Severe Thunderstorm Warning 195, NWS St. Louis, MO

Area Forecast Discussion
National Weather Service Saint Louis MO
219 PM CDT Wed Jun 24 2026

.KEY MESSAGES...

- Showers and thunderstorms capable of heavy rainfall are expected
  Thursday afternoon into Friday morning. A Flash Flood Watch has
  been issued for portions of the area.

- Heat and humidity will build into the region starting Sunday
  and continue through next week.

&&

.SHORT TERM...  (Through Late Thursday Night)
Issued at 207 PM CDT Wed Jun 24 2026

Current water vapor imagery shows broad, shallow troughing across
the eastern CONUS with an upper level ridge over the Desert
Southwest. This has the CWA beneath weak northwesterly flow aloft,
with multiple disturbances within this flow moving into the
Midwest. A surface low associated with one of these disturbances
is traversing the Midwest well north of the CWA, placing the CWA
beneath warm air advection. This warm air advection is responsible
for the ongoing isolated to scattered showers and thunderstorms.
As this wing of warm air advection pivots eastward, the focus for
the convection will also shift east-southeastward through this
evening, dissipating by mid evening with instability decreasing.
The main threats with this convection will be brief heavy rainfall
and frequent lightning. Current ACARS soundings and SPC
Mesoanalysis show that this convection is occurring on the leading
edge of more favorable instability and shear, and is expected to
remain outside of these favorable conditions due to a lack of
forcing.

The aforementioned surface low's attendant cold front will slowly sag
southward toward the CWA overnight, and has a low chance of forcing
weak convection for areas along and north of Highway 36/I-72.
However, the main period of focus for convection along this front is
Thursday afternoon into Friday. Then, the front will have stalled
somewhere along the I-70 corridor as disturbances within the zonal
flow aloft ripple overhead. This will lead to multiple rounds of
showers and thunderstorms along the front as early as mid-day
Thursday and continuing through late Thursday night.

Guidance consensus is that SBCAPE will be around 1,500 J/kg and deep-
layer shear around 25 knots will support a low chance for brief
updraft organization along and south of the front. A majority of
deterministic soundings show a weak inverted "V" signature near the
surface, favoring isolated damaging wind gusts. However, with PWAT
around 1.75" (~75th percentile), warm cloud depths, and slower storm
motion, the main threat with convection will be heavy rainfall and
flash flooding. The zonal flow aloft paired with the stalling front
leads to uncertainty in specifics on the exact location of this
threat, with there still being enough lead time for it to shift
north or south. Where confidence is greatest in totals quickly
reaching 2-3", with isolated 4-6", a Flash Flood Watch has been
issued through Friday morning.

Elmore

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 207 PM CDT Wed Jun 24 2026

The pattern on Friday is forecast to be similar to that on Thursday,
leading to continuing chances for rainfall through the day. However,
confidence is very low in the coverage and intensity of rainfall.
Some guidance has an MCS moving through the CWA very late Thursday
night/early Friday morning, and if this happens, then rain chances,
coverage, and intensity will remain lower through the day Friday.
However, if this does occur as some guidance suggests, then
additional showers and thunderstorms will occur through the day
along the front. These will be capable of heavy rainfall, and would
prompt an extension of the Flash Flood Watch. The front will begin
to buckle northward on Saturday as upper-level ridging noses into
the lower Midwest, though how quickly it moves northward varies
among guidance. A slower moving front favors rain chances continuing
through much of the day, while a faster front will end rain chances
by late morning.

Starting Sunday, both deterministic guidance and ensemble clusters
depict the upper-level ridge rapidly amplifying over the Midwest.
This will pull the front responsible for the late week and early
weekend rain chances well to the north, opening the door for deep
warm air advection. Ensemble means are clustered around 850mb
temperatures in the low to mid 20s C, climatologically favoring
surface temperatures in the low to mid 90s F. With a majority of
guidance favoring this solution, confidence is high in a multi-day
stretch of hot and humid conditions next week. Impacts from
potential convection at this point seem few to none thanks to the
ridge's axis being centered over or just east of the CWA and the
crest of the ridge well north of the area, keeping robust convective
activity well displaced from the CWA.

Elmore

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1221 PM CDT Wed Jun 24 2026

The main concern for impacts this period is the threat for
isolated to scattered showers and thunderstorms this afternoon and
early evening. Heavy rainfall will be capable of reducing
visibilities. There is a low chance for fog at all local terminals
late tonight, and a low chance for low stratus at KCOU and KJEF.
Given the low confidence in these threats and their occurrence
near the end of the period, impacts have been left out of the TAFs
for now.

Elmore

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch from Thursday afternoon through Friday morning 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 from Thursday afternoon through Friday morning for
     Bond IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey
     IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
     Saint Clair IL-Washington IL.

&&

WFO LSX