670
FXUS63 KLSX 021722
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1222 PM CDT Tue Sep 2 2025

.KEY MESSAGES...

-  A strong cold front will move through the region late
   Wednesday afternoon/evening and bring a good chance (50-70%) of
   thunderstorms. Behind the front, mostly dry weather with well-
   below normal temperatures are forecast through the weekend.


&&

.SHORT TERM...  (Through Late Tonight)
Issued at 257 AM CDT Tue Sep 2 2025

Isolated light rain showers, mainly in central and east-central
Missouri, are expected to continue through late morning. Shower
activity should gradually shift southward this morning before
dissipating just before midday. A stray afternoon shower or two
cannot be ruled out in southeast Missouri, but largely dry weather
is expected areawide after this morning. High temperatures are
expected to reach into the low 80s for most of the area, though
there may be some pockets of upper 70s in parts across parts of
central/southeast Missouri where midlevel clouds linger into the
afternoon.

Seasonably cool temperatures are expected overnight tonight as lows
drop back into the mid 50s to low 60s. The coolest locations are
expected to be in parts of northeast Missouri and western Illinois
where less cloud cover is expected. Conversely, more clouds are
expected in parts of east central and southeast Missouri as well as
southwest Illinois. These clouds should keep lows a bit more on the
milder side. Dry weather is expected overnight, but a few stray
light rain showers aren`t out of the question. Deterministic model
guidance shows some very weak low-level moisture convergence
overnight. A couple of CAMs are picking up on some spotty light rain
showers, but confidence wasn`t high enough to introduce any PoPs.
Regardless, any shower activity would be very light and not
impactful.


Gosselin

&&

.LONG TERM...  (Wednesday through Monday)
Issued at 257 AM CDT Tue Sep 2 2025

(Wednesday - Wednesday Night)

A strong mid/upper level low is expected to dive southeast toward
the Great Lakes by early Wednesday evening. This trough will induce
surface cyclogenesis downstream, with a low moving toward the
southern tip of Hudson Bay. A cold front will be draped to the
southwest of this low and move southward during the day on
Wednesday. The front is expected to move through the mid-Mississippi
Valley between late afternoon and mid/late evening on Wednesday. The
combination of weak surface convergence along the boundary and
mid/upper level diffluence should lead to fairly widespread showers
and some thunderstorms with this front. Chances of measurable
rainfall look very high across much of the area with the latest LREF
showing probabilities over a 24-hour period ending late Wednesday
night of 80-90+ percent. The bad news? The front will be moving
through pretty fast so there is not a long temporal window for more
beneficial rainfall. Probabilities for at least 0.25" of rain drop
dramatically (20-35%), with less than a 10% chance of 0.50" or more
of rainfall. In other words, this front is not expected to bring
widespread significant relief to the ongoing dryness over the past
month.

In terms of the threat for any strong to severe thunderstorms, the
better threat looks to stay to our west/southwest (more over eastern
Kansas) where deep-layer shear and instability both should be
higher. The stronger shear lags a bit more behind the cold front in
our area, with weaker midlevel lapse rates/instability as well.
Probabilities from the HREF for at least 500 J/kg and 30 knots of
deep-layer shear only peak out in the 20-40% range across northeast
Missouri through 0Z Thursday. As the front continues to progress,
the limited instability should fade after sunset which should
portend to weakening convection.

Ahead of the front, temperatures should warm nicely into the low to
mid 80s. Some upper 80s may even be possible in parts of east
central and southeast Missouri as well as southwest Illinois based
on the timing of the front. Surface winds are also expected to veer
more to the west/southwest, which is favorable for downslope flow
off of the Ozark Plateau.


(Thursday - Monday)

Much cooler temperatures are in store for Thursday due to strong low-
level cold air advection behind the cold front. Highs in the low to
mid 70s are forecast, or about 10-12 degrees below normal for the
date.

A secondary cold front is expected to move through between late
Thursday night and late Friday morning. There are some subtle (~3
hours) timing differences, but this could impact high temperatures
especially in southeast Missouri and southwest Illinois. This is
where the IQR of the NBM is higher, more on the order of 5-7 degrees
for highs. Prior experience with cold frontal passages in northwest
flow aloft tends to be on the quicker end of the guidance envelop,
so highs in the 70s (at or below the 25th percentile of the NBM) are
more likely.

Behind the front, increasing low to midlevel frontogenesis may yield
some light rain showers Friday/Friday evening. The ECMWF is much
stronger with this frontogenetical forcing than the GFS.
Consequently, it has more widespread rainfall. Regardless, any
rainfall looks light with LREF probabilities for even 0.10" or more
of rain only in the 10-20% range.

The secondary front will reinforce the much-below normal
temperatures through the remainder of the forecast period. Highs
mainly in the mid 70s with lows in the mid 40s to low 50s are
expected. These values would be about 10-15+ degrees below normal
for early September. The coolest mornings are likely to be Saturday
and Sunday morning as a strong surface high (~1026 hPa; >90th
percentile of climatology) moves down the Mississippi Valley.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1216 PM CDT Tue Sep 2 2025

Mid to high clouds and a few lingering showers continue to move
south of the terminals this afternoon. BKN/OVC skies that extend
from central Missouri eastward through southern Illinois all
remain above VFR with clearing skies northward through KUIN. Light
and variable surface winds persist through mid-morning Wednesday
with increasing westerlies ahead of an approaching cold front.

The frontal boundary, while a little quicker in most recent data,
looks like it should hold off at KUIN until after 18z Wednesday.
From there, the front progresses southward through the day with
increasing chances for showers and few thunderstorms. SHRA/TSRA
was initially entertained at KSTL but confidence was too low to
make a definitive call in the last couple of hours in the 30 hour
period. VFR is favored to hold through the end of the period.

Maples

&&

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

$$

WFO LSX