occur until the storms have
exited our forecast area during the early evening.

Carney

&&

.LONG TERM...  (Wednesday through Monday)
Issued at 305 AM CDT Tue May 20 2025

The weather for Wednesday through next Monday looks much more benign
than what we've been experiencing for the past several days.  A high
amplitude pattern dominated by a long wave trough over eastern North
America and a ridge over much of the central and western portion of
the continent.  Medium range guidance keeps our forecast area in
northwest flow for most of the period.  At least two or three short
waves pass over the area, one Friday, and another two Saturday into
Sunday.  These waves force cyclogenesis over the Plains and turn the
low level flow back to the south across the Lower and Mid
Mississippi Valley producing moderate to strong low level moisture
convergence.  The deterministic GFS and ECMWF print out several
inches of precipitation across parts of Missouri into southern
Illinois.  However, the LREF and NBM focus more on southwest
Missouri into the Lower Mississippi Valley.  The 75th and 90th
percentiles do paint a broad swath of 2+ inch QPF along and south of
I-70 though, so there could be some water issues to deal with this
weekend.  The good news is that our forecast area stays on the cool
side of the surface boundary.  Very little instability will be
available, and while there may be thunderstorms at times, the storms
should not be severe.  Temperatures will run cool through the period
with highs mainly in the mid 60s to around 70 and lows in the mid
40s to low and mid 50s.

Carney

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 634 AM CDT Tue May 20 2025

An area of MVFR ceilings may affect parts of central and
northeast Missouri for a few hours this morning. Elsewhere
and otherwise, expect VFR flight conditions to prevail. There's a
chance that scattered thunderstorms will develop ahead of a cold
front this afternoon, mainly in eastern Missouri and
southwest/south central Illinois. These storms could be severe
with wind gusts to 50kts and hail in excess of 1 inch in diameter.
A tornado or two is also possible, though not likely. MVFR
ceilings may affect parts of northeast Missouri into west central
Illinois late tonight, but VFR conditions are expected for the
rest of the area.

Carney

&&

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

WFO LSX

Hazardous Weather Outlook
National Weather Service St Louis MO
455 AM CDT Tue May 20 2025

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-211000-
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-
455 AM CDT Tue May 20 2025

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

.DAY ONE...Today and tonight.

There is a chance for thunderstorms today. A few thunderstorms could
be strong to severe during the afternoon and early evening. Large
hail and damaging winds are the primary threats. A tornado or two is
also possible.

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

There is a chance for thunderstorms late Friday through Monday
across parts of the area.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed this afternoon and evening.

Carney

Preliminary Local Storm Report
National Weather Service St Louis MO
933 AM CDT Tue May 20 2025

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

0856 PM     Hail             2 WNW Hazelwood         38.79N 90.39W
05/19/2025  M1.50 Inch       St. Louis          MO   Public

            Via Facebook: 1.5 inch hail measured with
            ruler.

&&

LSJ

Preliminary Local Storm Report
National Weather Service St Louis MO
936 AM CDT Tue May 20 2025

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

0856 PM     Tstm Wnd Dmg     6 WSW Belgrade          37.74N 90.94W
05/19/2025                   Washington         MO   Public

            Via Facebook: Numerous downed and damaged
            trees along and blocking Highway DD on the
            Washington/Iron County line. Report notes
            that this is the case for miles along this
            road.

&&

LSJ

Preliminary Local Storm Report
National Weather Service St Louis MO
940 AM CDT Tue May 20 2025

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

0831 AM     Rain             1 S Florissant          38.79N 90.32W
05/20/2025  M3.75 Inch       St. Louis          MO   Public

            3.75 inches of rain beginning at about
            8:38pm 5/19 CDT and measured at 8:30am CDT
            5/20.

&&

LSJ