107
FXUS63 KLSX 222320
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
620 PM CDT Fri Aug 22 2025

.KEY MESSAGES...

- Fall like conditions start Sunday behind a cold front and
  continue through the end of next week.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 236 PM CDT Fri Aug 22 2025

A ridge of surface high pressure remains in place across the mid-
Mississippi and Ohio Valleys. Diurnal cumulus/stratocumulus
should largely dissipate after sunset, leaving a mostly clear sky
and light winds into the early overnight hours. These conditions
are very favorable for radiational cooling, and should allow for
temperatures to drop back into the low 60s overnight. Exceptions
include parts of metropolitan St. Louis (urban heat island) and
northeast Missouri and west-central Illinois (increasing mid/high
level clouds).

An area of showers and a few thunderstorms is expected to be ongoing
across the mid-Missouri Valley. This activity does not look
like it will be particularly organized nor strong, and is still
expected to weaken and dissipate before it reaches parts of central
and northeast Missouri. This dissipation is expected due to the
showers/storms moving into a less favorable environment (low
instability) and weakening moisture convergence as the nocturnal low-
level jet fades after sunrise.

The cold front moves through the CWA during the afternoon/evening
hours from northwest to southeast. Ahead of this front, temperatures
are expected to warm into the mid to upper 80s or near to slightly
above normal for the date. The front is expected to pass through
largely dry, though some increasing instability and weak surface
convergence suggests a few isolated showers (and perhaps even a
rumble of thunder) are possible along the front very late
Saturday afternoon into the evening hours.


Gosselin

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 236 PM CDT Fri Aug 22 2025

The transition to abnormally cool temperatures gets underway on
Sunday as strong low-level cold air advection intensifies behind the
initial cold front. The mid/upper level pattern across the CONUS is
quite amplified for late August, with anomalous mid/upper level
troughing across the Great Lakes/southeast Canada. Individual
shortwaves continue to rotate cyclonically around the periphery of
the longwave trough, providing reinforcing shots of cool Canadian
air into the region.

The overall air mass coming in early next week continues to look
very impressive. Multiple indicators such as 850-hPa temperatures
~1st percentile of climatology), surface high pressure (>99th
percentile), and dewpoints (near daily record lows) all point toward
very cool temperatures through much of next week. The most anomalous
temperatures are expected between Monday and Wednesday as 850-hPa
temperatures drop below 10C across much of the mid-Mississippi
Valley. The dryness of the air mass and the incoming surface high
also suggest that low temperatures will be more anomalous than
daytime highs. Lows ranging from the upper 40s to low 50s are
expected both Monday and Tuesday night, and these readings may
threaten daily record lows each day. Please see the CLIMO section
below for more details. As alluded to above, the air mass certainly
seems capable of producing at least near-record lows, and if the
timing of this strong surface high is favorable and the sky is
clear, it would not surprise me at all if at least one record is
tied/broken next week.

The pattern across the CONUS deamplifies a bit heading toward late
next week, with 850-hPa temperatures climbing back closer to +15C on
the EPS/GEFS mean. This points toward moderating temperatures,
though they should still stay at least slightly cooler than normal.
Even the 75th percentile of the NBM stays slightly below normal for
temperatures all the way through Friday.

Rain chances are very low through Wednesday, with cool/dry air
across our region near the center of the surface anticyclone.
Chances may return later in the work week as a northwest flow
midlevel shortwave trough moves out of the mid-Missouri
Valley/Upper Midwest. Guidance differs quite a bit however with the
timing/track of this feature as well as the availability of moisture
associated with this system. Probabilities for measurable rainfall
ending Friday evening over a 24-hour period from the grand ensemble
are only in the 20-30% range, mainly confined to parts of central
and southeast Missouri.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 616 PM CDT Fri Aug 22 2025

River valley fog is forecast tonight in portions of eastern
Missouri including KSUS. IFR conditions are expected while fog is
in place and conditions will improve shortly after sunrise. A cold
front will move through the forecast area tomorrow, and winds will
switch to northwesterly as it approaches.

Jaja

&&

.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