968
FXUS63 KLSX 152003
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
303 PM CDT Sat Aug 15 2026

.KEY MESSAGES...
- Widespread relief from the heat is expected on Monday.

- There is a low chance for scattered showers and thunderstorms
  tonight and then again tomorrow afternoon and evening. A
  storm or two during both rounds will be capable of strong wind
  gusts.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 301 PM CDT Sat Aug 15 2026

The MCV that was at least partially responsible for last
night`s rainfall can be seen in visible satellite imagery moving
through northern Indiana into Ohio. While this features is now
well east of the CWA, convective debris from it has lingered
across portions of the area into this afternoon particularly
along and north of I-70. This has slowed the diurnal warming
trend for some areas, causing where the greatest heat index
values are located to be nebulous. As of this writing, most
locations are around 100 degrees, and with a a couple hours left
of heating, confidence is high that most locations will top out
at 100- 110 in the Advisory and Warning areas.

Our attention then turns to another MCV that is currently over the
Central Plains. It will continue moving eastward into tonight as a
low-level jet once again ramps up and noses into the Midwest. Both
features will force scattered convection, but tonight`s jet is not
forecast to be as strong compared to previous nights and is expected
to be more veered. Therefore, convective coverage will not be as
great was previous nights. Deep-layer shear around 25 kts will allow
for a brief strong storm capable gusty winds. The threat for severe
wind gusts remains low. Despite PWAT hovering around 2", progressive
storm motions and limited storm coverage will mitigate a flash flood
threat.

Tomorrow, a cold front will gradually sag southward through the CWA
during the day. The front will not be fast enough, nor the post-
frontal airmass potent enough to put an end to the heat for the
whole CWA. Southeastern Missouri and southwestern Illinois will see
another day of afternoon temperatures in the mid to upper 90s, with
heat index values of 105-110 degrees. Along the I-70 corridor,
confidence is lower in how high temperatures will climb due to
slight differences in the speed of the front among guidance and
uncertainty in convective trends overnight. If guidance trends more
aggressively with the southward extent of the effective front, the
current Heat Advisory and Excessive Heat Warning may need to be
trimmed.

The degree of heat and humidity south of the front will yield 2,500-
3,000 J/kg of SBCAPE per the 12z HREF and REFS means among 25 kts of
deep-layer shear during Sunday afternoon. A weak impulse will move
into the Middle Mississippi Valley during the afternoon, providing
additional lift along and ahead of the front. Guidance varies in the
exact track and magnitude of this impulse, with some guidance having
it too far behind the front to provide much warm-sector lift.
Additionally, some model soundings show subsidence in the low-
levels, which would limit convective strength. Therefore, our
confidence is low (20-30% chance) in showers and thunderstorms along
and ahead of the front. If thunderstorms are able to form, there is
a very low chance that a storm or two may briefly be capable of
damaging wind gusts given the degree of deep-layer shear.&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Issued at 301 PM CDT Sat Aug 15 2026

The consensus among ensemble clusters and deterministic models is
that the upper-levels across the CONUS through the workweek will be
characterized by some degree of ridging in the west and troughing in
the east. With the Middle Mississippi Valley positioned between
these two features and beneath northwesterly flow aloft, our local
weather will remain active. Confidence is high that the
aforementioned front will have cleared the CWA by Monday morning,
though with flow aloft in its wake being only weakly northwesterly,
leading to meager advection of cooler air. While most of the CWA is
expected to see a break in the heat, with afternoon temperatures
remaining in the 80s, portions of far southeastern Missouri still
have a 50-70% chance of reaching and exceeding 90 degrees per the
NBM.

The post-frontal air mass will quickly shift eastward on Tuesday,
returning south-southwesterly flow and warmer air to the CWA through
Wednesday. Where 850mb temperatures are warmest along and south of I-
70, afternoon temperatures both days have a medium chance (50-70%
per the NBM) of reaching and exceeding 90 degrees.

A series of shortwaves will move through the Midwest via the
northwesterly flow Wednesday through Thursday signaled by a FROPA on
Wednesday. This FROPA will serve has the most focused chance for
rain during the extended period, though there is large spread among
guidance on where this rain will occur near and within the CWA and
how quickly the front moves through the area. While most of the area
has at least a low chance (20-30%) for lighter rainfall, wherever
rainfall is able to be focused, upwards of 1-2" can be expected.
This front will also mark the beginning of at least a few days of
temperatures through the end of the workweek and early weekend.

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1224 PM CDT Sat Aug 15 2026

Dry and VFR flight conditions will persist at all local
terminals through this evening. Then a cold front will slowly
enter the region from the north, bringing a threat for scattered
showers and thunderstorms to KUIN. There is a very low chance
that the remaining local terminals could see a brief shower
and/or thunderstorm tonight, but the better chances look to be
tomorrow afternoon after 18z.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 9 PM CDT Sunday for MOZ035-036-042-052.
     Extreme Heat Warning until 9 PM CDT Sunday for MOZ041-047>051-
     059>065-072>075-084-085-099.
     Heat Advisory until 9 PM CDT this evening for MOZ034.
IL...Heat Advisory until 9 PM CDT Sunday for ILZ058>060-064-065-098-
     099.
     Extreme Heat Warning until 9 PM CDT Sunday for ILZ069-070-074-
     079-100>102.

&&

$$

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