437
FXUS63 KLSX 041939
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
239 PM CDT Fri Jul 4 2025

.KEY MESSAGES...

- Warm, humid conditions will persist through Saturday before
  temperatures cool closer to average by next week.

- Nearly daily chances of showers and thunderstorms exist from
  Saturday afternoon (central, northeastern MO and west-central
  IL) through next week. A few stronger thunderstorms could
  produce gusty winds and locally heavy rainfall Sunday afternoon.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 238 PM CDT Fri Jul 4 2025

An upper-level ridge has become positioned over the Mississippi
River Valley today, resulting in largely dry conditions aside from
isolated showers in more agitated cumulus in a corridor of weak low-
level convergence across central MO this afternoon. Otherwise,
another seasonably warm day is underway with similar dewpoints to
Thursday, lowering with deep BL mixing, keeping heat index values
from becoming excessive. The only thing noteworthy for tonight will
be patchy smoke in the St. Louis metro from 4th of July fireworks,
trapped beneath a shallow temperature inversion developing after
sunset with light southerly winds. This smoke could transition to
fog in river valleys by Saturday morning. However, confidence is low
that visibilities (smoke or fog) will be low enough to have any
significant impact.

The upper-level ridge will shift eastward through the Great Lakes
into the Northeast/Mid-Atlantic on Saturday, permitting a weakening
trough to reach the Mid-Mississippi River Valley Saturday
night/Sunday that forces a cold front into the CWA. Around 20 to 40
percent of HREF membership depicts showers and thunderstorms across
central, northeastern MO and west-central IL during the afternoon
and evening, both likely from showers and thunderstorms or their
remnants arriving from northern and western MO. CAMs indicate that a
modest (20 kt) southwesterly LLJ and the incoming cold front will
allow showers and thunderstorms Saturday evening to survive longer
into the CWA at night. Saturday`s high temperatures will be similar
to today`s aside from greater clouds and scattered showers and
thunderstorms keep temperatures slightly cooler in central,
northeastern MO and west-central IL.

Pfahler

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 238 PM CDT Fri Jul 4 2025

On Sunday, there will likely be a minimum in showers and
thunderstorms during the morning, before redevelopment occurs during
the afternoon. The greatest focus of showers and thunderstorms will
be near the cold front and any outflow that lingers, which 50 to 60
ensemble model guidance indicates will be generally near and south
of the I-44 and I-70 corridors. Coverage still remains an
uncertainty with weak forcing but some transient clustering of
thunderstorms could lead to areas of more numerous coverage. With
weak deep-layer wind shear around 10 kt, thunderstorms will be
disorganized, pulse type in nature, which is not favorable for
widespread severe weather. That being said, PW will be around 2" and
forecast soundings have "inverted V" signatures that point to very
localized heavy rainfall with slow-moving thunderstorms and gusty
winds with potential isolated microbursts. Showers, thunderstorms,
and clouds along with the frontal passage in the northwestern half
of the CWA will lead to further cooling of high temperatures,
ranging from the mid-80s to low-90s F.

A relatively active pattern will persist through most of next week
with nearly daily opportunities for showers and thunderstorms as
upper-level flow varies from quasi-zonal to northwesterly, navigated
by a series of mid-level perturbations and/or MCVs. The specifics of
this pattern are difficult to determine, including the timing of
each of these features and the wavering of the front throughout the
week but it will not rain the entire week like our current PoPs
could be interpreted. Ensemble model probabilities of measurable
rainfall are between 30 and 60 percent across most of the CWA each
day. But there has been a trend toward lower probabilities (20
percent or less) on Monday north of I-70 and the entire CWA Monday
night due to the front temporarily shifting southward and the area
being between waves. The NBM high temperature distribution is
generally centered on average through next week with the
interquartile range increasing from around 3 F during the first half
to 7 F in the second half as uncertainties in the exact position of
the front and presence/coverage of showers and thunderstorms
increase.

Pfahler

&&

.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