097
FXUS63 KLSX 120848
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
348 AM CDT Sat Jul 12 2025

.KEY MESSAGES...

- Cooler temperatures will move into the region over the weekend,
  providing some relief from the widespread 90s.

- Variable chances for showers and thunderstorms exist over the
  upcoming week. There will be plenty of dry time between rounds
  of wet weather.

&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 343 AM CDT Sat Jul 12 2025

Radar mosaics show very little left of what was a solid line of
thunderstorms that once stretched from northeastern to central
Missouri late last evening. Outflow from these thunderstorms has
raced well to the south and has become less discernible as it
pushes into southeastern Missouri and southern Illinois. A few
thunderstorms continue over sections of south-central Missouri at
the base of a mid-level trough. This activity is well under severe
limits and should continue to fade as it runs into more stable
air to the east.

Though surface observations showed winds briefly turn out of the
north/northwest behind the outflow, the cold front lags behind. As
of 08z, the front was sagging southeast through northeastern and
central Missouri. The front is expected to continue to progress to
the southeast this morning into this afternoon before stalling to
the south. A secondary boundary follows close behind, marking a
separation between dewpoints in the mid-60s/low-70s from 50s to low-
60s over the central and northern Plains. While the lead front will
be more of a player in today`s forecast, the secondary front will
have more of an impact on Sunday`s temperatures/dewpoints.

The lead front is expected to sink into the southeastern quadrant of
the CWA early this afternoon before the conditions destabilize once
again. Unlike Friday, Saturday`s thunderstorms will lose much of the
mid/upper ascent as the upper trough moves into the Great Lakes.
HREF shows mean SBCAPE climbing to around 2000 J/kg with weak deep
layer shear less than 15 knots. By the time conditions destabilize
enough to give rise to thunderstorms, the front should limit
scattered thunderstorms to southeastern Missouri and southwestern
Illinois. Considering the moisture-loaded environment, isolated
gusty winds are possible, but this potential will have spatial and
temporal limitations as the front continues to move southeast
through the afternoon. The secondary front continues to draw in
cooler and drier air tonight into Sunday with relief being more
noticeable over northern sections of Missouri into west-central
Illinois, where dewpoints drop into the 60s with highs in the 80s
across the area.

On Sunday, another shortwave enters the southern Plains, briefly
getting hung up over eastern sections of Oklahoma. An arm of
mid/upper level ascent/vorticity extend eastward through central
Missouri and Illinois Sunday afternoon, riding over top of the
meandering surface boundary. Though it will be cooler (80s), a pool
of modest instability with MLCAPE of 1000-1500 J/kg builds south
of I-70. Scattered showers and a few thunderstorms develop mainly
during the afternoon into the evening before fading once again
Sunday night.


Maples

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 343 AM CDT Sat Jul 12 2025

The pattern over the course of the upcoming week will include
variable chances of showers and thunderstorms. The upper level
pattern transitions to quasi-zonal flow, placing the region
underneath predominately weak flow. A wavering surface boundary will
be the focus for diurnally driven thunderstorm potential, while
upper level shortwaves enhance rainfall potential on occasion.

By late evening Sunday, a longwave, low amplitude upper level trough
moves east of the region with a stalled surface boundary extending
across southern sections of Missouri and Illinois. Showers linger
into the evening, remnant of decaying thunderstorms. Dry conditions
are favored Sunday night into early Monday as the surface boundary
looks like it is shunted southward into northern Arkansas.

Anti-cyclonic flow around an expansive ridge over the southeastern
U.S. will draw warm, moist air into the central Plains with lobes of
vorticity hugging the northwest periphery of the ridge. It will be
somewhat challenging to nail down rain chances as medium/long range
guidance struggle at resolving the spatial and temporal aspects of
these systems in the weak flow. Generally speaking, the surface
boundary initially remains south of the region with diurnally driven
convection holding higher PoPs to the south. Meanwhile, we await a
shortwave over the Plains that briefly gets hung up over eastern
section of Oklahoma and Kansas. As the shortwave approaches, it
attempts to push the boundary back to the north late Monday, but
maybe not soon enough to provide ascent for anything more than
isolated to widely scattered showers/thunderstorms Monday afternoon.
NBM probabilities cap chances at 20-40%, while 40-60% of the LREF
ensembles show measurable rainfall along and south of I-70.

The shortwave lifts northeast out of the Plains late Monday into
Tuesday, along with the northward jaunt in the surface boundary.
NAM/GFS/ECM vary in their depiction of the breadth and position of a
mid-level trough that accompanies the upper level shortwave, which
results in some spread with rainfall potential. LREF shows the
greatest spread running up the I-44 corridor in Missouri and along
the I-70 corridor in Illinois with roughly 0.25"-0.30" departure
between the 25th-75th percentiles in mean 24 hour precipitation
amounts. As has been the case over the last couple of events, PWATs
remain high (2-2.5 sigma above climatological norms) around 2".
Thunderstorms will likely be efficient rainfall producers once
again. However, NBM may be too dispersed with 50-60% probabilities,
dependent on the location of the surface boundary and evolution of
the shortwave. I`d expect this to be fine-tuned over the next couple
of days.

This remains the theme over the course of the week with the back-and-
forth in the surface boundary and tracking additional vort lobes up
the western side of the southeastern ridge. Meanwhile, the northern
CONUS remains relatively active, showing the zonal flow giving way
to an upper level trough and flattening in the southeastern ridge.
Global guidance diverges in the amplitude/timing of these features,
explaining the rapid increase in spread among NBM interquartile
ranges beyond Wednesday. This results in lower confidence with
regard to specific temperatures as the interquartile data ranges
from the low/mid-80s to the low/mid-90s through the end of the
period.


Maples

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1235 AM CDT Sat Jul 12 2025

A decaying line of showers and thunderstorms is moving through the
region, bringing impacts to all local terminals through the first
few hours of the TAF period. Given the decaying nature of the line,
the threat of impacts will rapidly decrease, and could end earlier
than forecast. The cold front responsible for the convection will
sag southward overnight through Saturday, with low stratus along it
bringing a brief bought of MVFR ceilings in the morning, thinning and
lifting around mid-day. The front is expected to have sunk far enough
south so that confidence is high that additional convection will
form south of the local terminals Saturday afternoon.

Elmore

&&

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

$$

WFO LSX