333
FXUS63 KLSX 111733
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1233 PM CDT Fri Jul 11 2025

.KEY MESSAGES...

- Today will be hot and humid with increasing chances for
  thunderstorms late this afternoon/evening. A few thunderstorms
  may be strong to severe, mainly over northeastern Missouri and
  west- central Illinois.

- The active pattern continues with temperatures trending cooler
  Saturday into Sunday.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 343 AM CDT Fri Jul 11 2025

The northern U.S. will remain active with multiple systems tracking
west to east in the vicinity of the U.S./Canadian border. Further
south, showers and thunderstorms will navigate around two expansive
ridges. One of the ridges is parked over southern California/Baja
Peninsula as the second is in control of the western Atlantic. The
mid-Mississippi Valley is sandwiched between these features, which
will exhibit some degree of influence through the next several days.

At the surface, a stationary boundary stretches east to west from
the New England Region into eastern Nebraska. Scattered
thunderstorms accompany the boundary over northern sections of
Illinois with broader coverage being supported by a surface low over
western sections of Iowa. Much of this is expected to remain north
with the exception of a shower/isolated thunderstorm skimming the
far northern fringes of the CWA (NE MO/WC IL). The surface low will
lead to modest warm air advection over MO/IL today in conjunction
with persistent, but weak south/southwesterly flow at the western
periphery of the Atlantic ridge. HREF H8 mean temperatures in the
upper teens (C) will warm into the low-20s under mostly sunny skies.
This comes with an increase in surface moisture, noted by dewpoints
climbing back into the low-70s. The combination will make for a hot
and humid day with highs in the low to mid-90s and heat indices in
the upper 90s to 105 degrees.

Aside from a short-lived, stray shower/thunderstorm, most of the day
remains dry with little/no ascent aloft, weak shear (<10 knots), and
marginal mid-level lapse rates (6C). The best potential for
thunderstorms arrives sometime between late this afternoon into this
evening as the surface low and mid-level trough move eastward out of
Nebraska. CAMs show another cluster/complex of thunderstorm
development over Iowa late this morning into early afternoon with
additional development stretching down a trailing cold front that
extends southwestward into eastern Kansas and western Oklahoma.
Paintball plots (>40 dBZ sim. reflectivity) show CAMs fairly well
clustered along and north of the Missouri/Iowa border with the
broader complex of thunderstorms (potential MCS). Being that much of
the lift associated with the low/trough will move north,
thunderstorms along the front will largely rely on frontal
convergence and diurnal instability. Development along the front
looks to enter the northwestern CWA between 21z-23z. At the peak of
diurnal instability, HREF mean SBCAPE values reach 2500-3000 J/kg in
conjunction with mid-level lapse rates near 7C and slight backing in
surface winds. The limiting factor is the weak flow component with
marginal shear values of 20-25 knots. Initial development along the
front may support mini-supercells or organized multicells with all
hazards possible. The unstable environment may pose the potential to
overcome the weaker shear to produce short-lived rotation and
damaging hail during the late afternoon/early evening, though
hodographs are not elongated to support sustained, strong rotation.
While the tornado potential is low, it may extend into northeastern
Missouri/west-central Illinois. As thunderstorms evolve downstream,
damaging winds become the primary threat as instability wanes and
the system becomes outflow dominant through midnight or so.

By early Saturday, the cold front drifts along or just south of I-
70, becoming more east-west oriented with scattered showers and
thunderstorms along and south of the front. The trough and
associated surface low eject northeast into the Great Lakes,
removing the better ascent associated with today`s thunderstorms.
However, MLCAPE climbs back to 2000-3000 J/kg with plentiful
moisture to tap into within a weakly sheared environment.
Thunderstorms reemerges Saturday afternoon/evening with much of the
convective potential focused along the front. RAP guidance shows
weaker lobes of mid-level vorticity rotating around the southern end
of the broader trough with isolated/widely scattered convection over
top the boundary which looks more diffuse in time. With PWATs of 1.5-
2", any of the thunderstorms will be capable of producing locally
heavy rain and localized downburts with damaging wind potential.
Thunderstorms weaken once again through the late evening/overnight
with decreasing instability.


Maples

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 343 AM CDT Fri Jul 11 2025

Sunday into early next week will continue an active stretch of
weather as the southeastern surface/mid-level ridge shifts
westward into the Gulf States and steers multiple shortwaves
northeastward through the central U.S. The first of these arrives
Sunday, along with a weak surface low that nudges the surface
boundary northward slightly. Though rain probabilities generally
increase from north to south (20% near KUIN to 40-45% near KFAM),
timing of the shortwave may very well be factor in further
adjustments to PoPs in subsequent updates. Considering the diurnal
trends with instability, the position of the shortwave is likely
to support relatively better coverage with afternoon development.
As of now, the best potential looks like it will be just east of
central Missouri into east- central/southeastern Missouri, as well
as southwestern Illinois. Another round of thunderstorms with
weak shear (20 knots) and high PWATs (around 1.8") is possible. I
would not be all too surprised to see PoPs increase in later
updates, mainly along and south of I-70 along and east of the
Mississippi River.

The remainder of the period remains rather active as the surface
boundary seems to meander through the region. Additional shortwaves
track around the southeastern ridge and over top the surface
boundary with diurnal peaks in thunderstorms potential. Where
exactly the highest potential sets up is less certain with scattered
convection in the vicinity of the boundary, aided by ascent from
each consecutive shortwave. NBM interquartile ranges remain
tightly clustered through Tuesday, but then show greater spread
(10 degrees) by mid to late week with guidance likely having
trouble resolving the position/timing of surface features and
resulting precipitation potential. The one day that may end up
mostly dry is Monday, as the boundary is shunted further south in
the wake of the first shortwave. Precipitation potential then
trends upward through the remainder of the week with a broad
spread in PoPs and temperatures with less confidence in the exact
position/track of surface features and upper shortwaves.
Nonetheless, the pattern favors more active weather through the
end of the period with slightly cooler temperatures as a result of
precipitation potential.


Maples

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1232 PM CDT Fri Jul 11 2025

There is a low chance (20 percent) of an isolated shower or
thunderstorm in the St. Louis metro this afternoon, but much higher
chances exist this evening into tonight as a broken cluster of
showers and thunderstorms is expected to develop across IA and
northern MO before progressing southeastward with time.
Thunderstorms will be capable of strong winds and IFR flight
conditions as they impact KUIN. Although thunderstorms are expected
to decrease in strength as they reach the remainder of terminals, a
period of degraded flight conditions and gusty outflow winds are
possible. Ahead of thunderstorms, gusty southerly or southwesterly
winds will persist into this evening.

A cold front will pass through KUIN, KCOU, and KJEF Saturday
morning, accompanied by veering winds to the west and some MVFR post-
frontal stratocumulus. Redevelopment of showers and thunderstorms is
forecast Saturday afternoon with the highest chances at St. Louis
metro terminals close to the front, but coverage and placement are
too uncertain for inclusion at this point.

Pfahler

&&

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

$$

WFO LSX