Area Forecast Discussion

407
FXUS63 KLSX 300806
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
306 AM CDT Sat Aug 30 2025

.KEY MESSAGES...

- Mostly dry conditions will continue, with temperatures falling
  below normal after today, resulting in a very comfortable Labor
  Day Weekend. A more dramatic, albeit temporary, cooldown is all
  but certain on Thursday.

&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 306 AM CDT Sat Aug 30 2025

The Mid-Mississippi Valley remains vaguely under the influence of
a surface ridge centered across the Great Lakes, though mid-level
shortwave impulses continue to stream south amidst the cyclonic
flow aloft. These features are promoting anything from mid-level
clouds to isolated, weak showers generally along the Mississippi
River where low-level moisture is locally-higher. So far tonight,
all activity is staying north of the forecast area in eastern Iowa
and northwest Illinois. However, most deterministic guidance does
feature continued weak, persistent mid-level vorticity maxima
along the Mississippi River today. Along with the local maximum in
low- level moisture, another day of isolated showers is in the
cards. Those in far northern Missouri and west-central Illinois
may hear a rumble of thunder, with MUCAPE values pushing 1000 J/kg
there, but most of the convection will be too shallow to glaciate
and produce lightning. Otherwise, a largely dry and seasonable
day will mark the first full day of the Labor Day weekend.

Showery activity will linger into the early evening until the loss
of daytime instability ends this threat. Winds back a bit overnight
but remain somewhat easterly to southeasterly, keeping any warm air
advection at bay. Sunday`s temperatures will likely (80% chance) be
stunted a bit compared to today`s due to increased cloud cover and a
low (10-15%) chance for light rain in northeast and central
Missouri. These low rain chances will be forced by a slow-moving mid-
level wave that will attenuate and stall in the region through early
next week, keeping pesky rain chances (15-35%) in place.

MRB

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 306 AM CDT Sat Aug 30 2025

The wave that stalls somewhere in western Missouri Sunday, according
to a majority of available guidance, will continue to promote
showery activity focused mainly to our west. While a few showers may
manage to drift east enough to bring light rain to central and
southeast Missouri through Monday, the bigger impact will be the
cooler temperatures as a result of more abundant cloud cover. The
beginning of climatological autumn will certainly feel like it.
Temperatures monday will struggle to warm above 80 degrees north and
west of St. Louis, and won`t be much warmer than 80 for St. Louis
and south/east. By Tuesday, a more well-defined shortwave develops
in most guidance, which will result in a more concrete threat for
precipitation, particularly south of I-70 where atmospheric moisture
will be most readily available. That said, the moisture profile
looks fairly unimpressive even in the worst-case scenario, which
will very likely (90%+ chance) prevent any heavy rain threat.

Right behind this, the much-advertised cold front that will usher in
a more substantial cooldown will cross the region on Wednesday. With
limited moisture available to it and the potent closed ULL, showers
will be scattered at best and any rain that falls will be on the
lighter side (chances of seeing 0.50 inches of rain are virtually
zero). Very anomalously-cold air aloft and at the surface will rush
in behind the front, keeping temperatures up to 15 degrees below
early-September normals on Thursday. Overnight lows will also be
comparatively chilly, with all areas outside of the immediate St.
Louis metropolitan area forecast to fall into the 40s early Friday
morning. Temperatures do moderate a tad as weak warm-air advection
attempts to establish by next weekend, but below-normal temperatures
and dry conditions will likely persist nonetheless. While this
spells picturesque sensible weather for outdoor activities, we are
entering our driest time of year having seen very little rain
recently. While evaporative demand will be fairly low with
temperatures so cool, the drought will not be alleviated by any
stretch given the current forecast.

MRB

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1031 PM CDT Fri Aug 29 2025

VFR conditions are expected through the 06Z TAF period. Light east
to southeast winds can be expected throughout the period, with
mainly high level cirrus followed by scattered to locally broken
cumulus clouds tomorrow afternoon. A few stray showers will be
possible tomorrow afternoon, mostly near UIN, but the probability
for local impacts to the terminal remain too low (20%) for mention
in the TAF at this time. Otherwise, some patchy steam fog is also
possible near SUS early this morning, but is not likely to impact
the terminal.

BRC

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion