Rocheport, Missouri Weather Forecast Discussion

514
FXUS63 KLSX 201714
AFDLSX
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1214 PM 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 18z TAFs through 18z Sunday Afternoon)
Issued at 1157 AM CDT Sat Sep 20 2025
A weak frontal boundary is draped from central Missouri to the St.
Louis metro and is contributing to isolated showers and
thunderstorms popping up across the St. Louis metro. The best
chance for showers and thunderstorms will be this afternoon in the
St. Louis metro when upper level support combines with the
boundary. Brief periods of MVFR visibility are expected underneath
thunderstorms, but storms are expected to be progressive enough to
quickly move out of the terminal space. 30-40 mph gusts are
possible if a storm moves directly over the terminal.
Elsewhere (KUIN, KCOU, KJEF) confidence in thunderstorm
development is lower so I have left a mention out of the TAF for
now.
Another round of showers and thunderstorms is expected tomorrow
afternoon with similar conditions to today. Isolated to scattered
thunderstorms will be capable of brief MVFR visibilities and
30-40mph gusts. Confidence in the exact timing and location of
storms is still uncertain, so have kept a mention of this out of
the TAF for now.
Winds are northeasterly north of the boundary (KUIN), and south to
southwesterly south of the boundary. The boundary will lift
northward through the overnight hours causing all sites to become
southwesterly by tomorrow morning.
SPECIFICS FOR KSTL:
Isolated chances for showers and thunderstorms through the course
of the day with the best chances during the afternoon when the
TEMPO group is. Confidence is high that thunderstorms will develop
along a boundary near the St. Louis metro today, but exactly
where and when they form is still uncertain. Uncertainty remains
in the timing and location of afternoon thunderstorms tomorrow, so
mention has been left out of the TAF for now.
Delia
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&
$$
WFO LSX

NWS LSX Office Area Forecast Discussion