232
FXUS63 KLSX 212335
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
635 PM CDT Thu Aug 21 2025

.KEY MESSAGES...

- Near normal temperatures are expected the next couple of days
  with highs in the 80s. A cold front moves through late Saturday
  and ushers in temperatures more typical of those observed
  during the first week of October.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 132 PM CDT Thu Aug 21 2025

Latest surface analysis shows a seasonably strong surface ridge
of high pressure across the Great Lakes extending southwest into
the mid-Mississippi Valley. This feature is expected to gradually
weaken and slide southward over the next 24 hours, helping provide
dry weather with near to slightly below normal temperatures.

Most areas will be slightly cooler tonight compared to last night,
with more widespread clearing and even less in the way of wind. Lows
in the low 60s are expected, though some spots of upper 50s may
occur in favored river valleys. Within the urban corridor of St.
Louis, lows will be a bit warmer and more in the mid to upper 60s.
Highs on Friday afternoon will moderate about 2-3 degrees compared
to this afternoon due to air mass modification and slightly higher
850-hPa temperatures. Temperatures should top out in the low to mid
80s, or about a degree or two below normal for the date.


Gosselin

&&

.LONG TERM...  (Friday Night through Next Thursday)
Issued at 132 PM CDT Thu Aug 21 2025

(Friday Night - Saturday)

A cold front is expected to move through the area on Saturday as a
surface low crosses just south of Hudson Bay. Ahead of this front,
moderating temperatures will continue as lows Friday night only drop
back into the mid 60s for most locations. Saturday afternoon highs
are expected to range from the low to upper 80s from northwest to
southeast. There is still some uncertainty with the exact timing of
the frontal passage, but overall confidence in temperatures is still
pretty high because the low-level cold air advection does not ramp
up until several hours after the passage of the surface cold front.

This initial cold front is likely to move through dry, with stronger
mid/upper level forcing for ascent staying well to our northeast
closer to the track of the shortwave trough. Convergence along the
front in our area is very weak, with stronger low-level support
staying across eastern Kansas. Moisture convergence late Friday
night also weakens as the nocturnal low-level jet fades Saturday
morning. This weakening combined with a much more stable environment
across our area strongly suggests any upstream convection will
dissipate before reaching our central/northeast Missouri counties.
This is a long way of saying that there is little/no signal for any
widespread significant rainfall with this frontal passage, and any
measurable precipitation looks unlikely. Probabilities for
measurable rainfall on the LREF have dropped on average about 20-25
percent over the past 24 hours. Given these trends, slight chance
PoPs were removed from the forecast.


(Saturday Night - Next Thursday)

Anomalous mid/upper level troughing continues to amplify across the
Great Lakes late this weekend into early next week, with multiple
reinforcing shots of cooler air spilling into the CONUS. The
ECMWF/EPS has trended more toward the cooler GFS/GEFS during this
time period, increasing confidence in well-below normal temperatures
for next week. The overall pattern is very reminiscent of one you
see in the cool season, and the air mass moving into the mid-
Mississippi Valley out of Canada looks quite impressive.
Temperatures from the NAEFS at 850 hPa drop to near or slightly
below +10C, or near the 1st percentile of climatology. The surface
high (~1022 hPa) sliding down Tuesday into the area is also near the
99th percentile of climatology. Forecast dewpoints are near record
lows at St. Louis, Columbia, and Quincy. In other words, this looks
to be the type of air mass that is very rare for late August and one
that looks capable at least of threatening daily record low
temperatures. High temperatures won`t be as extreme due to dry
weather and plenty of sunshine expected, generally topping out in
the 70s. Those values would still be some 10-12 degrees below normal
however. Widespread lows in the 50s with some upper 40s are likely
next week. The coolest nights are likely to be Monday and Tuesday
night as the strong surface high slides down into the mid-
Mississippi Valley. While the calendar will still say August, it
will feel more like the first full week of October next week.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 630 PM CDT Thu Aug 21 2025

With cool air in place, another round of fog is expected mainly in
river valleys late tonight and early tomorrow morning. Confidence
is greatest in impacts at KUIN, KJEF, KSUS, and KCPS, with the
worst the greatest impacts most likely at the typical trouble
spots of KSUS and KJEF. Fog that develops will dissipate with the
rising sun tomorrow morning, with another round of low-based diurnal
cumulus or stratocumulus developing mid to late morning, possibly
leading to brief MVFR flight conditions.

Elmore

&&

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

$$

WFO LSX