Herculaneum, Missouri Weather Forecast Discussion

344
FXUS63 KLSX 201120
AFDLSX
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
620 AM CDT Sat Sep 20 2025
.KEY MESSAGES...
- There is a 30-40% chance of isolated to scattered showers and
  thunderstorms this afternoon. Thunderstorms will be capable of
  frequent lightning, gusty winds, and locally heavy rainfall.
- Our active pattern will continue through the middle of next
  week, with rain chances peaking late Sunday/Sunday night (60%)
  and Tuesday (60-70%).
&&
.SHORT TERM... (Through Sunday Night)
Issued at 237 AM CDT Sat Sep 20 2025
Two areas of convection are west of the CWA early this morning:
one over the Middle Missouri Valley associated with a shortwave
moving through the Midwest, the other over the Central Plains at the
nose of the low-level jet. The former is expected to continue
eastward, but begin pivoting northeastward with the shortwave as it
enters northeastern Missouri. As for the Plains convection, it is
expected to continue tracking east-southeastward as the jet veers
early this morning, gradually weakening as the jet winds down. All
this to say, much of the CWA is expected to remain dry through the
morning hours.
Our chance for convection this afternoon is very questionable. The
main forcing mechanisms will be subtle disturbances within the quasi-
zonal flow aloft and outflow boundaries from the morning convection.
That being said, there are several factors going against convection
this afternoon: weak subsidence in the wake of the previously
mentioned shortwave, moderate to poor mid-level lapse rates, and
cloud cover. The latter may limit already meager forecasted
instability, with many hi-res guidance sources focusing in on SBCAPE
around 750 J/kg. At worst, the 90th percentile of the 00z HREF shows
1,000-1,500 J/kg across the CWA at peak heating - notable, but not
extreme. For any updrafts that are able to develop and maintain for
at least a brief while, they will have more deep-layer shear to
utilize than as of late, with guidance consensus showing 25-30 kts
of 0-6km bulk shear. Therefore, the chance for scattered showers and
thunderstorms across the area is around 30-40% this afternoon, and
while confidence is low in strong storms, isolated gusty winds
thanks to the deep-layer shear, steep low-level lapse rates, and a
dry boundary layer are possible.
Our chance for overnight convection will hinge on how this afternoon
and evening plays out. More widespread convection this afternoon and
evening will have a stabilizing effect on the atmosphere overnight,
whereas less activity during the daylight hours may allow for
isolated to scattered showers and perhaps a thunderstorm overnight
as weak disturbances ripple overhead.
On Sunday, a relatively stronger shortwave passes through the Middle
Mississippi Valley. Thanks to the better forcing from this wave and
falling heights, the potential for widespread convection increases
during the day, but particular during the evening and overnight
hours as the low-level jet ramps up over the region.
Elmore
&&
.LONG TERM... (Monday through Friday)
Issued at 237 AM CDT Sat Sep 20 2025
While there is a general consensus on the evolution of the upper-
level pattern through the extended period, specific scenarios vary
drastically among deterministic guidance and ensemble clusters. For
Monday into Tuesday, either quazi-zonal flow will persist or subtle
troughing will develop aloft over the Middle Mississippi Valley.
Either solution points to a continuation of our active pattern, with
subtle disturbances serving as drivers of at least one round of
convection. Late Monday into Tuesday, a majority of guidance has
consistently been taking a shortwave from the Northern Rockies and
having it dig southward into the Central Plains and then generally
into the Middle Mississippi and Ohio vallies. The main occurrence
with this shortwave is that the front that has been positioned just
to our west-northwest for the last several days will finally be
pushed southward through the region. This will serve at least as a
final round of a low chance for rain; however, recent guidance
trends have been for a low to develop along the front that passes
through or near the region, increasing our odds of receiving a
soaking rain if this solution occurs.
In the wake of the front Wednesday toward the end of the week,
another recent trend in guidance is that the aforementioned
shortwave tangles with one or more additional shortwaves over the
Midwest, developing a cutoff low that stalls somewhere over the
eastern CONUS. While the position and strength of this cutoff varies
among ensemble clusters and deterministic guidance, the majority of
solutions show the Middle Mississippi Valley is either beneath the
cutoff or just to its west. This position places the CWA in
favorable territory for temperatures running at or below
climatological normals (highs in mid to upper 70s, lows in the mid
to upper 50s) and will keep the door open for at least periodic low
rain chances through the end of the week.
Elmore
&&
.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 613 AM CDT Sat Sep 20 2025
Patchy low VFR/high MVFR fog has developed across the region this
morning. Given we are at sunrise at the time of this writing,
confidence is low that fog will worsen enough to produce prolonged
impacts at local terminals. A storm system moving just north of
the region this morning may also clip KUIN with light showers at
times, with rain not expected to be heavy enough to reduce
visbilities.
Another round of isolated to scattered showers and thunderstorms
is forecast this afternoon and evening, though confidence on the
exact timing and placement of this convection relative to the
local terminals is low. Any shower or storm that impacts the local
terminals directly will be capable of producing reduced
visibilities and erratic winds.
Elmore
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&
$$
WFO LSX

NWS LSX Office Area Forecast Discussion