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FXUS63 KLSX 041728
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1228 PM CDT Fri Jul 4 2025

.KEY MESSAGES...

- Warm and humid conditions are expected today through Saturday.
  Temperatures fall slightly Sunday into Monday and remain fairly
  steady through next week.

- Mainly dry conditions are expected through Saturday before
  chances of showers and thunderstorms increase late Saturday into
  Sunday. Probabilities for additional showers and thunderstorms
  extend through the upcoming week.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 224 AM CDT Fri Jul 4 2025

A look around the region shows the center of surface high pressure
sliding just to the east of the local area. In response, surface
winds are veering out of the southeast to south from eastern
Missouri westward through eastern Kansas. The surface high is
trailed by amplified mid/upper level ridging that will position
overhead today through Saturday.

Southerly flow at the western side of the ridge is drawing moisture
northward into the eastern Plains and will eventually shift eastward
into the local area in coming days. Until then, subsidence will
maintain dry conditions with mostly clear to partly cloudy skies.
Expect temperatures to climb into the 90s areawide with urban areas
squarely in the mid-90s. Heat index values will only be a few
degrees warmer than actual air temperatures, along with an increase
in surface flow. This may counter the moisture somewhat, serving to
cool those outdoors better than the calm/variable winds of the last
24 hours.

Saturday is another warm day with much of the daylight hours
remaining dry. Latest data suggests slight chances (15-20%) will
nudge into the western edge of the CWA by mid-morning Saturday. If
this did occur, it`s likely to be remnant activity from the west,
which would be expected to be going through a dissipating phase
considering it being during the diurnal lull in instability and
model sounding show air quickly drying from west to east. Timing
tools show the best estimate for measurable rainfall will be around
5 p.m. Saturday evening and I`d say this may be at the earlier stage
of potential as LREF shows 10-30% probabilities for measurable
rainfall.

Regardless of rainfall, cloud cover will be thicker to the west
during the day Saturday. This keeps temperatures slightly cooler
from central Missouri through west-central Illinois, where upper 80s
are forecast. 90s will largely be confined to southeastern and east-
central Missouri and southwestern Illinois. Lows will range from the
upper 60s to low-70s.


Maples

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 224 AM CDT Fri Jul 4 2025

Aside from the modest spread in temperatures , ensembles have been
rather tightly clustered along with consistency in deterministic
long range guidance with respect to increasing rain chances Saturday
night into Sunday. This is the timeframe that the upper level ridge
overhead begins to move east and deamplify as a train of
troughs/shortwaves traverse the northern sections of the Lower 48.

Guidance continues to show the first in the series of shortwaves
moving into the Great Lakes while pushing a weakening cold front
southward into the region. Potential for showers and thunderstorms
will increase from north to south late Saturday into Sunday. What
initially arrives is likely to be in the form of showers or
weakening convection as surface instability drops in the overnight
period. Sunday will include an increase in cloud cover from
aforementioned activity, which will hold temperatures down slightly
from Saturday. The front encounters a warm, moist air mass, but one
that is initially capped Sunday morning and includes weak
flow/shear. Once air become uncapped, scattered showers and
thunderstorms reemerge during the afternoon and evening.

The stalled boundary remains the focus for additional chances in the
early portion of the week with diurnally driven thunderstorms that
lack any real organization as a result of weak flow. The biggest
question may center on Tuesday as surface high pressure passes to
the northeast but may have enough influence to reduce the potential
over eastern sections of the CWA. It seems that NBM guidance may be
picking up on this a little more as the broad spread of low PoPs
have come down some and the relatively higher potential to the
southwest form a horseshoe around the influence of the surface ridge.
I expect this to be resolved in time to provide better detail to the
spread and magnitude of potential as Tuesday approaches.

An additional consideration will be the mean upper level flow as
ridging builds westward and strengthens over the Four Corners Region
through midweek. Quasi-zonal upper level flow transitions to
northwest flow over the central U.S., which may be conducive for
driving shortwaves around the northern periphery of the ridge and
into the Plains/Mississippi Valley. This is more likely to be the
case by midweek, while early week potential is attributed to the
stalled surface boundary. If so, the best potential for rainfall may
actually be mid to late week.


Maples

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1227 PM CDT Fri Jul 4 2025

Predominantly dry and VFR flight conditions are expected through the
TAF period, with the exception of 4th of July fireworks smoke in
the St. Louis metro forecast to impact KSTL, KSUS, and KCPS this
evening. There is some uncertainty in the exact visibilities
resulting from smoke, but a shallow inversion after sunset should
be able to trap the smoke as it is carried northward by light
southerly winds. At KSUS and KCPS, this smoke could transition to
fog through sunrise Saturday morning, with confidence greater at
KSUS. On Saturday, winds will become more southerly with chances
of showers and thunderstorms increasing just beyond the TAF period
during the evening.

Pfahler

&&

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

$$

WFO LSX