571
FXUS63 KLSX 020833
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
333 AM CDT Wed Jul 2 2025

.KEY MESSAGES...

- Seasonable temperatures will persist through Thursday. Warmer,
  more humid air returns on the 4th of July into the holiday
  weekend.

- The potential for showers and thunderstorms will increase over
  the holiday weekend into early next week.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 330 AM CDT Wed Jul 2 2025

Regional analysis shows high pressure centered over southwestern
Missouri and northwestern Arkansas early this morning. Skies were
mostly clear across much of the CWA with high clouds extending west
to east across southern Missouri into the western Tennessee Valley.
Surface observations reflect a slight uptick in dewpoints since
Tuesday, which were largely in the mid-60s at 07z.

Today and Thursday look nearly identical with limited impacts and
little sensible change in the 24-36 hour period. The first item to
address is the potential for patchy fog this morning, mainly in the
low lying areas and river valleys. RAP soundings show a steep
temperature inversion just off the surface with light/variable
surface flow underneath the surface ridge. Temperatures ranged from
the mid-60s to low-70s, which will drop a few more degrees under
before sunrise with clear skies overhead. The steep temperature
inversion, light/calm winds, shrinking dewpoint depressions and
nighttime microphysics products already show some semblance of
patchy fog over central Missouri, further validating the argument.
What fog does develop should erode rather quickly after sunrise
(13z).

The axis to an amplified upper level ridge extends north/south
through the central Plains today. A couple of shortwaves are
expected to round the northern periphery of the ridge, resulting in
a couple of weakly organized clusters of thunderstorms well to the
north. Much of this is expected to weaken as it closes in on
northern Missouri, but may introduce some mid/high clouds to
northeast Missouri and portions of west-central Illinois. While much
of it is not expected to have a big influence on forecast
parameters, mainly temperatures, HREF guidance does show up to a 70%
probability for scattered ceiling east of the Mississippi River.
Otherwise, most areas should only see few diurnal, mid-level clouds
through peak heating.

We sort of hit the repeat button for tonight into Thursday with
another episode of patchy fog in low lying areas. The only
difference will be a slight increase in surface winds, albeit small,
and shift out of the south/southwest with a slight eastward shift in
the surface ridge. This may limit the extent/magnitude in
comparison to this morning`s potential. This will also give way to a
bump in high temperatures with upper 80s and low-90s Thursday
afternoon.


Maples

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 330 AM CDT Wed Jul 2 2025

Ensemble guidance remains tightly clustered through Friday with a
continuation of the stair-step trend in temperatures. Surface high
pressure moves further southeast as the axis to the upper level
ridge parallels the Mississippi River Valley. Warmer temperatures
will be accompanied by an ever-so-slight eastward shift in the
north/south oriented corridor of higher moisture currently extending
through the Plains. Dewpoints range from the mid-60s over
southwestern Illinois, increasing to the low-70s in Missouri. It
will feel a bit less comfortable, though not necessarily oppressive.
LREF shows low probabilities (15-25%) for heat index values above 95
degrees, making outdoor plans less difficult to manage than triple
digit indices. All things considered, it will be a rather typical,
rain-free July 4th.

Ensemble guidance, while in decent agreement with respect to the
synoptic pattern, shows the greatest question will be precipitation
chances and temperatures from Saturday onward. A series of
shortwaves/troughs is projected to track generally west to east over
the northern section of the Lower 48 states. In addition, the
western side of the upper level ridge becomes the focus for active
weather with several compact upper vort lobes ejecting northeast out
of the Desert Southwest. The eastward translation of the synoptic
pattern will only increase chances for showers and thunderstorms
through the weekend. The noticeable trends has been the delay in the
arrival of higher PoPs with the slow eastward shift in mean pressure
patterns. NBM slight chances (15-25%) now only cover northeast
Missouri and far west-central Illinois, possibly leaning toward a
mostly dry forecast as the ridge maintains enough influence of the
area.

By Sunday, the trough to the north begins to flatten as it moves
over the Great Lakes Region. This lead trough initially looked like
it would be enough of a motivator to push a cold front through the
area, but that has only delayed in speed, too. The front now slowly
sinks southward through Sunday afternoon/evening, providing weak to
marginal surface convergence for showers and thunderstorms that peak
during the diurnal max, provided the limited ascent in the absence
of upper lift. NBM spreads are justified as LREF ensemble data shows
greater variability in the progress and position of the surface
boundary, likely washing out somewhere in the region. Additionally,
the upper ridge flattens as multiple shortwave track west to east
over the northern U.S. Chances for showers and thunderstorms
stretch into early next week with a 5-7 degree spread between NBM
interquartile ranges through the end of the period.

Despite better moisture profiles and increase in instability, the
west/east orientation of the front early next week results in less
convergence with time. On top of this, weak flow/shear will likely
lean on diurnally driven behavior for opportunities to receive
measurable rainfall. Surface high pressure eventually builds
southeastward from the northern Plains into the Great Lakes late
Monday with the ridge axis extending southwestward into the mid-
Mississippi Valley. The exact position of the surface high and its
southwest extent will be something to watch, as it may be enough to
push the surface boundary further south and result in a downward
trend in Monday`s PoPs in subsequent forecasts.


Maples

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1017 PM CDT Tue Jul 1 2025

Mainly dry and VFR conditions with light and variable winds are
expected through the period. The only exception will be river fog
late tonight into early Wednesday that will likely affect
JEF/SUS/CPS, so will continue with the TEMPO groups at these
airports between 08-13Z.

Britt

&&

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

$$

WFO LSX