328
FXUS63 KLSX 240813
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
313 AM CDT Tue Jun 24 2025

.KEY MESSAGES...

- Heat impacts will likely continue through at least Thursday, with
  heat index values reaching the upper 90s to near 105 degrees.

- Isolated to widely scattered thunderstorms will be possible this
  afternoon and evening.

- Scattered showers and thunderstorms are expected (30 to 50%)
  Friday through the weekend along with potentially a slight cool
  down.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 307 AM CDT Tue Jun 24 2025

Temperatures will continue to be the main concern over the next
few days and associated heat index values. Persistence is often a
good starting point in these stagnant patterns. However, there are
a couple of subtle differences. For one, there will be more
mid/high level cloud cover across parts of northeast Missouri and
west- central Illinois. This area will be just south of the
stalled frontal boundary and widespread convective debris clouds
have spilled gradually southeast. Therefore, have cooled highs in
northern sections of the area back into the upper 80s to near 90
degrees. Further southeast, highs are likely to be very similar to
yesterday. The mid/upper level cloud cover is most likely to be
thinner and any diurnal cumulus will not really become broken
until after 1800 UTC. By that time, temperature readings are
already very near daytime highs. Readings are expected to top out
in the low to mid 90s. These temperatures combined with dewpoints
in the low 70s are expected to yield peak heat index values of
100-105 degrees.

Thunderstorms are the other concern today, mainly during the
afternoon and early evening hours. Isolated to widely scattered
showers and thunderstorms are expected to develop by early
afternoon, mainly across parts of southeast Missouri and southwest
Illinois. A lack of deep-layer shear (~10 knots) means there won`t
be any organization to the convection today, with pulse storms
expected. These should essentially rain themselves out as rain-
cooled downdrafts kill their own updrafts. However, steep low-level
lapse rates and high DCAPE values suggests some gusty winds may be
possible in addition to brief, heavy downpours. By this evening, the
surface frontal boundary is expected to ebb southward to near the
Missouri-Iowa border. Scattered thunderstorms are also expected
across parts of northeast Missouri and west-central Illinois, mainly
between 03 and 08 UTC. Deep-layer shear is still pretty weak up
there even this evening, closer to 15 knots. With weakening low-
level lapse rates and instability in general, hard to see any
thunderstorms from getting to strong in these areas.

The front is forecast to move back northward on Wednesday, with
rising mid/upper level heights and ever so slightly warmer
temperatures at 850 hPa. There also doesn`t appear to be nearly as
strong of a signal for widespread mid/upper level cloudiness like
there is today in our north. In conclusion, temperatures on
Wednesday should be at least slightly warmer than this afternoon.
More widespread low to mid 90s are forecast areawide, with peak heat
index values once again of 100-105. Some locations may top 105F
in/around the St. Louis metro area and adjacent southwest Illinois.

Isolated to widely scattered showers and thunderstorms are expected
to develop once again on Wednesday by early/mid afternoon. Pulse
thunderstorms will be the convective mode given the lack of deep-
layer shear, with brief downpours and gusty winds possible in weaker
donwnbursts.


Gosselin

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 307 AM CDT Tue Jun 24 2025

(Wednesday Night - Thursday Night)

The stagnant hot and humid weather pattern shows no sign of changing
through Thursday night. Mild lows in the 70s are forecast both
Wednesday and Thursday nights with highs Thursday nearly a carbon
copy of Wednesday. Peak heat index values mostly of 100-105F with a
few spots just above 105F are expected. The current heat advisory
looks to be in good shape through the Thursday.


(Friday - Sunday Night)

Forecast uncertainty begins to increase on Friday with respect to
temperatures. While any air mass changes are negligible at best,
there is a signal for a midlevel shortwave trough to move across the
mid-Mississippi Valley. This feature may help yield more widespread
convection (and cloud debris), which could limit the temperature
rise. Highs are most likely to cool at least a couple of degrees in
northeast Missouri and west-central Illinois, but the impacts of
this shortwave and associated convection could also be felt further
to the south. The latest NBM spread between the 25th/75th percentile
is around 4-6F for highs Friday. Given this uncertainty, there are
no plans to extend the heat advisory at this time. That being said,
the air mass is supportive of another day with highs well into the
90s with peak heat index values of 100-105F+ in the absence of
afternoon cloud cover/convection.

Saturday has slightly less temperature uncertainty as the mid/upper
level ridge builds slightly back northward. Highs in the low to mid
90s with peak heat index values of 100-105F seem like a better bet
compared to Friday. Sunday looks similar to Saturday, though both
the EPS/GEFS suggest some slight weakening of the mid/upper level
ridge. This could increase the potential for more widespread
convection as deterministic guidance such as the ECMWF even suggest
a subtle midlevel shortwave trough moving overhead. Temperature
uncertainty not surprisingly is higher Sunday afternoon compared to
Saturday afternoon.


(Monday)

Ensemble guidance is in pretty good agreement that a mid/upper level
trough will swing through the Great Lakes/southeast Canada early
next week. However, there are both timing and amplitude differences
evident on clusters of the 500-hPa height pattern. At least slight
cooling is favored by Monday as even a weaker mid/upper level trough
moving through the Great Lakes/southeast Canada should help bring
down a cold front into the region. Whether this front makes it
through the area however and brings more substantial relief from the
heat is unknown.


Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1028 PM CDT Mon Jun 23 2025

Light and variable surface flow will remain light and turn out of
the west-southwest Tuesday. A few thunderstorms could impact KUIN
with the best potential arriving during peak diurnal heating,
accounted for with PROB30. Thunderstorm potential is very low
further south with any potential likely isolated at best. If, by
chance, a thunderstorm impacts any site aside from KUIN, it`ll
likely need to be addressed via short term amendments. VFR
conditions are heavily favored through the period with cloud bases
remaining well above MVFR categories.

Maples

&&

.CLIMATE...
Issued at 1225 PM CDT Thu Jun 19 2025

Record maximum temperatures at St. Louis, MO (STL), Columbia, MO (COU),
and Quincy, IL (UIN) along with the year of the record:

    |    6/24    |    6/25
-------------------------------------------------------
STL | 102 (1988) | 103 (2024)
    |            |            |
COU | 103 (1988) | 102 (1988)
    |            |            |
UIN | 99 (1988)  | 102 (1931)


Record high minimum temperatures at St. Louis, MO (STL), Columbia, MO (COU),
and Quincy, IL (UIN) along with the year of the record:

    |    6/24    |    6/25
------------------------------------------------------
STL | 80 (1914)  | 82 (1988)
    |            |            |
COU | 77 (1937)  | 77 (1988)
    |            |            |
UIN | 79 (1931)  | 80 (1937)


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 9 PM CDT Thursday 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 Thursday 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