382
FXUS63 KLSX 021954
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
254 PM CDT Tue Sep 2 2025

.KEY MESSAGES...

-  A strong cold front will move through the region Wednesday
   afternoon/evening with showers and a few thunderstorms likely
   (50-70%) over much of the area. Temperatures behind the front
   will be roughly 10 degrees below normal Thursday.

-  A second cold front brings additional chances for rain Friday,
   reinforcing the seasonably cool airmass. The cool pattern
   persists into early next week.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 248 PM CDT Tue Sep 2 2025

(Thru Tonight)

This morning`s showers have pushed into southeast Missouri and
southwest Illinois early this afternoon, becoming less numerous in
time. The extent of thicker cloud cover has decreased, as well, but
where it remains, temperatures have struggled to climb above 70 by
the noon hour with KCOU/KJEF remaining in the low-70s at 19z. Cloud
cover follows a saturated mid-level layer (850-700mb) closely
resembled by the RAP guidance. Drier air has worked in to the
northern side of the activity with clearing skies north of I-70
ever so slowly expanding southward. Where skies have cleared,
temperatures have warmed well into the 70s to near 80 degrees.

Showers have hung on a little longer than expected with overall
diminishing trends going in the right direction. Hi-res guidance
holds onto spotty activity late this evening and overnight across
sections of central and southeast Missouri in an area of weak mid-
level forcing. I cannot entirely rule out a stray shower with some
of the guidance hanging onto the shallow layer of mid-level
saturation with relatively better chances over southeast Missouri.
Temperatures will cool more effectively to the north where skies
clear (upper 50s) with low a touch warmer underneath cloud cover
over southeast Missouri.

(Wednesday-Wednesday Night)

Mostly clear skies will bring ample sunshine Wednesday for all but
northeast Missouri and west-central Illinois where clouds increase
from mid to late morning ahead of a strong cold front. Ahead of the
front, pre-frontal westerlies draw from milder air over the Plains,
pushing 15-18C at 850mb as far east as southwest Illinois. This has
been consistent in the last several runs, giving higher confidence
in warmer temperatures generally south of I-70 where highs are
expected to climb well into the 80s.

Confidence trails off a bit to the north with the latest guidance
coming in a little quicker as the cold front reaches KUIN as early
as 2-3 p.m. While this seems to be a little too quick, it is not out
of the realm of possibility with LREF timing tools in the last
couple of runs showing the earliest arrival just after the noon
hour. As mentioned in this morning`s discussion, the quicker trends
in these scenarios aren`t uncommon. Latest HREF guidance shows the
greatest spread (10-13 degrees) around KUIN by 2 p.m., progressing
southward toward I-70 by 5 p.m. Considering this is core of peak
heating on a typical diurnal curve, these areas could see forecast
highs trend cooler if trends continue in this direction.

All that being said, the front now looks to arrive over the northern
sections of the CWA near peak heating. This provides a little boost
in available CAPE, now showing MUCAPE upwards of 1500-2000 J/kg
running along/ahead of the advancing front. 0-6km shear of 20-25kts
at the eastern sections of the CWA increases to around 30kts with
the strongest shear lagging just behind the front and corridor of
higher CAPE. Mid-level lapse rates have come up, but remain marginal
(6-6.5C). With timing of the front being in question, even as minor
as it sounds, the dislocation of these parameters and marginal
magnitude don`t make a strong argument for severe potential with
better support remaining over western Missouri and Kansas. A few of
the storms could conceivably show a bit of promise early on, but as
the front progresses toward I-70 near sunset, much of what potential
does exist will wane with the loss of instability. The best bet is
that some thunderstorms produce brief downpours in the narrow
corridor of 1.25-1.5" PWATs and sub-severe gusts, but lose intensity
as they make their way south of I-70 during the late evening.

Winds turn out of the north-northwest with much cooler air spilling
in Wednesday night. Lows in the upper 50s over southeast Missouri
and southwest Illinois trend to the mid to upper 40s over northeast
Missouri and west-central Illinois.


Maples

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 248 PM CDT Tue Sep 2 2025

(Thursday-Friday)

Surface high pressure builds in behind Wednesday`s cold front with
cold air advection persisting through early afternoon Thursday.
Dewpoints fall into the upper 40s to low-50s with light and variable
surface winds under mostly clear skies. Highs in the 70s, along with
the dry nature of the airmass will resemble more an early October
day with temperatures running ~10 degrees below normal.

A deep upper level low rotates south of the Hudson Bay late Thursday
into Thursday night as a surface low navigates the southern portion
of the longwave trough. Another cold front extends southwest of the
surface low and sweeps southward through the area late Thursday
night into Friday morning. As spokes of mid/upper level vorticity
spoke around the longwave trough, a few showers may accompany the
front. The biggest change is to Friday afternoon as medium range
guidance seems to pick up on a mid-level vort feature ejecting
eastward out of the Intermountain West. NBM has trended higher with
PoPs (30-50%) south of the I-70 corridor. LREF guidance has made a
significant jump with 40-60% of its members bringing measurable
rainfall from central Missouri through southwest Illinois late
Friday afternoon into Friday evening. These probabilities have
essentially doubled in the the last 24 hours. While it doesn`t look
like a significant amount of rain, LREF IQR show 0.25" to perhaps up
to 0.50" (75th percentile) over the far southern fringes of the CWA
by late Friday night.

(Saturday-Tuesday)

Much of the long range is uneventful as surface high pressure builds
southeastward from the northern Plains and keeps things quiet
through the upcoming weekend. As the surface high crosses the region
the base of the longwave trough becomes less amplified as the
upper low ejects northeast. Long range guidance is in fairly good
agreement as this distance in the forecast with respect to the
orientation and progression of the surface ridge. This maintains a
dry, seasonably cool forecast through the weekend with cooler
temperatures extending into early next week.

The disagreement begins to appear heading into Monday as Tuesday as
the surface ridge moves into the New England Region. Difference in
the upper wave pattern, along with the introduction of a couple of
shortwaves over the central sections of the country may not be all
that well handled at this length in time. The spread between long
range guidance cause NBM to introduce slight chance PoPs as early
as Monday morning over central Missouri with slight chances
through Tuesday. This isn`t all that uncommon as the pattern goes
through transition and smaller details are more difficult to
grasp. Therefore, confidence in rain chances isn`t all that high
later in the forecast period.


Maples

&&

.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