417
FXUS63 KLSX 200356
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1056 PM CDT Sat Sep 19 2026

.KEY MESSAGES...

- A cold front will slowly drift southward through the region,
bringing widespread showers and thunderstorms late Sunday into
Monday.

- The pattern shift will result in cooler, more seasonable
temperatures through next week.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 236 PM CDT Sat Sep 19 2026

A cold front will begin to sink southward through the later half of
the weekend with increasing chances for showers and thunderstorms
Sunday into Monday. Precipitation potential peaks (70-90%) late
Sunday night into Monday, when coverage is expected to become
widespread. This will mark the beginning of a pattern shift that
will include much cooler, more seasonable temperatures behind the
cold front.

Extensive cloud cover coincides with a stationary boundary that has
occasionally meandered north/south over the course of the last few
days or so. South of the boundary, weak mid-level impulses have
given rise to isolated showers and a few thunderstorms. Cells east
of the Mississippi River may briefly show some intensity this
afternoon with a brief, localized downpour, gusty winds, and
lightning. However, this potential looks short-lived with weak
vertical and directional shear (<20 kts) profiles. The stationary
boundary to the north will finally begin to drift southward Sunday
as mid/upper level ridging begins to break down and a longwave upper
level trough approaches from the Plains. Showers and a few
thunderstorms generally track west to east within the westerly flow,
some of which skim the northern sections of Missouri/west central
Illinois by early Sunday morning, roughly along and north of Hwy 36
and I-72. South of the front, showers/thunderstorms are expected to
be more isolated in nature with similar characteristics as today`s
isolated activity with little change in shear/weak westerly flow.
Localized heavy rainfall, gusty winds, and lightning will accompany
more robust thunderstorms, but coverage remains limited where CAPE
values around 2000 J/kg might otherwise support stronger
thunderstorms. Severe potential is low and isolated. Therefore,
severe potential is not being widely messaged.

Increasing chances will track along the front as a second upper
shortwave rounds the mid-level ridge and ejects northeastward from
the southern Plains into Missouri and Illinois late Sunday. Ascent
increases with the arrival of the parent upper trough Sunday night
into Monday with deep saturation and broken/overcast skies. Much of
the ascent is elevated with mean surface instability limited to
around 500-1000 J/kg, so expect showers and a few elevated
thunderstorms through Monday. The greatest uncertainty, with regard
to temperatures, follows the progression of the boundary Sunday
(north of I-70), dictating where surface heating is able to achieve
90s for highs. Otherwise, a much cooler air mass brings needed
relief for all areas by Monday. The warmest temperatures (80s)
linger over southeast Missouri and southwest Illinois, while 60s and
70s run along and north of I-70. probabilistic LREF data shows aerial
averages of 0.25-1.00 of total QPF by late Monday evening. Everyone
gets in on some rainfall, though higher totals are favored around
locations that get under thunderstorms supported by PWATS above 1.5
inches.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
Issued at 236 PM CDT Sat Sep 19 2026

For now, it`s beginning to look as though the anomalous heat and
humidity will give way to the seasonal transition over the course of
the next week. This cool down may seem cool or even chilly to some,
as we shave 20-30 degrees from both high and low temperatures for
several days.

Surface high pressure and mid-level ridging develop over the Great
Lakes into the southeastern Canadian Provinces as the axis to an
upper ridge over the Plains edge eastward toward the Mississippi
River Valley. Global guidance (ECM/GFS) both show some version of
this progression, though they begin to diverge in its evolution from
mid to late week. GFS shows the upper wave getting pinched off and
momentarily becoming stationary west of Lake Michigan, while ECM is
more progressive, taking the upper wave into the Ohio Valley. In
both scenarios, easterly flow is funneled into the region with
limited moisture and mid-level temperatures in the single digits to
around 10C. Despite some of the spread in the long range synoptic
evolution, surface temperature spreads are pretty reasonable,
running around 5 degrees through much of the period. Additionally,
long range teleconnection outlooks give a bit of a boost in a shift
toward sustaining fall-like temperatures as projections show the PNA
sharply trending more positive and AO favoring negative trends. This
would lean in favor of eastern CONUS troughing with northwesterly
flow maintaining more of a fall-like feel through the later part of
the month into early October.

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 1047 PM CDT Sat Sep 19 2026

Area of showers and a few thunderstorms over north central
Missouri is expected to move just north of UIN over the next
several hours, however still expect the chances for showers
will increase at UIN late tonight into Sunday with a chance
(30-40%) between 09-14Z. There is an additional chance at UIN,
after that but confidence is too low in timing to include in
the TAF at this time. Expect MVFR ceilings to move into UIN by
mid-morning remain through the rest of the period with a chance
(30-40%) for IFR ceilings after 00Z. There are also low chances
(20-30%) for showers at COU/JEF between 12-15Z. Chances for
showers remain low at the St. Louis metro terminals until after
00Z on Sunday evening when I have continued a PROB30 group
starting at 02Z. I have only included showers at this point as
the coverage of thunderstorms is expected to be limited, though
I cannot rule out thunderstorms at any of the sites during the
time I have included rain chances. Ceilings will drop to MVFR
after 06Z on Saturday night.  Winds will generally be light,
particularly outside of thunderstorms.

&&

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

&&

$$

SHORT TERM...Maples
LONG TERM...Maples
AVIATION...Britt