218
FXUS63 KLSX 072353
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
653 PM CDT Sat Jun 7 2025

.KEY MESSAGES...

- Thunderstorms moving into the region this afternoon will pose an isolated
  damaging wind threat and low tornado threat through early this
  evening across central and southeastern Missouri.

- Another round of thunderstorms is expected along a cold front tomorrow
  afternoon and evening. A few storms will become strong to
  severe, with the main threats being damaging wind gusts and
  large hail.

- Relatively calm weather is expected Monday through Wednesday before
  an active pattern returns at the end of the week into the
  weekend.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 206 PM CDT Sat Jun 7 2025

Water vapor imagery this afternoon shows a compact shortwave moving
into the Lower Missouri Valley, with its surface reflection moving
into central Missouri. The early morning convective complex that
tracked through the Mid-South produced a blanket of mid to upper-
level cloud cover over the CWA ahead of the shortwave, which when
combined with low stratus from the earlier fog, has limited surface
heating over much of the area. A brief bout of warm air advection
via southwesterly low to mid level flow ahead of the wave will
offset the effect of this cloud cover to some degree over portions
of central and southeastern Missouri. Here, an area of 1,000-1,500
J/kg of MLCAPE is nosing into central Missouri among about 50 kts of
effective shear. These attributes are favorable for strong to severe
storms, though the CAPE profile is tall and thin, with soundings
showing a weak inversion at about 500 mb. This may keep updrafts
shallow and limit their ability to fully recognize the amount of
shear in place. That being said, this system resembles an MCV, and
these are notorious for overperforming relative to the environment
they are in. As line segments associated with this system move
through west-northwestern Missouri, weak mesovortices have been
noted along the leading edge. Shear in the 0-3km layer is weaker -
about 20 kts - which is not favorable for robust mesovortex
development. However, SPC mesoanalysis shows that the shear
increases with southern extent, and any east-northeastward oriented
segments will be able to utilize for mesovortex development, posing
an isolated damaging wind and low tornado threat. This threat will
extend from central Missouri southeastward in time through
southeastern Missouri into the early evening.

With abundant low level moisture, weak winds, and some clearing,
another round of fog, possibly dense, is expected tonight. The most
favorable area for this is across central Missouri thanks to more
clearing forecast there. Other portions of the area further eastward
may also experience fog, though how dense remains uncertain.

Through Sunday, the shortwave will depart the region to the east-
northeast as a deep upper-level trough begins to dig into the
Northern Plains and Midwest. This trough will send a cold front
surging south-southeastward through the Midwest through the day,
entering the CWA sometime during the afternoon. Mostly clear skies
ahead of the front will allow for surface heating and MLCAPE to
around 1,500 J/kg among about 30 kts of 0-6 km shear. Convergence
along the front will be weak, though enough to pair with falling
heights to produce isolated to scattered convection. Given the shear
profile, a multicellular storm mode with transient supercellular
structures are favored. Updrafts will be tall, favoring enough CAPE
in the hail growth zone to lead to a large hail threat initially,
and high LCLs/inverted V soundings support a damaging wind threat.
Deep-layer shear vectors becoming increasingly parallel to the
advancing front will favor updrafts to be undercut and possibly
congeal with time, favoring a transition to mainly a damaging wind
threat. Given convection likely being undercut with time and
uncertainty in frontal timing, how far southward the threat for
storms extends into the CWA is uncertain. Current indications are
that a corridor from central Missouri northeastward into west-
central Illinois is the most favorable to see storms capable of
isolated to scattered severe threats. Some deterministic soundings
show a warm nose/capping through most of the day, so there is a low
chance that convection could struggle to form altogether. High LCLs
are expected to mitigate a tornado threat.

Elmore

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 206 PM CDT Sat Jun 7 2025

To start the workweek, guidance is unanimous in an anomalously deep
trough digging equatorward over the central CONUS. This trough will
send yet another cold front through the CWA sometime during the day
on Monday. Given the previous FROPA resulting in minimal moisture
over the CWA and weak convergence along the front, confidence is
high (90%) in a dry FROPA. In the wake of the front, deep
northwesterly flow will keep our weather quiet and temperatures just
on the cool side of climatological normals, with ensembles tightly
clustered around 80 degrees for highs through Tuesday. Our weather
stays calm through Wednesday, though temperatures will begin warming
then as the upper-level trough shift east, allowing ridging to build
into the region, supporting southwesterly mid-level flow and warmer
850 mb temperatures.

Toward the end of the week into the weekend, guidance depicts a
series of shortwaves/cutoffs moving beneath the ridge and/or along
its eastern periphery impacting the Middle Mississippi Valley. This
regime will return active weather to the region, with periods of
showers and thunderstorms forecast Thursday through Saturday. There
will likely be plenty of dry time within this stretch, but guidance
variability in the phasing of the shortwaves and thusly, rain
chances, causes blended guidance such as the NBM (our forecast) to
have a broad smattering of low to medium precipitation chances over
a prolonged period of time. The warming temperatures and increasing
moisture over the area will build instability, but guidance
consensus is that deep layer shear will remain meager beneath the
ridge, mitigating a more notable severe thunderstorm threat as of
now.

Elmore

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 644 PM CDT Sat Jun 7 2025

Light showers are expected to linger around STL/SUS/CPS through
02Z. Then expect stratus with ceilings between 1500-2500ft AGL at
COU through 02Z and STL/SUS/CPS through 14Z. There will also be
fog lingering through the night with IFR/MVFR visibilites at
UIN/COU/JEF/SUS. Visibilities will improve on Sunday morning, but
there will be a chance (30-40%) of thunderstorms at UIN/COU/JEF
after 18Z and at STL after 00Z.

Britt

&&

.CLIMATE...
Issued at 227 PM CDT Fri Jun 6 2025

St Louis has still not seen 90 degrees yet in 2025. The average
first 90 degree reading is May 20. The most recent late 90-degree
start was June 10, 2021. If St Louis doesn`t hit 90 by June 10,
it will be the latest first 90 degree day since at least 1995
(June 19). The latest date of first 90 degrees on record was July
4 in both 1961 and 1912.

Columbia has also not yet seen a 90 degree reading in 2025, however
the average first date is much later (May 31). The latest date of
first 90 degrees on record was July 14, 1904.

Quincy reached 90 degrees on May 15 of this year, about 3 weeks
earlier than their average first 90 degrees (June 2).

Kimble


&&

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

$$

WFO LSX