515
FXUS63 KLSX 132338
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
638 PM CDT Thu Aug 13 2026

.KEY MESSAGES...

- Small chance for a few showers and storms along an outflow
  boundary in the northern half of the area.

- Dangerous heat and humidity is expected to continue into at
  least Saturday.

- Relief from the heat and humidity is expected early next week
  with the passing of a cold front Sunday into Monday. Showers
  and storms look likely off and on through the weekend.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 1242 PM CDT Thu Aug 13 2026

Oppressive heat and humidity continues again today. A cluster of
thunderstorms has passed just north of Quincy and Knox City and
set down a fairly robust outflow boundary that has shifted winds
to the north there and provided a modest temperature reprieve.
South of that boundary there is no real letup in sight and
expanded the heat advisory to the north slightly from the
earlier arrangement to account for heat and humidity trends with
dangerous heat and humidity (>80-90% chance of advisory/warning
criteria) expected to continue through Saturday.

Watching the outflow boundary to see if a shower/storm or two
can spark along it but model soundings show LFC heights that are
probably just a bit too high to get anything going. The
developing MCS to the northeast looks to be the strongest
shortwave in town right now and while weak ripples in the
WNW flow aloft could spark additional showers it does look to be
trending slightly drier overnight but have maintained at least
chance (20-40%) PoPs from about 50 miles north of I-70 and
points northward for overnight tonight into early Friday
morning.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
Issued at 1242 PM CDT Thu Aug 13 2026

Flow aloft stays mostly zonal to slightly northwesterly through
the period. A more trackable shortwave works towards the area
for the weekend. This tries to nudge cold front through slowly
through the weekend. This should eventually work to limit the
oppressive heat, at least for a bit. Showers and storms chances
continue both Saturday and Sunday before looking to start to
decrease early next week.

Mid and upper level ridging works back in from the west by
Tuesday into Wednesday that in the current deterministic
synoptic picture looks to put the area back into hot and humid
conditions with thunderstorms developing near or just north of
the area, somewhat similarly to the current pattern.

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 626 PM CDT Thu Aug 13 2026

Mainly dry and VFR conditions are expected through the period.
There is a low chance (20%) that showers and thunderstorms
could develop late tonight and early on Friday (between 09-15Z)
and affect UIN as well as the St. Louis metro area terminals.
This chance is too low to include in the TAFS at this time. If
they to do develop, they could bring MVFR (possible IFR)
conditions in brief heavy rain. Otherwise winds are expected to
remain southerly through the period.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 9 PM CDT Saturday for MOZ026-027-034>036-
     042-052.
     Extreme Heat Warning until 9 PM CDT Saturday for MOZ041-
     047>051-059>065-072>075-084-085-099.
IL...Heat Advisory until 9 PM CDT Saturday for ILZ058>060-064-065-
     070-097>099.
     Extreme Heat Warning until 9 PM CDT Saturday for ILZ069-074-
     079-100>102.

&&

$$

SHORT TERM...JGG
LONG TERM...JGG
AVIATION...Britt