482
FXUS63 KLSX 102310
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
610 PM CDT Mon Aug 10 2026

.KEY MESSAGES...

- Dangerous heat and humidity will continue areawide through Tuesday.

- An approaching cold front is expected to bring increased cloud
  cover and rain chances Wednesday through Friday. That means
  there is less certainty temperatures and heat index values for
  mid/late week.

- There is a strong signal for widespread dangerous
  heat/humidity to return areawide this weekend.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 231 PM CDT Mon Aug 10 2026

Intermittent cloud cover has helped keep temperatures and heat
index values down slightly compared to what was expected
along/north of I- 70. However, temperatures have climbed closer
to the 90 degree mark (even with the cloud cover) with heat
index values nearing the century mark. Lows tonight should be a
couple of degrees warmer than last night, with temperatures
only bottoming out in the mid 70s to near 80 degrees. These
values would be about 10 degrees above normal for the middle of
August.

Tuesday should be the hottest day of the work week, with plenty
of sunshine expected and surface winds veering a bit more to
the southwest. Far northeast sections of the forecast area may
see some mid/high level clouds to the southwest of an MCS, which
is expected to travel across northern Illinois into Indiana. A
few showers are also possible in northeast Missouri/west central
Illinois due to weak low-level moisture convergence. Therefore,
there may be some impacts to temperature over these parts of
the CWA, but elsewhere there is very high confidence in high
temperatures in the mid to upper 90s with peak heat index values
of 105-110F.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
Issued at 231 PM CDT Mon Aug 10 2026

(Tuesday Night - Friday Night)

Deterministic guidance shows the mid/upper level ridge weakening and
shifting slightly to the southwest Tuesday night into Wednesday.
There is also a pretty coherent signal of another "ridge-runner"
midlevel shortwave trough to move across the Mississippi River early
Wednesday morning. Widespread showers and thunderstorms are expected
downstream of this feature, which should be at least slightly
farther southwest than the MCS expected Tuesday morning. The core of
this MCS should miss us to the northwest, but parts along/east of
the Mississippi River may see these showers and storms on the
southwestern flank. In addition, weak moisture convergence on the
nose of a veering low-level jet should get at least some scattered
showers and thunderstorms farther to the west. The combination of
increased mid/high level cloud cover and aforementioned rain chances
really puts into question high temperatures and peak heat index
values across much of the region. Areas in southeast
Missouri/southwest Illinois are least likely to be impacted by
morning clouds/rain chances and therefore have the best chances of
seeing another day with 105+ heat index values. There were no
changes made to the heat headlines, but at least parts of the area
may need to be cancelled early if current indications of
clouds/storms hold true.

There remains uncertainty with how the synoptic front evolves mid-
late week. A preponderance of guidance shifts the boundary southward
into the area late Wednesday/Wednesday night. The front should then
stall out somewhere across the region through Friday, but it may be
modulated by subsequent rounds of convection. Speaking of which,
there is a strong signal for widespread convection along/north
of the quasi-stationary front Thursday night into Friday
morning. Parts of northeast Missouri and west-central Illinois
have the best chance (50-70%) of showers and thunderstorms
during this time period. This convection could in turn help push
the front back southward a bit and lead to cooler temperatures
on Friday, but the exact location of the front is not certain at
this time range. Needless to say, the precise high temperatures
(and heat index values) really each day Wednesday through
Friday are pretty uncertain. The inter-quartile range of the NBM
shows this uncertainty quite well, with about a 5-10F range
between the 25th/75th percentiles and 10- 20F between the
10th/90th percentiles.


(Saturday - Sunday)

Ensemble guidance is in good agreement that the mid/upper level
ridge will amplify back over the Mississippi Valley early this
weekend, with leeside cyclogenesis across the Plains. This should
help push the quasi-stationary boundary back north of the area as a
warm front. Widespread heat and humidity should return to the bi-
state area as a result, with highs most likely rebounding into the
mid to upper 90s for many locations. Peak heat index values of 105+
are expected each day roughly along/south of I-70.


(Sunday Night - Next Monday)

The synoptic pattern may cycle once again heading into early next
week as ensemble guidance portrays a flattening mid/upper level
ridge. This likely will allow a cold front to push into the area
sometime Sunday night into next Monday, bringing with it a returning
chance (20-40%) of showers and thunderstorms along with temperatures
(and heat index values) cooling back closer to normal.

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 550 PM CDT Mon Aug 10 2026

VFR conditions are expected throughout the 00Z TAF period. Light
south to southwest winds will persist overnight, with a slight
increase in speeds from mid morning through late afternoon
tomorrow. At JEF, some LLWS is possible very late
tonight/early tomorrow morning as winds may fully decouple and
go calm at the surface, with a 40 kt southwesterly low level jet
between 1500-2000ft. This is less of a factor at other
terminals, where either surface winds are likely to stay high
enough, or the low level jet weak enough, to mitigate this
concern. Otherwise, only afternoon VFR cumulus is expected
tomorrow afternoon.

&&

.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...Gosselin
AVIATION...BRC