153
FXUS63 KLSX 291718
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1218 PM CDT Sun Jun 29 2025

.KEY MESSAGES...

- Multiple rounds of showers and thunderstorms are expected
  through Monday, those these rounds will be separated by plenty
  of dry time.

- Seasonable temperatures are expected through midweek before
  heat builds back into the region to end the week.


&&

.SHORT TERM... (Through Monday Night)
Issued at 320 AM CDT Sun Jun 29 2025

Early morning water vapor imagery shows an upper-level trough
beginning to edge into the Northern Plains as subtle disturbances
move through quasi-zonal flow across the northern half of the CONUS.
One such disturbance, an MCV, is slowly edging eastward across
southern Missouri this morning and has kicked off convection over
the last couple hours. This convection spans from central to
southeastern Missouri and is expected to remain weak given the lack
of instability and shear over the area. However, the slow moving
nature of the convection and precipitable water nearing 2" still
posses a threat of flash flooding, and the current Flood Watch
across Reynolds County remains in place.

The convective forecast through the day and into tonight is rather
complex. The following scenarios are the most likely as of right
now, but it doesn`t take much to alter convective potential is this
type of pattern. First, the ongoing convection is generally expected
to weaken and dissipate with eastward extent as the forcing from the
MCV focuses further southward. However, there is a low chance (less
than 20%) that this MCV continues to produce convection throughout
the CWA during the day, limiting instability for later rounds.
Second, there is the ongoing thunderstorm complex moving south-
southeastward across Nebraska. The remnants of this complex or the
complex itself, if it can be maintained, will be moving into central
Missouri during the mid to late afternoon. The other solution is
that the Nebraska complex fails to survive the night-day transition,
with no impacts from it occurring in the CWA. This solution favors
additional convection developing at the nose of the low-level jet
this evening and tonight over the Middle Missouri Valley and
traveling southward into the CWA overnight.

In either of the latter scenarios, instability will be ample and on
the order of several thousand J/kg of SB or MUCAPE. Deep layer
shear, however, will remain weak and limit storm strength and
longevity baring convection being able to congeal and grow upscale.
If this does happen, storms will be capable of damaging wind gusts
primarily west of the Mississippi River where convective coverage is
expected to be greatest. Confidence in damaging wind gusts is low,
as several deterministic soundings show capping around 850mb,
leading to convection struggling to be maintained or it being
elevated, limiting the damaging wind potential.

How convection evolves tonight will dictate our shower and
thunderstorm chances Monday as a cold front slides through the CWA.
The front is already expected to be nearing the southeastern border
of the CWA around peak heating when convection is expected to
initiate along it. If overnight convection lingers too long into the
early morning hours, the effective boundary may be pushed just
southeastward of the CWA and/or convective debris may limit
convective initiation along the front in the CWA. In the case of
destabilization and that convection is able to form along the front
in the CWA, it will do so among an air mass characterized by 2,500-
3,000 J/kg of SBCAPE and around 20 kts of 0-6 km shear. For updrafts
that are able to congeal, this environment will lead to an isolated
damaging wind threat. Given the conditionality of this already low
threat, we will continue to not message the SPC Day 2 Marginal
publicly.

Elmore

&&

.LONG TERM... (Tuesday through Saturday)
Issued at 320 AM CDT Sun Jun 29 2025

An upper-level trough will be digging into the Ohio Valley on
Tuesday per deterministic guidance and ensemble clusters. With the
axis of this trough east of the CWA, the area will be beneath deep
northwesterly flow that will advect high pressure in. The result is
seasonable temperatures, with global ensembles continuing to tightly
cluster around temperatures in the mid to upper 80s. Wednesday is
expected to be similar, though a degree or two warmer as the surface
high shifts eastward and low-level flow becomes increasingly
southwesterly.

For Thursday into the weekend, confidence is high that upper-level
ridging will build back into the Midwest, but guidance continues to
vary notably on the speed and amplitude of the ridge. While this
pattern will favor warming temperatures, the spread on the amplitude
has the difference between the 25th and 75th percentiles of ensemble
guidance varying by about 5 degrees. While this spread may seem
insignificant, it is the difference between heat index values
remaining below or exceeding 100 degrees come Friday and Saturday.

How much the ridge amplifies will also determine our rain chances
for the holiday weekend. A more amplified ridge will keep rain
chances subdued, while a flatter ridge will allow for more quasi-
zonal flow near the region and possible disturbances that will bring
rain chances to the area. Guidance is currently leaning toward a
more amplified ridge, and the going forecast reflects that with rain
chances only topping out 20-30% Friday and Saturday.

Elmore

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1215 PM CDT Sun Jun 29 2025

Fairly low confidence forecast with respect to thunderstorms in
the near term and extending into Monday. A few different
thunderstorm complexes are currently ongoing across the region,
with Quincy actually the most likely to be affected in the next
few hours. Otherwise persistent rain from a dying thunderstorm
complex has led to MVFR to IFR ceilings in and out from central
Missouri to the St Louis metro. Expect these conditions to
gradually improve with at least some heating this afternoon,
although even that is limited by extensive cloud cover.

There`s somewhat good agreement in the high resolution guidance
that a new thunderstorm complex develops across western/central
Missouri overnight, though most of this high resolution guidance
also doesn`t have a good handle on current conditions so
confidence in the consensus output is not as high as it would
seem. The most likely area impacted by this would be in central
Missouri between 6-12Z.

A cold front gradually pushes southward through the region
tomorrow. Additional thunderstorms may develop along this front as
it moves through, though this depends in part on earlier storms
and clouds which may prevent the development of instability ahead
of the front. That said, while confidence in overnight convection
is lower, confidence in convection near the front on Monday is a
little higher. As a result, this is the time period in which the
St Louis metro is most likely to see a thunderstorm, although that
chance is still low enough at this point that only a PROB30
designation will be used in the extended portion of the STL TAF.

Kimble

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch until 1 PM CDT this afternoon for Reynolds MO.

IL...None.
&&

$$

WFO LSX