297
FXUS63 KLSX 071957
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
257 PM CDT Mon Jul 7 2025

.KEY MESSAGES...

- Scattered thunderstorms are possible this afternoon and evening
  from the eastern Ozarks through southwestern Illinois. Isolated
  to perhaps scattered thunderstorms are possible over central
  Missouri.

- Another round of thunderstorms is possible Tuesday. A few
  thunderstorms may become strong with isolated severe potential.
  The primary threat will be damaging wind gusts to 60 mph and a
  localized threat for heavy rain.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 231 PM CDT Mon Jul 7 2025

Regional analysis shows a diffuse or remnant cold front slowly
sinking southward across sections of southern Missouri and Illinois.
Scattered showers and thunderstorms have largely been focused along
this boundary early this afternoon. Curving back to the northwest
through central Missouri is isolated activity that is largely
supported by mid/upper level vorticity riding over top the washed
out surface boundary and theta-e gradient that extends through
mid- Missouri from NW to SE.

The boundary, which is essentially washed out/quasi stationary, will
remain the primary driver for additional showers and thunderstorms
through this evening. Trends over the last few hours have showed a
slight reinforcement toward the south as a result of cooler air
produced by ongoing thunderstorms in this region. If the cooler air
produced by thunderstorms continues to push the boundary south,
which come CAMs suggest, much of it will shift that direction over
the next few hours. That leaves us with isolated to perhaps
scattered convection over central Missouri, where mid/upper
vorticity is currently tracking east-southeast along a theta-e
gradient that separates the moist atmosphere from the copious
moisture further southwest. Fortunately, thunderstorms are developing
in a weak shear environment and other than the semi-linear
orientation along the diffuse boundary, there isn`t much
organization expected. Therefore, should a thunderstorm become
strong, the primary threat would be gusty winds. Even that potential
is low considering the inverted-V signature isn`t as pronounced as
I`d like to otherwise see (marked by 10 dewpoint depressions).

Thunderstorm chances never really zero out, but will wane late this
evening as diurnal instability is lost after sunset. Tuesday sets up
another complicated scenario which poses slightly better chances for
stronger thunderstorms. CAMs show some semblance of an MCS taking
shape over sections of eastern Nebraska, then sliding southeast near
the IA/MO border overnight. By Tuesday morning, the complex weakens,
if not entirely dissipates as it outruns its H8 theta-e source and
become outflow dominant. Slight chance/chance PoPs cover this
potential over central MO into west-central IL. The biggest question
was how far east to spread slight chance PoPs considering the
uncertainty with how long the complex survives. This will certainly
be an area of opportunity for fine tuning in subsequent updates.

The best potential for thunderstorms comes Tuesday afternoon into
Tuesday evening with the caveat that prior convection will be long
gone, allowing instability to build through the afternoon. The
complex addressed in the prior paragraph will send and outflow
boundary southward, encountering dewpoints in the low to mid-70s and
SBCAPE values of 2500-3500 J/kg per HRRR/RAP/NAM guidance, as well
as interaction with the remnant surface boundary to its south. How
it plays out is dependent on timing and placement, which has not
been so consistent across individual guidance. Should a slow
solution play out, thunderstorms would initialize further northwest
and take advantage of an unstable atmosphere that is largely
undisturbed or merely riddles with isolated to scattered activity.
Should the quicker solution play out, the interaction with the
surface boundary could result in development sooner, which would
have broader geographical coverage that Monday. This would then lead
to competing updrafts in a low shear (20 knots or less) environment
with moisture loaded thunderstorms falling into a surface layer that
is marginally supportive of wet downburst. Therefore, the SPC`s
marginal outlook seems reasonable with the potential for isolated
damaging wind gust to around 60 mph.

Additionally, PWAT values around 2 inches suggest heavy rainfall
will be possible within thunderstorms. However, these look somewhat
progressive and while localized ponding or nuisance flooding is
possible, it does not look like a widespread issue at this time.


Maples

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 231 PM CDT Mon Jul 7 2025

The main features that draw attention in the long term period are
the strengthening upper level ridge over the Desert Southwest and
the stacked ridge centered over the western Atlantic that extends
westward through the Gulf States.

By Wednesday, the surface front that aids in thunderstorm
development Tuesday into Tuesday evening will be reinforces
southward as the mid/upper trough passes over the mid-Mississippi
Valley. As is typical for mid-summer patterns, guidance varies
on the exact position and resulting precipitation potential.
Nonetheless, the general consensus is that thunderstorms will be
concentrated along and south of I-70 as high pressure builds in
front the northwest and northwesterly flow draws in slightly
cooler and drier air.

The remainder of the forecast is somewhat of a squeeze play, as
the strong upper level ridge over the southwestern CONUS begins
to flatten shift slightly west. What initially steers the active
track over the northern Rockies into the Plains, will eventually
give way to zonal flow aloft. Meanwhile, the ridge over the
southeastern U.S. could be a player in convective trends further
out. In summer patterns like this, convection will often get
hinged up at the northwest side of the surface/mid-level ridge,
but can also be invigorated by shortwave/troughs that are
introduced from the west. Considering that upper level flow turns
zonal in time, additional chances for thunderstorms will press
through the region, along with the enhancement of southerly flow
at the back side of the southeastern ridge and ahead of
approaching systems. While no single day looks completely dry, the
most of the dry time might end up being Wednesday afternoon into
Thursday as surface high pressure crosses the region.

By late week into next weekend, a better organized trough tracks
west to east across the central Rockies, resulting in surface
cyclogenesis over the Plains. A cold front trails the system and
could be best chance at showers and thunderstorms beyond
Wednesday.


Maples

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1232 PM CDT Mon Jul 7 2025

Despite the dry forecast at all but KUIN (PROB30), there are
several caveats to what is an uncertain forecast with respect to
point-specific impacts. What is essentially a washed out cold
front over southeastern MO and southwestern IL will be the primary
focus for afternoon/evening thunderstorm potential. Then, another
area in question is central MO, where isolated showers and perhaps
a thunderstorm or two is being supported by ascent from mid/upper
level vorticity.

The best potential for thunderstorms will ride along the surface
boundary south of the terminals this afternoon. The one exception
will be central MO, where trends will have to be monitored, but as
of now, showers have remained north of KCOU/KJEF with Little
indication that it will move much further south. KUIN`s PROB30
addresses the potential that isolated/widely scattered
thunderstorms impact areas of northeastern MO and west-central IL
between late morning/early afternoon Tuesday as a weakening
complex progresses southeast out of the Plains. Considering the
summer pattern, each round will dependent on the prior round,
which results in a higher degrees of uncertainty at any given
point. That said, better probabilities for thunderstorms will be
near the end or just beyond the 24 hour TAF period Tuesday, when
additional convection redevelops and present potential for broader
coverage in scattered thunderstorms. Until then, VFR is favored.

Maples

&&

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

$$

WFO LSX