518
FXUS63 KLSX 261713
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1213 PM CDT Thu Jun 26 2025

.KEY MESSAGES...

- Another hot and humid day is forecast today with peak heat index
  values of 100-105 degrees in the heat advisory.

- Increasing clouds and thunderstorm chances cast doubt on
  temperatures and associated heat index values Friday - Sunday.

- There is a stronger signal for cooler and less humid conditions
  for early/mid next week.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 327 AM CDT Thu Jun 26 2025

Persistence is the theme again for today`s weather with little/no
change in the ambient air mass. There should be a bit less
overall cloud cover and only isolated showers and weak
thunderstorms developing this afternoon. As a result, most
locations should be ever so slightly warmer than yesterday
afternoon. Highs in the low to mid 90s with peak heat index values
in the 100-105F range are forecast this afternoon.

Widespread showers and thunderstorms are forecast along a cold front
this afternoon/early evening stretching from the south-central
Plains northeast into the Upper Midwest. This activity is forecast
to attempt to move into parts of northeast Missouri after about 9 or
10 PM. Thunderstorms are expected to weaken and decrease in
coverage due to several factors including: 1) convergence along
the surface boundary abating, 2) diurnal instability decreasing
after sunset, and propensity for thunderstorms to become more
outflow dominant as they encounter a very low shear environment
(~10-15 knots). In addition, the stronger moisture convergence on
the nose of a 20-30 knot low-level jet weakens overnight and is
mainly focused in western Missouri. All that being said, scattered
showers and weak thunderstorms entering northeast Missouri late
this evening should slowly fade as they attempt to move off to the
south and east. Much of the activity after 0600 UTC may not even
have lightning.

Temperatures and associated heat index values are the main concern
on Friday. The expectation is for morning activity to gradually
increase in coverage across east central and southeast Missouri as
well as southwest Illinois by midday. Showers and thunderstorms
should be focused along a remnant outflow boundary from Thursday
night`s convection. A midlevel shortwave trough is also forecast to
help aid in convective development as it heads across the
Mississippi River by early afternoon. This of course all casts doubt
on exactly how warm it will get Friday afternoon. The high humidity
aided by dewpoints in the low 70s are a near certainty, so it will
really come down to the temperatures. Given earlier development of
showers and thunderstorms (by late morning) and convective debris
clouds, not to mention a possible outflow boundary, highs may stay
in the upper 80s to near 90 degrees. Given all of the possible
negating factors, did not extend the heat advisory anywhere.
However, there is at least a low chance that heat index values
around 100 degrees are possible in/around metropolitan St. Louis and
adjacent southwest Illinois. This would be more likely if any
outflow boundary does not move through and convection develops
either further northwest than anticipated or waits until mid
afternoon to develop.


Gosselin

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 327 AM CDT Thu Jun 26 2025

(Friday Night - Sunday Night)

Trends are coming into better agreement through the weekend in terms
of thunderstorm chances and temperatures. Confidence is increasing
that there will be more dry time on Saturday, with rising mid/upper
level heights and increasing low-level warm air advection. Any
thunderstorms on Saturday looks to be diurnally driven, and mainly
during the afternoon/early evening hours. In other words, convection
and any debris clouds really should not negatively impact
temperatures on Saturday. Highs in the low 90s with peak heat index
values around 100 degrees are expected.

Sunday looks very similar to Friday, with a midlevel shortwave
trough moving across the mid-Mississippi Valley. This feature should
help develop more widespread convection, which may help at least
limit temperatures that afternoon. Highs in the upper 80s to low 90s
are forecast, but if confidence increases in more widespread
convection early enough in the day, temperatures may need to be
dropped another couple of degrees.


(Monday - Wednesday)

Confidence is also increasing in a change in the synoptic pattern
across much of the CONUS early next week. Trends have shown a
slightly stronger mid/upper level wave crossing the Great
Lakes/southeast Canada, with a cold front moving through our region
late Monday/Monday night. Widespread showers and thunderstorms are
expected with the frontal passage. Cannot totally rule out a few
stronger storms with around 20 knots of deep-layer shear. The bigger
deal though with this front will be the cooler and less humid air
moving into the region. Temperatures and humidity levels are
expected to at least drop back to normal levels for early July, if
not a touch below. Dry weather is also favored behind this front.
All in all, it looks like at least a 2-3 day period leading up to
the 4th with nice weather.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1213 PM CDT Thu Jun 26 2025

Predominantly VFR flight conditions are expected through the TAF
period, but there are chances of showers and thunderstorms at times.
Through this afternoon showers and weak thunderstorms are expected
to be isolated, limiting confidence in any direct impacts. Tonight
into early Friday morning, a weakening cluster of thunderstorms is
forecast traverse northeastern MO, potentially surviving long enough
to reach KUIN, KCOU, and KJEF. Following a lull in precipitation,
redevelopment of showers and thunderstorms will take place midday
into afternoon on Friday with the greatest chances at St. Louis
metro terminals. Thunderstorms could contain brief downpours with
MVFR to IFR flight conditions and gusty winds.

Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 9 PM CDT this evening for Callaway MO-Cole
     MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson
     MO-Lincoln MO-Madison MO-Montgomery MO-Osage MO-Pike MO-
     Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis
     City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-
     Washington MO.

IL...Heat Advisory until 9 PM CDT this evening for Adams IL-Bond IL-
     Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey
     IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
     Pike IL-Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX