320
FXUS63 KLSX 112001
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
301 PM CDT Tue Aug 11 2026

.KEY MESSAGES...

- Dangerous heat and humidity will continue through Wednesday.
  Another round of excessive heat is possible Friday through
  Sunday.

- Scattered showers and thunderstorms are forecast Wednesday
  afternoon and evening, mainly in south-central Illinois. A few
  storms may be strong to severe with damaging winds the main
  threat.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 259 PM CDT Tue Aug 11 2026

Dry conditions are expected to continue this evening into the
early overnight hours with unseasonably warm temperatures. Lows
are only expected to drop back into the mid to upper 70s for
most locations, with near 80 degree readings within the urban
heat island of metropolitan St. Louis. Very late tonight into
early Wednesday morning, there may be some isolated to scattered
showers/weak thunderstorms across northeast Missouri and west-
central Illinois. This is where low-level moisture convergence
increases and is in close proximity to the surface cold front.
There also should be another MCS ongoing and that should be
farther southwest than this morning. However, the expectation is
for that to travel through central/southeast Iowa and miss us
once again to the north/northeast. Debris clouds from this MCS
along with the warm advection showers/thunderstorms very well
may hamper temperatures in northern counties on Wednesday, but
exactly how much is unknown. Therefore, kept the heat advisory
even in far northeast Missouri/west central Illinois though it
would not be shocking if highs stayed in the 80s and kept peak
heat index values below 100 degrees. Across the rest of the
area, confidence remains very high in another day with highs
well into the 90s (possibly flirting with the century mark in a
few locations) with heat index values peaking in the 105-110F
range.

Any morning elevated convection should slowly fade as the low-level
jet abates. Attention will then focus on the front itself and if any
diurnal convection can fire along the boundary. While surface
convergence is moderately strong, a couple of factors suggest
initiation may be difficult, including rising mid/upper level
heights and strong capping (particularly west of the Mississippi
River). The best chance of development is across eastern Illinois
where there is a weaker cap and where there will be ascent (and
falling heights) downstream of a midlevel shortwave trough/remnant
MCV. Any storms that develop out that way and congeal will develop a
cold pool, though forward-propagating Corfidi vectors suggest a
northwest to southeast movement, which would keep them away from our
south-central Illinois counties. However, the cap may be weak enough
for development farther to the west, with south-central Illinois
having the best chance (30-40%) of storms roughly between 3 and 9
PM. A few of these storms may be strong to severe during that time
frame, with damaging winds the primary threat. The threat is
certainly conditional on storms actually forming in or just
upstream, but the environment is very favorable for severe weather,
including supercells. The deep-layer shear is particularly strong
for the middle of August, on the order of 40-50+ knots. There could
be a discrete supercell or two at least initially. If this happens,
large hail and possibly a tornado would be possible.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Issued at 259 PM CDT Tue Aug 11 2026

Probabilities for showers and thunderstorms remain low across much
of the area Friday with limited chances (15-20%) along the northern
fringes of the CWA. Heat rebounds over the weekend with chances for
showers and thunderstorms returning next week.

The active convective pattern that has persisted across parts of the
region will persist through the next several days. The key will be
the anomalously strong ridge that is centered near the Gulf Coast,
along with surface features (e.g. outflow) induced by convective
behavior rounding the northern periphery of the ridge. NBM blends
hang onto slight chance PoPs over the far northern sections of the
CWA, which is reasonable considering the uncertainty involved with
these small features. LREF ensembles show 500mb mean heights
reaching near the 100th percentile, which slightly flattens as it
expands westward this weekend into next week. The initial impact is
the return of excessive heat with high well into the 90s and heat
index values approach 110F in some locations Friday through Sunday.
LREFs exceedance probabilities indicate a 60-70% chance for apparent
temperatures at or above 100 along and south of I-70. Even KUIN
exceeds 50%, though this region of the CWA stands the best chance
for adjustment given the active pattern to the north. If a shower or
thunderstorm makes it far enough south, it could at least briefly
alleviate some of the heat. By early next week, the center of the
ridge shift westward into the southern Plains with a NW/SE
orientation to the northern side of the ridge. Global guidance
shows multiple shortwaves rounding the ridge and could be an
early sign of shifting the MCS track southward, though the exact
track of these features is rarely certain this far out. The
greatest sensible impact, aside from thunderstorm chances, will
be cooler daytime temperatures.

High temperatures remain in the 90s through much of the period with
urban areas inching close to 100 degrees Saturday and Sunday. Low
temperatures will largely be in the 70s.

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1221 PM CDT Tue Aug 11 2026

Dry/VFR conditions are forecast to continue through tonight with
southwest winds. A cold front will approach northeast Missouri
and west-central Illinois Wednesday morning, bringing a chance
of showers and thunderstorms. Added a PROB30 group to KUIN
between 1400 and 1800 UTC. Another potential concern is for some
marginal LLWS overnight, mainly in the central Missouri
terminals. However, there looks to be little directional shear
and the magnitude of the LLJ may be not be strong enough to add
mention at this time.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT Wednesday for MOZ018-019-026-027-
     034>036-042-052-072>075-084-085-099.
     Extreme Heat Warning until 8 PM CDT Wednesday for MOZ041-
     047>051-059>065.
IL...Heat Advisory until 8 PM CDT Wednesday for ILZ058>060-064-065-
     070-095>099.
     Extreme Heat Warning until 8 PM CDT Wednesday for ILZ069-074-
     079-100>102.

&&

$$

SHORT TERM...Gosselin
LONG TERM...Maples
AVIATION...Gosselin