239
FXUS63 KLSX 232353
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
653 PM CDT Sat Aug 23 2025

.KEY MESSAGES...

- After today, a period of below average, fall-like temperatures
  and largely dry conditions will persist through mid-next week.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 239 PM CDT Sat Aug 23 2025

A broad upper-level trough will continue to pass over the CWA
through into this evening which has lead to broken band of upper/mid-
level clouds passing overhead throughout the day. HREF membership
support up to a 20 percent chance of showers this evening in
association with these clouds across central and northeastern MO and
south-central/southwestern IL with a cold front sweeping through the
area. That being said, nebulous/ill-defined forcing and weak frontal
convergence will be an overall limiting factor for any
precipitation. If showers develop, a stray lightning strike cannot
be ruled out with MUCAPE of 750 to 1500 J/kg noted on the latest SPC
mesoanalysis.

However, the main impact of the cold front will be the subsequent
cooler and drier airmass ushered by low-level northwesterly flow/CAA
into the region with today`s high temperatures in the 80s F cooling
25 to 30 F to the mid-50s to around 60 F by Sunday morning.
Increasingly efficient radiational cooling upon arrival of the
leading edge of a surface anticyclone will also aid in the cooling
and possibly support patchy fog in northeastern and central MO.
Temperatures at 850-hPa will be cooling below the 10th
climatological percentile on Sunday, leading to high temperatures in
the mid-70s to near 80 F despite insolation only competing with
diurnal cumulus.

Pfahler

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 239 PM CDT Sat Aug 23 2025

Monday through Wednesday, an anomalous upper-level longwave trough
across southeastern Canada and the Lower Great Lakes will be slow to
advance eastward, entrenching the Mid-Mississippi River Valley in
northwesterly flow above a strengthening surface anticyclone with
MSLP of 1024+ hPa nearly exceeding seasonal climatology. These
features will contribute to further cooling of 850 hPa temperatures
to below the 5th climatological percentile and deflection of most if
not all showers that attempt to reach the CWA at the edge of a
wetter pattern across the Central/Southern Plains. The end result
will be an extended period of below average high temperatures in the
70s F and low temperatures in the mid-40s to low-50s F, more akin to
early October. Low temperatures could threaten daily records at our
climate sites (see CLIMATE section) Tuesday and Wednesday mornings,
especially if clouds remain minimal with the surface anticyclone and
otherwise efficient radiational cooling favored.

Global model guidance begin to diverge after Wednesday on if the
upper-level longwave trough is able to depart further eastward or is
reinforced by additional troughs rounding the feature, making the
difference whether or not deep northwesterly flow persists or begins
to open to a more westerly, progressive pattern. Although there is
agreement in the NBM on a slight warming trend during the second
half of the week, interquartile temperature ranges span 5+ F by next
weekend as uncertainty increases. Depending on the position of the
longwave trough, the wetter pattern that will have been in place
across the Central/Southern Plains may shift slightly northeastward
as 20 to 30 percent of ensemble model membership has measurable
rainfall in central and southeastern MO. However, largely dry
conditions still have the most support.

Pfahler

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 646 PM CDT Sat Aug 23 2025

A cold front is moving south through the area tonight. Isolated
showers, possibly a thunderstorm or two, are possible ahead of the
front this evening. Any convection that does develop should be
weak, and is expected to dissipate before 06Z. The showers may
briefly reduce the visibility to less than 6SM. Otherwise VFR
flight conditions and northwest flow are expected to prevail
through the period.

Carney

&&

.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