177
FXUS63 KLSX 251715
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1215 PM CDT Sat Jul 25 2026

.KEY MESSAGES...

- Dangerous heat and humidity build into the region today into
  Sunday, lasting through at least Monday.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 250 AM CDT Sat Jul 25 2026

Water vapor imagery shows a subtle disturbance moving through the
Middle Mississippi Valley, and this disturbance is associated with
the ongoing scattered showers across the CWA. As the disturbance
gradually exits the region through this morning, we will see the
threat for showers decrease from west to east. In the immediate wake
of the disturbance, southwesterly flow is advecting warmer air into
the CWA. This will build MUCAPE through the morning, allowing for a
couple of the showers to strengthen enough to become thunderstorms.
Any of the stronger showers and thunderstorms will be capable of
locally heavy rainfall, but given the lack of training and residence
time for any single convective cell over a location, the threat for
flash flooding remains low.

The warm air advection signals the start of a notable period of heat
for the CWA. Most of the area will still see temperatures at or just
below seasonal normals today, especially in south-central Illinois
given the later onset of warm air advection and exiting cloud cover.
Here, temperatures this afternoon will peak in the low to mid 80s.
However, across central Missouri where warm air advection will be
more consistent through the day, temperatures will quickly warm into
the low 90s. This heat paired with dew points in the mid to upper
70s will produce heat index values to around 105 degrees. The Heat
Advisory remains in effect and on track for this portion of the
area due to these heat index values.

As an upper-level ridge builds over the western CONUS into Sunday, a
low-level thermal ridge will encroach upon the Middle Mississippi
Valley. With 850 mb temperatures ticking up through the low 20s
degrees C, all locations will see highs in the 90s, with mid to
upper 90s along and west of the Mississippi River. These
temperatures will yield heat index values around 105 to as high as
around 110. The expansion of the Heat Advisory across the entire CWA
remains warranted. There is at least a low chance for heat index
values warming several degrees above 110 degrees in central
Missouri, and depending on guidance trends over the next 24 hours,
this portion of the CWA may need upgrading to an Extreme Heat
Warning.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 250 AM CDT Sat Jul 25 2026

The upper-level ridge will still be holding over the western CONUS
on Monday per guidance consensus, with the worst of the thermal
ridge for the period shifting over the Middle Mississippi Valley.
This will result in 850 mb temperatures peaking around 25 degrees C,
which supports surface temperatures in the upper 90s to around 100
degrees. With very little ensemble spread and a warm start to the
day, confidence is high in this degree of heat. With dew points
still expected to be in the mid to upper 70s, heat index values will
top out in the 105-110 range during the afternoon for much of the
area.

Confidence decreases in forecast specifics starting Tuesday as a
shortwave moving through the Ohio Valley sends a backdoor cold front
toward and/or through the CWA. This will result in the thermal ridge
breaking down, but guidance varies in how quickly this happens. A
majority of guidance supports a decrease in afternoon temperatures
Tuesday compared to Monday, but the speed of the front will dictate
how much of the area experiences relief, with a slower front
favoring areas along and south of I-70 seeing another day of 100+
degree heat index values.

For Wednesday into the weekend, the upper-level ridge will remain
centered over the Rockies as northwesterly flow persists over much
of the Midwest. Within this flow, deterministic guidance depicts
multiple shortwaves diving southeastward through the region. These
disturbances will keep the thermal ridge focused west of the CWA,
allowing for temperatures to remain closer to seasonal normals
(highs around 90 degrees). Each of these disturbances will be
capable of producing rainfall; however, there is a lack of consensus
among guidance on if/when rainfall will occur across the CWA,
leading to low confidence and a mostly dry forecast for this portion
of the period.

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1159 AM CDT Sat Jul 25 2026

Stratocumulus is lifting early this afternoon, and should be
above 3 kft AGL areawide within the next couple of hours. Some
fog is possible late tonight, but held off on adding to any
terminals as there may be enough high clouds to prevent fog
development. Finally, there is a low chance (~20% or less) of
thunderstorms Sunday. The "best" chances seem to be pointed
toward the afternoon roughly along the Mississippi River and
after this TAF package. Therefore, kept TAFs dry.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from 11 AM Sunday to 9 PM CDT Monday for MOZ018-
     019-026-027-034>036-042-049>052-059>065-072>075-084-085-
     099.
     Heat Advisory until 9 PM CDT Monday for MOZ041-047-048.
IL...Heat Advisory from 11 AM Sunday to 9 PM CDT Monday for
     ILZ058>060-064-065-069-070-074-079-095>102.

&&

$$

SHORT TERM...Elmore
LONG TERM...Elmore
AVIATION...Gosselin