083
FXUS63 KLSX 120836
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
336 AM CDT Wed Aug 12 2026

.KEY MESSAGES...
- Dangerous heat and humidity continue to impact the area, and
  is expected to continue. The Heat Advisory and Extreme Heat
  Warning have been extended to Thursday at 8pm, with a future
  extension possible into the weekend.

- There is a chance of severe thunderstorms later this afternoon
  with the main threat of strong winds, primarily in our IL
  counties.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 326 AM CDT Wed Aug 12 2026

Radar indicates a convective system in southern SD and northern
NE tracking along a stationary front and periphery of an upper-
level ridge. This system will be the catalyst for our severe
weather threat this afternoon. There is a ridge-runner
shortwave that will drive this system to the east-southeast into
a region of favorable conditions with 4000+ j/kg of MUCAPE and
abundant surface moisture (>70 F dewpoints). As a result the
convective system will pass just to the north of our forecast
area in southeastern IA and northern IL.

Environmental conditions in our area will be favorable for
severe weather with 2000+ J/kg of SBCAPE and an excess of
surface moisture, but there is no synoptic support for lift.
According to CAMs, as this convection passes to the north it
will send an outflow boundary south through eastern MO and
western IL, which will provide the lift needed to spark some
isolated thunderstorms this afternoon. The convection is
expected to initiate in southwestern IL mid afternoon today, but
there remains a possibility that an isolated thunderstorm or
two could initiate earlier as far north as northeastern MO. This
convection will be isolated due to model guidance indicating
some weak inhibition, so thunderstorms will form in the areas
where the outflow boundary is able to provide enough lift to
break the cap. The main threat this afternoon will be damaging
winds from the strong downdrafts associated with the
thunderstorms enhanced by drier air aloft. A tornado cant be
ruled out as there is a brief period of time during the mid
afternoon where model soundings indicate better 0-1 km wind
shear before a strong cap starts to develop in the late
afternoon and the thunderstorms become elevated. This all being
said, SPC continues to have our IL and far eastern MO counties
in a Slight Risk and the rest of our MO counties in a Marginal
Risk associated with the thunderstorms this afternoon.

Todays temperatures wont be as bad as yesterdays, with
guidance indicating lower to mid-90s for surface temperatures
and low 70s dewpoints resulting in slightly lower heat index
values closer to 100-105 F. Due to cloud cover and rain chances
through the day today, northeastern MO and west central IL could
see surface temperatures as cool as the upper 80s, but could
experience slightly higher dewpoints due to lack of vertical
mixing. Thursday the front stays to our north, keeping the hot
and humid air in our region. Despite convective debris casting
clouds over our north, ensemble guidance indicates a 90% chance
of Thursday`s high temperatures being 95 F and dewpoints being
greater than 70 F. As a result, our Heat Advisory and Extreme
Heat Warning were extended through Thursday at 8pm CDT, with a
future extension possible into the weekend.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Issued at 326 AM CDT Wed Aug 12 2026

Thursday night into Friday, there is another chance of rain.
Over IA there will be synoptic support for lift in combination
with the abundant surface moisture pooling ahead of a stationary
front that will allow for convection to develop Thursday night.
Cluster analysis of the global ensemble members is in agreement
that the swath of heavy rain is going to remain to the north of
our forecast region in central IA and northern IL into northern
IN, with a 75% chance of east central IA seeing at least 1 inch
of rain. That being said, rain chances in our forecast region
arent completely nil. NBM guidance indicates a 25-50% chance of
northeastern MO and west central IL to receive 0.25 inches of
rain. Even lower is the chance of the St. Louis metropolitan
area seeing more than 0.1 inches of rain being 10-20%.

Despite the chances for rain Thursday night into Friday, no
long term heat relief is expected. Models continue to be in
consensus that the ridge will build north toward the end of the
week allowing for the hot and humid air mass to remain over the
central CONUS. Looking at the end of the week, ensemble guidance
indicates a 75-90% chance of high temperatures Friday through
Sunday being 95 F or higher, and a 60-90% chance of dewpoints
being greater than 70 F. On the brighter side, ensemble guidance
is continuing to show strong signs of a cold front moving
through the region at the beginning of next week. Cluster
analysis is in agreement that heights will fall and bring in
northerly flow starting Sunday night into Monday.

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1044 PM CDT Tue Aug 11 2026

While VFR conditions are expected to prevail for a majority of
the TAF period, chances for showers and a few thunderstorms are
increasing slightly at local terminals from late tomorrow
morning through the afternoon, particularly at UIN, STL, SUS,
and CPS.

Before we get to storm chances, a chance for marginal low level
wind shear persists at JEF late tonight and early tomorrow
morning, due to nearly calm surface winds in the river valley
underneath a 35-40kt southwesterly low level jet and nocturnal
inversion. Modeled wind speeds have weakened slightly since
this afternoon`s TAF package, but we will maintain a mention for
one more cycle and monitor this trend before completely
removing it.

Remnant showers and a few thunderstorms are increasingly likely
to reach UIN by mid morning tomorrow, and may gain strength as
they move southeast through the early afternoon. Confidence
remains low in the exact timing and evolution of these storms,
but probabilities have increased enough to add PROB30 mentions
of storms at all St. Louis area terminals. Storms may also
feature strong wind gusts and heavy rain. This activity should
move southeast of these terminals before sunset. Additional
showers are possible near the end of the period, but this has
been omitted due to low forecast confidence.

BRC


.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.

&&

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

&&

$$

SHORT TERM...Asel/MRB
LONG TERM...Asel/MRB
AVIATION...19