712
FXUS63 KLSX 101634
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1134 AM CDT Thu Sep 10 2026

.KEY MESSAGES...

- Periods of showers and thunderstorms, widespread at times,
  will continue through this morning into early afternoon.

- Locally heavy rainfall is possible this morning but isolated
  flash flooding cannot be ruled out in portions of east-central
  MO and southwestern IL.

- Temperatures will be seasonable through Friday before warming
  above average through the weekend and early next week. Low (20
  to 40 percent) chances of showers and thunderstorms also exist
  Friday through Sunday.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 329 AM CDT Thu Sep 10 2026

Upper-level quasi-zonal flow ahead of a trough over the Central
Plains will continue to support waves of showers and thunderstorms
through at least the morning as a series of mid-level vort
maxes/MCVs track eastward. Most of these showers and thunderstorms
are expected to be focused around the I-70 and I-44 corridors where
low-level FGEN is forecast to ebb and flow with multiple narrow LLJ
cores translating eastward. With decreasing (increasing) surface-
based instability (convective inhibition), the already low threat of
severe weather has further decreased, but locally heavy rainfall
still warrants monitoring through the morning. With anomalous PW
exceeding 2", deep-warm cloud depths, and around 1000 J/kg of
MUCAPE; ingredients are in place for efficient, high rainfall rates.
In fact, rates of 1.5 to 2.5"/hr were estimated with the strongest
thunderstorms earlier this evening. The 00z HREF LPMM of QPF valid
after 08z (3 am) this morning, indicates 1 to locally 3" of
additional rainfall near the I-70 and I-44 corridors. Soils are
still dry, but there is a swath of 1 to 3" of rainfall from last
evening that extends from near Washington, MO to Belleville, IL that
has elevated streamflows and could precondition soils for isolated
flash flooding if additional heavy rainfall overlaps that area.
Otherwise, the threat of flash flooding is still low with most, if
not all, of this rainfall being just beneficial.

It is not completely clear how quickly precipitation will end late
this morning through afternoon with the CWA still remaining just
ahead of the upper-level trough axis and in an equatorward jet
entrance region, but there should at least be a decrease in
thunderstorms and overall coverage of showers as the effective cold
front progresses further south of the CWA. That being said, there is
a weak, secondary cold front that advances southeastward into the
evening CWA that 15 to 30 percent of HREF membership has resulting
in additional isolated to scattered showers, perhaps a rumble of
thunder. With widespread clouds and precipitation lingering well
into the day, high temperatures will fail to warm above 80 F in many
areas.

The back edge of at least thicker cloud cover is progged to reach
some portion of the northwestern half of the CWA this evening into
tonight, which will help set the stage for a favorable setup for fog
as the aforementioned front washes out with residual BL moisture and
stagnating low-level flow in addition to moistened soils. The most
favored areas for fog development is central, northeastern MO and
west-central IL, where confidence is relatively higher in
thinner/fewer clouds. Dense fog cannot be ruled out.

An increasing amount of short-term model guidance has the tail-end
of the upper-level trough fracturing and then lifting across the Mid-
Mississippi River Valley or Lower Ohio Valley on Friday, which leads
to another round of scattered showers and a few thunderstorms mainly
across southeastern MO/southwestern IL where HREF probabilities of
measurable rainfall are highest (up to 40 percent). This evolution
will also prolong clouds across much of the CWA, resulting in high
temperatures only reaching the upper 70s to low-80s F, which are
actually close to average for mid-September.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 329 AM CDT Thu Sep 10 2026

After the upper-level trough finally accelerates and departs the Mid-
Mississippi River Valley, upper-level flow will become quasi-zonal
evening Saturday into Sunday with a cold front reaching the CWA late
Saturday and then stalling on Sunday. As a result, between 15 and 40
percent of ensemble model membership has showers and thunderstorms
at times late Friday night through Sunday, especially across central
and northeastern MO where large-scale forcing and nocturnal LLJs
will be strongest through the period. High temperatures will range
from the low-80s F north of the front and where there are
morning/daytime showers and thunderstorms, to the low-90s F south of
the front where there should be less clouds.

An upper-level ridge will briefly dominate the Mid-Mississippi River
Valley late Sunday through Monday, helping the front lift northward
out of the CWA which places more of the CWA firmly in the warm
sector with 850-hPa temperatures increasing to the 95th
climatological percentile. These factors support Monday being the
most confident hot day of the period for most of the CWA with high
temperatures reaching the upper 80s to mid-90s F.

Global models still vary in their depictions of the upper-level wave
pattern beyond Monday, but there is a growing signal for the upper-
level ridge to shift across the Southeast and Mid-Atlantic, placing
the Mid-Mississippi River Valley in a period of deeper southwesterly
flow for at least a couple days. What is still uncertain is how
quickly the ridge breaks down and allows a cold front to gradually
advance southeastward into the region, breaking hot temperatures
with showers and thunderstorms and a much cooler airmass behind the
front. As a result, the NBM interquartile range of high temperatures
increase to around 10 F Tuesday, onward ranging from persistent
seasonably warm or hot temperatures to temperatures closer to
seasonal averages. It is also worth noting that part of this
uncertain in the ridge break down is related to highly variable
solutions with tropical system development in the Atlantic Ocean
mid to late next week.

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1125 AM CDT Thu Sep 10 2026

Lingering showers and low ceilings are expected to gradually
diminish by the start of the 18Z period or very shortly
afterward. VFR ceilings should then prevail until evening,
although mid and high level cloud cover will be slow to clear
out completely. Overnight, the main concern is the potential for
low ceilings and fog, and confidence is low regarding the
relative coverage of each. Fog is most likely in
northern/western areas where mid/high clouds are less prevalent,
including at UIN/COU/JEF. IFR and even LIFR visibilities will
be possible wherever fog develops, but this will be less common
where low stratus cover develops instead. Where the latter
occurs, it will be low ceilings that impact flight categories
rather than visibilities, with MVFR ceilings at a minimum, and
the potential to drop lower at times. Ceilings should gradually
lift and scatter by the end of the period.

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

&&

$$

SHORT TERM...Pfahler
LONG TERM...Pfahler
AVIATION...BRC