077
FXUS63 KLSX 231106
AFDLSX

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

.KEY MESSAGES...

-There is a slight chance for showers and thunderstorms across
southwest Illinois this afternoon.

-Dry and cool conditions will kick off Sunday and linger into the
middle of next week.


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 248 AM CDT Sat Aug 23 2025

High clouds are moving into the forecast area early this morning,
stemming largely from enhanced cloud cover along a cold front
stretching from central Kansas into central Iowa. Driven by a mid-
level trough centered near the Great Lakes region, this boundary
will slide south through the forecast area today, though its passage
will be marked in most locations only by a wind shift. The slight
upward trend in instability and moisture convergence along the front
has continued, increasing confidence in isolated showers and
thunderstorms along the front in Illinois this afternoon where
moisture convergence is greatest. Yet convergence will still be too
weak to produce anything more than an isolated thunderstorm.

A secondary surface front, which is the true cold front, will lag
behind the first by roughly 12 hours before ushering the cool and
dry airmass into the region. Temperatures will drop dramatically
overnight as this airmass settles in, and morning lows Sunday will
be in the 50s across much of the area. The exception will be in the
far south and St. Louis metro areas, which will bottom out near 60.

A surface high will ooze into the region overnight and into Sunday,
helping to keep the sky largely clear during the day. The cool start
combined with cooling 850 mb temperatures will result in high
temperatures on Sunday 8-10 degrees cooler than today.

Delia

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 248 AM CDT Sat Aug 23 2025

The mid-level trough will continue to shift eastward into the start
of the work week, becoming locked in over the eastern CONUS, and
leaving the mid-Mississippi Valley within persistent northwest flow
through the end of the period. The surface high will likewise
persist over the northern Plains into the mid-Mississippi Valley,
keeping the region open to renewed surges of cool, dry Canadian air
through the work week.

As mentioned in previous forecasts, the 850 mb temperatures
associated with this airmass are anomalously cool, near the 1st
percentile for this time of year. The coolest part of the airmass
will engulf the area Monday-Wednesday. Forecasted values between 8-
11 degrees will result in high temperatures largely in the 70s,
though days with a mostly clear or clear sky will add a few degrees
to the highs. The dryness of the airmass (dewpoints in the upper 40s
to low 50s) will allow overnight temperatures to cool more
significantly into the low 50s or even upper 40s in our northern
areas and more sheltered locations. We will continue to watch the
potential for record low temperatures with this airmass, but the
overall effect will be another taste of fall, y`all.

By Thursday the mid-level pattern will weaken slightly and the
surface high will scoot to the east, opening the region back up to
moisture and temperature advection. How strong both of these are
remains uncertain and guidance has wavered slightly on how quickly
temperatures and moisture rebound. This is shown in the high
temperature interquartile spread which increases to 5-8 degrees
Thursday and beyond. The weakening trough influence will also open
the region up to disturbances that could bring rain back to the
region. Ensemble guidance increases precipitation chances Thursday
into the weekend, though the details on the timing and intensity of
the mid-level disturbance remains uncertain at this time.


Delia

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 558 AM CDT Sat Aug 23 2025

A cold front will drop through the region today, and there is a
15-20% chance for showers and thunderstorms along the front.
Confidence that showers and thunderstorms will develop is low, so
have not included a mention in the TAFs for now. Brief periods of MVFR
visibility is likely underneath any thunderstorms that develop.

Otherwise, dry and VFR flight conditions will continue. Light and
variable winds ahead of the front will become northerly and
slightly stronger in speed as the front passes. Afternoon gusts
are expected at KUIN but will diminish around sunset.

Delia

&&

.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