544
FXUS63 KLSX 301921
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
221 PM CDT Mon Jun 30 2025

.KEY MESSAGES...

- Scattered to numerous showers and thunderstorms are expected
  across portions of the area this evening. A few thunderstorms
  could be capable of damaging winds.

- Drier and seasonable conditions area expected Tuesday through
  Wednesday, followed by a return to heat and humidity on 4th of
  July into the holiday weekend.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 220 PM CDT Mon Jun 30 2025

An upper-level trough is evident on water vapor imagery just
upstream of the Mid-Mississippi River Valley, which is accompanied
by a southeastward-advancing cold front extending through
northeastern to west-central MO according to 19z surface
observations. Ahead of this front, scattering/clearing of clouds has
been most prominent along/east of the Mississippi River allowing
1000 to 2000 J/kg of MLCAPE to develop. However, modest low and mid-
level lapse rates and deep-layer wind shear around 20 kt will limit
the intensity and overall organization of thunderstorms to only
support sporadic microbursts with damaging winds with the threat
locally greater where there is higher instability. The latest CAMs
support thunderstorms largely initiating along the cold front across
north-central MO and west-central IL late this afternoon as the
trough arrives, but there also other mesoscale boundaries present
that cannot be ruled out as points of initiation including one
across east-central/southeastern MO from earlier showers and
thunderstorms. As thunderstorms propagate southeastward through the
CWA this evening, they could aggregate into loosely organized
multicell clusters, leading to higher coverage. Showers and
thunderstorms will not exit entirely until the cold front clears the
area tonight but the severe threat will be decreasing after sunset.

Deep northwesterly flow will usher a drier and cooler airmass into
the region late tonight through Tuesday beneath mid-level height
rises/subsidence. High temperatures will be similar or slightly
cooler on Tuesday compared to today and in the 80s F with less cloud
cover, despite 850-hPa temperatures around 4 C cooler. A more
significant change will be lower dewpoints in the 60s F.

Pfahler

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 220 PM CDT Mon Jun 30 2025

Northwesterly flow will remain dominant through Wednesday and
Thursday while an upper-level ridge crosses the Great Plains. This
pattern will maintain predominantly dry conditions through this
timeperiod accompanied by a gradual warming of temperatures as low-
level flow becomes increasingly southwesterly or southerly and
induces WAA in the wake of a low-level ridge that could also support
patchy river valley fog Wednesday morning. Accordingly, NBM
probabilities of high temperatures in the 90s F also increase to 50
to 80 percent across much of the CWA with highest probabilities in
the St. Louis metro.

Global model guidance are in agreement that the upper-level ridge
will be positioned over the Mid-Mississippi River Valley on
Friday/4th of July with continued warming of temperatures and
additionally a return of higher dewpoints. However, there is
disagreement on how quickly or even if the ridge deamplifies
subsequently over the weekend as multiple shortwave troughs arrive
from upstream, influencing the longevity of hot and humid conditions
through the weekend that have to contend with clouds and increase
probabilities of showers and thunderstorms. That being said, even
with the stronger ridge, at least diurnal, isolated showers and
thunderstorms are favored with the hot and moist airmass. The
majority of the NBM temperature distribution has high temperatures
in the 90s F Friday through Saturday with spread increasing by
Sunday as the 75th percentile remains steady and the 25th percent
cools closer to average. With these temperatures and higher
dewpoints, heat index values of 100+ F could be achieved, but global
ensemble model probabilities of these values are 10 to 30 percent
Friday and Saturday with the caveat that these probabilities are
underdone by the inability of the membership to fully capture peak
heating among other factors.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1223 PM CDT Mon Jun 30 2025

The main concern this TAF period will be showers and thunderstorms
this afternoon and evening. Thunderstorms ongoing across central IL
early this afternoon have been weakening, leaving the most likely
scenario for additional development and impacts to be ahead of the
cold front later this afternoon and evening. Thunderstorms could
result in MVFR to IFR flight conditions and contain gusty winds in
the strongest thunderstorms. Outside of thunderstorms, VFR flight
conditions are forecast but sporadic MVFR ceilings may persist this
afternoon. Westerly winds will become northwesterly with frontal
passage this evening, persisting through Tuesday along with some low-
VFR diurnal cumulus.

Pfahler

&&

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

$$

WFO LSX