880
FXUS63 KLSX 220345
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1045 PM CDT Mon Jul 21 2025

.KEY MESSAGES...

- After a relatively cooler day today, dangerous heat begins
  Tuesday and lasts through at least Thursday.

- There remains uncertainty with respect to temperatures (and
  therefore heat index values) starting on Friday due to chances
  of showers and thunderstorms returning to portions of the area.

&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 219 PM CDT Mon Jul 21 2025

Widespread showers and thunderstorms in our area have largely
dissipated across our area over the past few hours. Further west,
there is still quite a bit of stratiform rain, but this area too
has weakened with time and has struggled to push into central
Missouri. GOES-16 satellite imagery has shown some cloud top
warming, with regional radar showing weakening of the overall
structure of the MCS. Some light rain may make it into parts of
central Missouri with some isolated thunderstorms possible more
into southeast Missouri.

For tonight, a mostly dry forecast appears likely though there is
some weak low-level moisture convergence. With a lingering composite
outflow boundary also potentially moving back northward, did not
want to leave the forecast completely dry. The big negating factors
include the continuation of the mid/upper level ridge building
overhead and only a 10-15 knot low-level jet overnight tonight (much
weaker than last night). Otherwise, look for variable cloudiness
with lows ranging from the low to mid 70s.

While today`s rain and clouds really kept the heat mostly away, we
will not be as lucky on Tuesday. Confidence is very high that this
will be the first of what should be at least three very hot and
humid days with highs well into the 90s and dewpoints mainly between
75-80 degrees. The saying "it`s not the heat, it`s the humidity"
will ring true again with this event. While high temperatures will
not even get within 10-15 degrees of daily records, the
aforementioned dewpoints will be within striking distance. This will
be the big driver of the extreme heat index values beginning
tomorrow, with peak values ranging from 105-110F over most of the
area. The exception may be parts of south-central Illinois where
peak heat index values are expected to be closer to 100 degrees due
to slightly cooler high temperatures in that region. In those areas,
left the extreme heat watch as is, but did convert the remaining
watch over to an advisory starting tomorrow and lasting through
Thursday. No changes were made to the extreme heat warning.


Gosselin

&&

.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 219 PM CDT Mon Jul 21 2025

(Tuesday Night - Thursday Night)

As mentioned earlier in the discussion, there is very high
confidence in the extreme heat event continuing through at least
Thursday. This high confidence is due to the placement of the
mid/upper level ridge more directly overhead the mid-Mississippi
Valley. This is particularly true on Wednesday, as by Thursday
afternoon there is a subtle push more to the south and east. This
suggests at least the possibility of some mid/upper level clouds to
potentially temper high temperatures a couple of degrees Thursday
afternoon (mainly in northeast Missouri/west-central Illinois).
Regardless, we are expecting highs each day mostly in the mid to
upper 90s (probably a couple of degrees warmer than Tuesday).
Dewpoints may tick down a bit due to drying soils, but should still
remain in the 70s areawide. What does this mean for peak heat index
values? Well, not a whole lot of change as values climb well above
100 degrees areawide. The hottest (105-110F) conditions should be in
the warning area, but more specifically, in/around metropolitan St.
Louis where some 110+ values are possible.


(Friday - Next Monday)

By Friday, the west-east axis of the mid/upper level ridge is
centered more across the mid south, putting the mid-Mississippi
Valley near the "ring of fire". There or course is some
discrepancies with respect to the timing/track, but deterministic
guidance is showing a midlevel shortwave trough moving west to east.
This feature combined with increasing low-level moisture convergence
on the nose of a low-level jet could yield widespread showers and
thunderstorms Friday morning. This rain and associated cloud cover
could keep temperatures below 90 degrees and put (at least a
temporary) end to this heat event. However, there is basically no
change in the ambient air mass. This means in the absence of any
rain/cloud cover, high temperatures would be able to get warm enough
to yield heat index values at least above 100 degrees (advisory
criteria for duration- 4+ days). Given the uncertainty, did not
extend any heat headlines right now but it is possible this will be
needed in the coming days.

By late this weekend heading into early next week, both the EPS and
GEFS are in pretty good agreement that the mid/upper level
anticyclone will build back to the north and west. Exactly where it
is centered is unknown however. A further west placement would
continue to potentially yield "ridge-runners" and rain/cloud
chances. However, if the high is either stronger and/or further
east, this would yield dry weather and a return to the extreme heat.
Recent trends in ensemble guidance has been to a slightly stronger
mid/upper level high. If this trend holds water, confidence in a
return (or continuation if storms do not interrupt it
Friday/Saturday) of the heat would increase.


Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1042 PM CDT Mon Jul 21 2025

Dry, VFR conditions are likely at the terminals through Tuesday
night. Areas of fog are now forecast in parts of central and
south-central Illinois where significant rainfall occurred
yesterday and skies will clear tonight. This fog will not reach
the TAF sites, and will dissipate shortly after sunrise. Weak
southerly winds will establish through the morning and remain in
place through the rest of the period.

MRB

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from noon Tuesday to 7 PM CDT Thursday for Knox MO-
     Lewis MO-Marion MO-Monroe MO-Pike MO-Ralls MO-Shelby MO.

     Extreme Heat Warning until 7 PM CDT Thursday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Lincoln MO-Madison MO-
     Moniteau MO-Montgomery MO-Osage MO-Reynolds MO-Saint
     Charles MO-Saint Francois MO-Saint Louis City MO-Saint
     Louis MO-Sainte Genevieve MO-Warren MO-Washington MO.

IL...Extreme Heat Watch from Tuesday afternoon through Thursday
     evening for Bond IL-Fayette IL-Greene IL-Jersey IL-Macoupin
     IL-Marion IL-Montgomery IL.

     Heat Advisory from noon Tuesday to 7 PM CDT Thursday for Adams
     IL-Brown IL-Calhoun IL-Clinton IL-Pike IL-Washington IL.

     Extreme Heat Warning until 7 PM CDT Thursday for Madison IL-
     Monroe IL-Randolph IL-Saint Clair IL.

&&

$$

WFO LSX