791
FXUS63 KLSX 011929
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
229 PM CDT Mon Sep 1 2025

.KEY MESSAGES...

- Slight rain chances (15-20%) skim central and southeast Missouri
  through Tuesday afternoon. Better chances (60-70%) arrive along
  a cold front late Wednesday evening into Wednesday night.

- Below normal temperatures gradually warm to near normal
  Wednesday. Much cooler than normal temperatures arrive behind
  Wednesday night`s cold front. A second cold front brings an
  extension to the cooler than normal pattern next weekend.

&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 225 PM CDT Mon Sep 1 2025

A longwave upper trough virtually enshrouds the eastern 2/3rds of
the U.S. mainland with surface to mid-level ridging extending from
the New England Region westward through the Great Lakes and into the
Upper Mississippi Valley. A weakening upper level shortwave and
corresponding mid-level low continue to track southeastward through
the eastern Plains and down the western side of the upper ridge.
Scattered/broken high clouds over west-central and southwest
Illinois thicken to the west with overcast skies and a few light
showers that continue to weaken over central Missouri.

The weakening upper level shortwave is complimented by a mid-level
low that is expected to traverse the Kansas/Missouri border today
into Tuesday. As the mid/upper levels (above 800 mb) saturate west
of the Mississippi River, the depth of the dry layer increases
significantly over Illinois. Surface dewpoints are relatively
uniform with low to mid-50s well-represented across the area this
afternoon. This changes very little through Tuesday with RAP
soundings providing better insight to the rain potential, albeit low
(15-20%). While the HRRR/RAP/NAM all favor scattered, light showers
working north to south tonight into Tuesday, RAP model soundings
provide better insight on rain potential with little saturation over
our Illinois Counties, if any at all over interior sections of the
state.

Showers may skim the central Missouri counties in the CWA tonight,
progress into southeast Missouri Tuesday. An isolated clap of
thunder is possible Tuesday afternoon, but I`m hard pressed to see
much panning out north and east from there. Even where saturation is
deeper aloft of 800 mb, only 30-40% of HREF members suggest
measurable rainfall (0.01") as surface dewpoint depressions of 15-
25 degrees persist through the end of the period. Outside of minor
rain chances, clouds will play a role in temperatures with slightly
milder nighttime lows (upper 50s/low-60s) tonight and slightly
cooler highs (upper 70s) where clouds remain broken to overcast
(central/southeast Missouri). Subtract/add on a couple of degrees
for lows/highs where clouds are less prevalent.


Maples

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 225 PM CDT Mon Sep 1 2025

Medium to long range guidance is in pretty decent agreement with
respect to a well-organized upper level, closed low that becomes
centered over the Upper Midwest near the Canadian border by midweek.
As the system rotates well to our north, west-southwest flow, around
the base of the trough, advects milder mid-level air from the Plains
into the Mid-Mississippi Valley. 850mb temperatures warm from the
low-teens (C) to the mid and upper teens ahead of the front
Wednesday afternoon. This equates to highs in the low to mid-80s,
which is right on par with normal temperatures at this time of the
year.

Much of Wednesday remains dry with LREF timing probs showing a
relatively narrow spread in the arrival time of the front. Unlike
many of the weak, slow-moving fronts that stall in summer, this is
one of the first impressive fronts we`ve seen to the start of
meteorological fall (SEP-NOV). The front brings strong cold air
advection with PWATs of 1.25-1.50" running along and just ahead of
the boundary. This would sometimes lead to inquiry about the
potential for locally heavy rain and/or strong thunderstorms.
However, convective potential is limited by the lack of directional
shear with unidirectional flow through the depth of the sounding and
0-6km bulk shear of 20-25 knots not lending much more support.
SBCAPE values well under 1000 J/kg and mid-level lapse rates below
6C do not suggest much more than showers and a few thunderstorms
will be possible. The fact that the boundary arrives late in the day
with LREF timing probs suggesting 5 p.m.-7 p.m. Wednesday evening
near KUIN will further limit potential as diurnal trends take over.
Unfortunately, this will be a quick hit with activity weakening in
the late night period. More precipitation favors the northern CWA
with amounts trailing off to the south. Latest LREF data has
roughing 30-40% of the members >0.25" from KCOU to KUIN, which
quickly tapers to less than 20% from Rolla to St. Louis through
Effingham (IL) and points south.

The front clears much of the area by early Thursday with some
lingering precipitation over far southern sections of the CWA. The
bigger story will be some of the coolest temperatures of the season
thus far. Current NBM forecast highs in the low to mid-70s are
notably cool and not far off from record territory (record cold
highs). St. Louis would come the closes with a record of 68 degrees
set back in 1974, while Columbia is 65 in 1974 and Quincy is 62 in
1994. While Quincy looks out of the question, the 25th percentile in
LREF spreads land in the upper 60s with only 4 degrees separating
the 25th/75th percentile. Current forecast lean closer to the 75th
percentile with decreasing clouds through the afternoon.

Another shortwave rotates down the western side of the longwave
trough late in the week, along with a weak surface low and second,
reinforcing cold front. Temperatures will only warm slightly ahead
of the front (low/mid-80s), but timing of the front will determine
just how far north the milder air makes it Friday. The front isn`t
necessarily an airmass that is cooler than Thurssday`s low-70s, but
is cooler than Friday and most notably an extension to the below
normal pattern we`ve seemed to settle into recently. As noted in the
prior discussion, this may not last too much longer with some
growing signals of a return to the 90s just beyond the end of the
long term period.


Maples

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1216 PM CDT Mon Sep 1 2025

VFR conditions are expected through the end of the TAF period as a
high pressure ridge maintains light easterly/variable surface
flow. An isolated showers is possible over central Missouri this
afternoon/evening, but dry air is favored to hold with any
precipitation being light and short-lived. Therefore, no
precipitation was included in the latest TAF update.

Maples

&&

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

$$

WFO LSX