229
FXUS63 KLSX 182241
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
541 PM CDT Thu Sep 18 2025

.KEY MESSAGES...

- Thunderstorms are likely (60-70%) early tonight across
  central/northeast Missouri. A few of these storms may be strong
  to severe between 6-10 PM in central Missouri. Damaging winds
  is the main threat.

- Multiple subsequent rounds of showers and thunderstorms are
  possible this weekend through mid next week, with the best
  chances (50-70%) across the area Sunday/Sunday night.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 228 PM CDT Thu Sep 18 2025

Showers and thunderstorms have just initiated across far eastern
Kansas and southwest Missouri in response to a midlevel
disturbance ejecting out of the south-central Plains. These
storms should increase in coverage as they move northeast into
parts of central Missouri by early/mid evening. A few of these
storms may be strong to severe in central Missouri between 6-10 PM
with damaging winds the main concern. There will be steep low-
level lapse rates and high DCAPE values, so the strongest cores
may be able to produce some microbursts as they collapse. By
late evening, there likely will be some type of composite outflow
that moves out ahead of the thunderstorms to the north and east.
This should occur due to a lack of balance between the cold pool
and the low-level shear. Storms behind the composite outflow
should then weaken with time. This internal factor combined with
weakening instability and the midlevel disturbance moving off into
the Upper Midwest late tonight mean more in the way of isolated
to widely scattered showers as activity moves toward the
Mississippi River.

Showers and thunderstorms are most likely (60-70%) across parts of
central/northeast Missouri through about midnight. These areas
should see at least some beneficial rainfall, with most locations
seeing over a quarter inch. Those directly impacted by thunderstorms
will see more, with some spots as high as 1.0-1.5" possible as
supported by the 12Z HREF LPMM.

Isolated showers remain possible early Friday morning across eastern
Missouri and western Illinois. This activity should completely
diminish by mid/late morning as the low-level jet fades. As diurnal
instability climbs, additional showers and thunderstorms should
redevelop by afternoon. They should develop along a southwest-
northeast axis, likely along a surface trough or remnant boundary
from the antecedent convection. This boundary is likely to be near
or just southeast of I-44/I-55 in Missouri and Illinois
respectively. Showers and storms should then drift east/northeast
and weaken/exit the area by late Friday evening.

Highs on Friday are expected to be on average about 5-10 degrees
cooler than this afternoon due in large part cloud cover, though 850-
hPa temperatures do cool about 4C as well. Readings are forecast to
reach into the 80s for most, with temperatures increasing as you
head further east.

Gosselin

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 228 PM CDT Thu Sep 18 2025

(Saturday - Sunday Night)

There remains quite a bit of ambiguity with respect to convective
trends this weekend. The key remains trying to time individual
disturbances within the quasi-zonal flow aloft. That is very
difficult, even within 72 hours. Multiple rounds of showers and
thunderstorms remain possible this weekend. Ensemble guidance is
hitting the Sunday afternoon/evening time frame hardest with respect
to the probabilities for measurable precipitation (60-70% on the
LREF). Saturday morning conversely is the most likely time to be
completely dry, with only 10-20% of members showing measurable
rainfall. The likelihood that Saturday morning is dry makes a lot of
sense with rising mid/upper level heights behind Friday`s
disturbance. I am a bit more leery on the high PoPs Sunday
afternoon/evening, in large part due to timing/track differences
with another midlevel disturbance, but also the possibility of
overnight convection late Saturday night into Sunday morning. There
is a signal from deterministic guidance of fairly widespread
convection during this time period, but vary widely with the
location. This round of convection will have at least some
ramifications on Sunday as well.


(Monday - Next Thursday)

Models show a midlevel shortwave trough digging into the northern
Rockies on Monday and becoming cutoff from the mean flow. The
overall signal for a cutoff mid/upper level low forming early next
week remains high. The most recent cluster analysis of the 500-hPa
height pattern now shows a cutoff low explicitly in 3 of the 4
clusters (~80% of LREF members). The big question remains where and
there is still a lot of spread. One cluster cuts it off more in the
Rocky Mountains while the other two are in the middle of the US. A
position to the west/southwest of the area would have the potential
to produce quite a bit of widespread beneficial rainfall as deep
Gulf moisture would brought north and remain potentially over the
area for several days. Mean total rainfall from the grand ensemble
is 1.25-1.75" of rain, but the 90th percentile jumps to 2.50-3.00".
Given the ongoing dryness/drought any widespread rain should be very
beneficial and there is the possibility that this occurs early/mid
next week. The caution is that these cutoff lows are notoriously
difficult to predict, in terms of where they first cutoff, but also
how they evolve with time as internal dynamics (Fujiwhara effect)
play an outsized role. There definitely is still a chance that
either the midlevel low does not cut off and is more progressive, or
cuts off to our east. Either of those two scenarios would not
produce that much rain. The 10th percentile of the LREF shows this
lower end rainfall potential well, showing only about 0.25-0.50" of
total rain through next Thursday.

Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 535 PM CDT Thu Sep 18 2025

The primary concern during the 00Z TAF window continues to be the
potential for showers and thunderstorms, particularly at
COU/JEF/UIN during the first 3 to 4 hours of the period. Numerous
thunderstorms are ongoing to the west of the area, and eventually
some of these storms are expected to drift east and may impact
these terminals in the evening. They will likely be weakening as
they arrive, but may still cause reduced visibilities and produce
occasional strong, erratic wind gusts. Overnight, these storms
will weaken, but a few showers may reach STL/SUS/CPS very late or
even near sunrise. However, this activity is not expected to be as
impactful, if any of it manages to make it this far east at all.

A break from the showers and storms is likely after this early
morning activity, and dry conditions are likely through early
afternoon. A few more showers and thunderstorms may develop late
tomorrow afternoon but sparse coverage continues to keep
probabilities below mentionable levels (less than 30%) at any
specific terminals.

BRC

&&

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

$$

WFO LSX