510
FXUS63 KLSX 141051
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
551 AM CDT Fri Aug 14 2026

.KEY MESSAGES...

- Dangerous heat and humidity will continue today and tomorrow
  at a minimum, with the potential to linger into Sunday in the
  Ozarks. Confidence in more widespread heat relief is highest
  Monday onward.

- While a few stray showers/thunderstorms are possible (20-30%)
  this morning and afternoon, better chances (30-50%) exist
  overnight tonight across northern MO and central IL, with
  additional chances area-wide (20-40%) Sunday.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 244 AM CDT Fri Aug 14 2026

While our northern fringes saw some modest heat relief yesterday
thanks to a persistent outflow boundary, dangerous heat continued
across much of the rest of the area, and this is expected to persist
today and again tomorrow. Much like yesterday, the main source of
uncertainty in the temperature/humidity forecast continues to be in
those same northern fringes, where once again the potential for
another outflow boundary looms. Latest high-resolution model data
does not seem to be factoring in the cooling effect of such a
boundary today, as even the four counties currently excluded from
heat headlines (Knox, Lewis, Adams, Brown) are forecast to reach a
heat index of near 105 today, driven largely by very high surface
dewpoints just shy of 80 degrees. Over the past several days, we
have seen this area repeatedly cooled by cloud cover and convective
outflows, with really only one day with a verified heat index of 105
degrees or more (and mostly below 100). Meanwhile, the potential for
lingering cloud cover and outflows from storms to the north persist
today as well, with even a modest potential (20%) for a few stray
showers further south in our area this morning and afternoon. Given
this week`s history and the lack of persistent heat in these areas,
and the fact that even a bullish forecast only barely reaches the
105 degree threshold for a single-day Heat Advisory, we have opted
not to add these counties to the headline for now. Still, if cloud
cover and/or outflow is not as persistent or southerly as it could
be, any little bit of sun will bring the heat index close to that
threshold given the very high moisture in place.

As for precipitation, nocturnal thunderstorms continue to percolate
just north of our area this morning, with a bit of an outflow-driven
southerly surge noted in central Illinois. This has all remained to
our north so far, but it would not be a shock to see a few showers
dip into our portion of Illinois this morning, with a few more
possible in northeast Missouri later this afternoon as well. We
don`t expect much more than a glancing blow from this activity
though, and our probabilities remain low overall (20-30%). Later in
the evening and overnight, though, a passing shortwave to our north
is likely to drive another MCS(s) across southern Iowa, and there is
some support for a more southerly track that would clip our northern
tier of counties as well. Wherever this MCS goes, damaging wind and
torrential rain rates would be the most significant hazards, the
latter being driven by exceptionally high atmospheric moisture (2.2+
inch precipitable water). This will need to be monitored closely for
slight changes in forecast track, as this will determine the impact
level in our forecast area.

Once showers/storms clear out Saturday morning, a similar story is
likely to unfold. Dangerous heat continues across a majority of our
area, with the possible exception of our northern fringes thanks to
(you guessed it) lingering outflow and cloud cover. As such, our
heat headlines remain unchanged and valid through Saturday evening.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Issued at 244 AM CDT Fri Aug 14 2026

Sunday and Sunday night, confidence continues to increase that a
shortwave will dig into the Great Lakes and send a weakening cold
front south and into our area. Forcing along this front does not
appear to be particularly strong, as it becomes far removed from the
shortwave and surface low as it moves through our area. As such, it
will likely lose momentum as it arrives, and there is some
uncertainty regarding both its timing and potential to drive another
round of showers and thunderstorms. Precipitation probabilities
remain modest as a result (20-40%), but Sunday afternoon/evening
represents the next best opportunity for showers and storms.

Much more likely is that this front will bring heat relief to the
majority of the area (with a few possible exceptions) early next
week, although this may not completely be felt until Monday across
the Ozarks. Given the timing uncertainty we have opted not to extend
any heat headlines into Sunday just yet, but that potential does
exist mainly south of I-70. There is also a very real possibility
that this front will stall somewhere near our southern fringes and
wash out, which may prevent our far south (Reynolds, Iron, Madison,
etc.) from truly seeing substantial heat relief before perhaps a
brief rebound on Wednesday. Meanwhile, rain chances diminish behind
the front thanks to a push of drier air, although rain chances may
linger in the vicinity of the front wherever it stalls. By mid week
(WED/THU) a reinforcing shortwave is likely to send another front
into our area, renewing modest chances for showers and
thunderstorms, and reinforcing heat relief late in the week.

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 546 AM CDT Fri Aug 14 2026

VFR categories are expected for a majority of the 12Z TAF
period, but there are a few opportunities for showers and
thunderstorms at times. Isolated and largely weak convective
showers are percolating near St. Louis area terminals and UIN at
the start of the period, and this may continue a few more hours
before this activity moves east of the area. Mostly dry
conditions are expected the rest of the day, aside from perhaps
a few more stray showers/weak thunderstorms near UIN during the
afternoon. However, the best chance for terminal impacts due to
thunderstorms will be late this evening and overnight at UIN.
While the best chances for storms remain north of the terminal,
probabilities are high enough (30-50%) to include a PROB30 group
over the last 6 hours of the period. Gusty winds and heavy rain
will be the main threats with storms if they do impact the
terminals, including visibility reductions.

BRC

&&

.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...19
LONG TERM...19
AVIATION...19