283
FXUS63 KLSX 242313
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
613 PM CDT Sun Aug 24 2025

.KEY MESSAGES...

- Below average, fall-like temperatures and largely dry
  conditions are expected to persist into at least mid-week.


&&

.SHORT TERM...  (Through Wednesday)
Issued at 222 PM CDT Sun Aug 24 2025

The upper-level flow pattern through Wednesday will involve an
expansive longwave trough/low across eastern North America and
northwesterly flow over the Mid-Mississippi River Valley. This flow
pattern in tandem with an anomalous surface anticyclone building
into the area will reinforce the cool and dry post-frontal airmass
already in place today as 850-hPa temperatures fall below the 1st
climatological percentile and PW falls below the 10th. This airmass
will equate to an extended period of below average, fall-like
temperatures with highs in the 70s to near 80 and lows in the mid-
40s to low-50s F, similar values to the anomalously low dewpoints
also present. In the core of the cool airmass and with efficient
radiational cooling, low temperatures Tuesday and Wednesday morning
are expected to approach daily records at our climate sites (KSTL,
KCOU, KUIN; see CLIMATE section). The Mid-Mississippi River
Valley will also be at the northeastern periphery of a wetter
pattern along a weak baroclinic zone across the Central and
Southern Plains, including associated upper and mid-level clouds
spreading over the southwestern half of the CWA on Monday. Some
virga could fall from these clouds across central/southeastern MO
as well, but the dry low- level airmass and little (less than 20
percent) HREF and global ensemble support of measurable
precipitation decreases confidence that anything reaching the
ground would be more than sprinkles.

Pfahler

&&

.LONG TERM...  (Wednesday Night through Next Sunday)
Issued at 222 PM CDT Sun Aug 24 2025

Global model guidance are coming into better agreement that the
aforementioned upper-level longwave trough will shift eastward
enough to allow the baroclinic zone in the Central/Southern Plains
to advance northeastward enough to increase the opportunity of
showers or light rain across central and southeastern MO Wednesday
night through Thursday. Around 30 to 50 percent of ensemble model
guidance supports this scenario, but much higher probabilities of
rainfall exist further to the southwest. Thereafter, the longwave
trough will continue to propagate eastward, eventually allowing a
few potential passing troughs over the weekend as the flow opens up,
at least slightly, to a more westerly component. There is some
uncertainty on how much temperatures moderate through late week into
the weekend with disagreement on whether or not low-level flow
becomes southerly, as NBM interquartile temperature ranges reach
around 5 to 8 F during that time. How much precipitation and clouds
accompany the troughs will also be a factor, but only around 10 to
20 percent of ensemble membership has any showers and thunderstorms,
meaning dry conditions are most likely through the weekend.

Pfahler

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 612 PM CDT Sun Aug 24 2025

Confidence is high that dry and VFR flight conditions will
continue at all local terminals through the TAF period. Wind will
quickly die down at sunset but pick up out of the northwest again
tomorrow morning.

Elmore

&&

.CLIMATE...
Issued at 236 PM CDT Fri Aug 22 2025

Daily record lows for:
                       KSTL         KCOU            KUIN
Tuesday 8/26        53F (1934)     48F (1910)      46F (1910)
Wednesday 8/27      51F (1968)     46F (1910)      48F (1986)


&&

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

$$

WFO LSX