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FXUS63 KLSX 281113
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
613 AM CDT Thu May 28 2026

.KEY MESSAGES...

- Motorists may encounter patchy dense fog in river valleys early
  this morning, particularly in southwest IL and east-central MO.

- Seasonably warm and dry conditions are expected today, but
  scattered showers and a few weak thunderstorms return to the
  Ozarks and central Missouri tomorrow and possibly Saturday.
  Lightning and brief bursts of heavy rain are the main hazards.

- A steady drying trend is expected next week, with a continuation
  of near average temperatures.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 322 AM CDT Thu May 28 2026

In the immediate term, quiet conditions are generally in place with
precipitation currently remaining well to the southwest. However, in
spite of widespread cirrus cover, sporadic surface observations
reveal that fog is beginning to develop in some low-lying river
valleys, especially across east-central MO and southwest Illinois.
While we don`t expect fog to become widespread enough to warrant a
dense fog advisory locally, we can`t rule out a few instances of 1/4
mile visibility, especially across south-central Illinois. In fact,
areas just to our east ARE included in a dense fog advisory, so
motorists traveling in that direction should prepare for a
steadily increasing potential for dense fog.

While there will still be a couple of opportunities for showers in a
few places before the end of the week, today is expected to be
mostly dry across our forecast area thanks to an intrusion of dry
air from the northeast behind a weak back-door cold front. This
feature has essentially pushed all of the rich Gulf moisture (50+
percentile precipitable water) back to the southwest, keeping
today`s showers and thunderstorms across mainly southwest Missouri.
While a broad arc of showers and weak thunderstorms currently in
this area will gradually move north through the day, it will be
continuously battling dry air as it does so, and will likely
struggle to reach our area until sometime overnight or tomorrow
morning. As a result, expect a seasonably warm and dry day today,
with highs generally in the mid 80s with comfortable humidity
levels.

Over the course of the day tomorrow, an elongated weak upper trough
will continue to slowly lift north and into the Missouri River
basin, and will push the previously mentioned higher PWAT air north
with it. While this more humid airmass will be constantly battling
the much drier airmass to the northeast, eventually showers and a
few thunderstorms are expected to once again move into parts of
central and southeast Missouri, likely by sometime tomorrow morning.
These areas are expected to see off and on showers throughout the
day tomorrow, but exactly how much further north these showers reach
is a bit less certain. There are some indications in high-resolution
guidance (particularly the HRRR) that a few convective cells may
reach as far northeast as the St. Louis metro, although these
solutions are generally in the minority. In any case, we do expect
that at least some of this activity will move into our portion of
the Ozarks tomorrow at a minimum. As has been the case with all of
this activity, lightning and brief bursts of heavy rain will be the
most likely hazards in the strongest cells, but flooding and severe
thunderstorm hazards not expected due to a number of factors (short
duration of heavy rain rates, weak wind shear). Meanwhile, the rest
of the area can expect to see another day of seasonably warm
temperatures, with perhaps a bit more opaque cloud cover that keeps
temperatures ~5 degrees cooler than today.

BRC

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 322 AM CDT Thu May 28 2026

From Saturday through mid-week, the synoptic pattern across the
CONUS will feature a persistent "Omega Block", with a highly
amplified upper ridge across the central U.S. and Canada bookended
by troughs/closed lows. The eastern trough will be reinforced by a
digging shortwave across the Great Lakes over the weekend, and this
is expected to drive the previously discussed dry air and surface
high pressure back to the southwest. Because of this, it appears
that our potential for additional showers and thunderstorms will
progressively decrease, although we do maintain some low-end
chances for lingering showers throughout the day Saturday.
Otherwise, precipitation probabilities drop below 20% Sunday
onward, and by Tuesday, ensemble mean precipitable water values
drop below the 25th percentile as well. While there is some
uncertainty regarding exactly when our precipitation chances will
conclusively end along our southwestern fringes, the trend for
drier conditions over time is clear.

Meanwhile, this pattern will lock in semi-permanent east-
northeasterly low level flow, which climatologically favors milder
temperatures, or at the very least limits our ceiling for heat. Not
surprisingly, ensemble temperature forecasts maintain near average
temperatures throughout the forecast period with narrow spreads,
although it should be noted that previous suggestions of a
noteworthy cooling trend have also largely diminished as well.
Instead, current indications are that temperatures will remain
largely steady-state for several days, with highs generally in the
upper 70s to low 80s, with perhaps a slow decreasing trend in
overnight lows as drier air and more favorable radiative cooling
conditions become more firmly established.

BRC

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 603 AM CDT Thu May 28 2026

Fog impacting KSUS/KCPS will erode early this morning. Once fog
dissipates, VFR is expected through the period. MVFR ceilings
flirt with KCOU/KJEF Friday morning. However, the two terminals
straddle the line. There is little skill on margins this narrow at
24 hours out in addition to dry, easterly flow impeding the
eastward progress of moisture. Therefore, MVFR was not included
in the prevailing groups, but may be added in later updates.

Maples

&&

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

$$

WFO LSX