021
FXUS63 KLSX 241723
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1223 PM CDT Sun Aug 24 2025

.KEY MESSAGES...

- -Fall like conditions will kick off today and linger into the
  mid week period.


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 127 AM CDT Sun Aug 24 2025

The cold front is exiting the forecast area this morning, and cool,
dry air is making its way into the region. As of 1 AM temperatures
range from the upper 50s across northeast Missouri and west-
central Illinois to the mid 60s across the St. Louis metro, southeast
Missouri, and southwest Illinois. These temperatures will continue
to cool through the night before reaching their minimums around
sunrise. As the cold air advection continues today, 850 mb
temperatures will drop into the low teens (C), resulting in high
temperatures in the mid 70s to low 80s today despite the mostly
clear sky.

The unseasonably cool and dry air is driven in part by an
anomalously deep mid-level trough that will continue to dig into
the eastern half of the CONUS into next week. At the surface a
strong high is pushing southeast into the region from the Northern
Plains, and is the other contributing factor to our fall like
temperatures. 850 mb temperatures will continue to cool on Monday
as the main mass of the cool Canadian air settles into the mid-
Mississippi Valley. 850 mb temperatures will be roughly 8-11
degrees (C), and will result in high temperatures solidly in the
70s. Areas that see heavier cloud cover Monday may drop an
additional degree or two.

Delia

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 127 AM CDT Sun Aug 24 2025

Through Wednesday the anomalously strong mid-level trough and
surface high pressure will keep the cool, dry Canadian airmass
fixed over the region. High temperatures through the mid-week
period will be similar to Monday, though overnight lows will be
considerably cooler than normal thanks to the low dewpoints within
this airmass. As mentioned in previous forecasts, we`ll be
dancing with record low minimum temperatures Tuesday and Wednesday
morning (see Climate Section below), particularly at KUIN. The
strength of the surface high will keep the area rain free through
Wednesday.

By Thursday the mid-level pattern will start to weaken, but the mid-
Mississippi Valley will remain in mid-level northwesterly flow. The
surface high will slide eastward during the same period, and the
result will be a rebound of temperatures and moisture across the
region for the end of the work week and into the holiday weekend.
Guidance is starting to key in on a shortwave sliding down the mid-
level northwesterly flow into the region over the weekend, and there
will be enough available moisture to generate some rain where it
tracks. However, a lot of uncertainty remains in the strength,
timing, and location of this feature, and so uncertainty remains in
who, if anyone, sees rain and how much. 850 mb temperatures will
warm as the surface high shifts eastward, returning to near normal
(14-16C). How warm high temperatures get will depend on the timing
and location of rain and associated cloud cover, and the uncertainty
in these details is highlighted in the 5F+ interquartile spread in
high temperatures. Despite this uncertainty, there is a clear signal
for warming into the weekend.

Delia

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1223 PM CDT Sun Aug 24 2025

Predominantly dry and VFR flight conditions are expected through the
TAF period at all terminals with a nearby high pressure center
dominating. Patchy river valley fog cannot be entirely ruled out
Monday morning, but confidence is low that there will be enough
moisture. Winds will remain generally northwesterly with gusts of 17
to 22 kt through this afternoon, but winds will become lighter and
periodically variable tonight.

Pfahler

&&

.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