131
FXUS63 KLSX 041051
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
551 AM CDT Mon Aug 4 2025

.KEY MESSAGES...

- Isolated to scattered showers and thunderstorms are forecast
  today and Tuesday, mainly during afternoon across south-
  central/southwestern IL. Locally heavy rainfall cannot be ruled
  out.

- There will be a gradual warming trend this week, with more
  typical summer temperatures and humidity returning during the
  second half of the week.


&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 240 AM CDT Mon Aug 4 2025

An upper-level trough across western MO early this morning will
slowly propagate eastward today, absorbing an MCV near Kansas City
in the process. Between upper/mid-level clouds associated with these
features, radiational cooling has been sufficient for patchy fog
development, mainly in Ozark river valleys. The trough will begin to
interact with a plume of moisture within a low-level inverted trough
progressing from KY and southern IN into IL by this afternoon. Weak,
broad large-scale ascent, the low-level inverted trough, and daytime
heating will help drive scattered showers and thunderstorms today,
mainly across south-central/southwestern IL per 30 to 50 percent of
HREF membership. Negligible deep-layer wind shear and modest
instability will preclude any threat of severe thunderstorms, but
very slow thunderstorm motions may support locally heavy rainfall.
That being said, modest PW of only around 1.5" thanks to a mid-level
layer of dry air and disorganization of thunderstorms decreases
confidence that rainfall rates will be high enough to produce any
flash flooding. With weak low-level WAA, high temperatures will be
slightly warmer than Sunday and in the low to mid-80s F, except
across portions of south-central/southwestern IL where precipitation
and more persistent clouds will keep temperatures in the 70s F.

With weak forcing, showers and thunderstorms will wane after sunset
this evening, leaving dry conditions tonight. With upper/mid-level
clouds also decreasing, there is a signal for a mixture stratus and
fog to develop across the region overnight into Tuesday morning.
Latest HREF probabilities of visibilities less than 1/4 mi are 30 to
50 percent across portions of northeastern, central, and
southeastern MO, supporting those areas being most favorable for
fog.

On Tuesday, the upper-level trough will continue eastward, leaving
the CWA beneath rising heights in its wake. Daytime heating and
residing moisture will result in another round of isolated to
scattered showers and thunderstorms during the afternoon, but more
nebulous forcing should lead to less coverage than today.
Accordingly, HREF membership with showers and thunderstorms is
around 10 percent lower. Morning stratus will become more cumuliform
and scatter through the day, allowing more widespread high
temperatures further into the 80s F than previous days.

Pfahler

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 240 AM CDT Mon Aug 4 2025

Wednesday through Friday, the Mid-Mississippi River Valley will be
dominated by the northeastern branch of a broad upper-level ridge
centered across the Four Corners Region and Southern Plains.
Therefore, dry conditions will accompany a continued, very gradual
warming trend with the footprint of high temperatures reaching the
90s F across the CWA increasing Thursday through Saturday, NBM
probabilities (70+ percent) of 90+ F serving as a proxy. Dewpoints
will also be increasing into the 70s F through this time, but
current indications are that they will not be quite as high as our
recent heatwave and could be enough to prevent heat index values
from becoming hazardous. This condition can be attributed to a lack
of direct access to the Gulf of Mexico moisture with our low-level
flow instead sourced continentally from the Southeast at the
southern flank of a low-level anticyclone.

Model guidance are in agreement that the upper-level ridge will
break down as a trough digs into the northern-tier of the CONUS over
the weekend. However, the amplitude and timing of this trough is
uncertain along with when an associated cold front arrives and how
far into the CWA it reaches. This pattern change and these
uncertainties are manifested on Sunday as probabilities of showers
and thunderstorms increasing to 20 to 40 percent in ensemble model
guidance and NBM interquartile high temperature ranges beginning to
increase, especially in northeastern MO/west-central IL.
Subsequently, there are signals for upper-level flow to become quasi-
zonal early next week with similar shower and thunderstorm
probabilities persisting as multiple, potential shortwave troughs
pass.

Pfahler

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 550 AM CDT Mon Aug 4 2025

Aside from brief fog at KSUS just before the start of the period
with patchy river valley fog, VFR flight conditions will continue
through this evening. However, there is a 30 to 50 percent chance of
showers and thunderstorms this afternoon and evening, mainly across
south-central/southwestern IL. Therefore, a PROB30 has been included
at KCPS late this afternoon. Conditions will become favorable for
the development of a mixture of stratus and fog overnight into
Tuesday morning. The coverage stratus and fog is uncertain along
with the spatial delineation between them; however, model guidance
suggests portions of northeastern, central, and southeastern MO are
most likely to see fog. This TAF package generally reflects this
idea, but only MVFR visibilities and/or ceilings have been included
for now.

Pfahler

&&

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

$$

WFO LSX