441
FXUS63 KLSX 161922
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
222 PM CDT Sat Aug 16 2025

.KEY MESSAGES...

- Dangerous heat and humidity will continue through Monday, and
  potentially into Tuesday for portions of the area.

- Isolated to scattered thunderstorms are forecast Sunday afternoon,
  but the best chances (30-50%) are associated with a cold front
  Tuesday night through Wednesday.


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 222 PM CDT Sat Aug 16 2025

GOES-16 satellite imagery shows cumulus becoming more agitated
early this afternoon mainly across central, east central, and
southeast Missouri. Isolated showers and thunderstorms are
expected to develop over the next 1-2 hours in this region. The
latest SPC mesoanalysis shows plenty of instability (2000-4000
J/kg of MLCAPE), but <15 knots of effective shear. Given that the
area is directly beneath the midlevel anticyclone, the lack of
shear is certainly not a surprise. Individual showers and
thunderstorms, like yesterday, should be very short lived as rain-
cooled downdrafts choke off their respective updrafts. There may
be a storm or to that does produce some gusty winds via weak
microbursts however given steep low-level lapse rates (>8 C/km)
and high DCAPE values (1000-1200 J/kg).

Lingering showers and thunderstorms should drift westward and
completely dissipate around/shortly after sunset. Dry weather is
then forecast for the remainder of the overnight period. Model
guidance, including most CAMs, show another complex of thunderstorms
moving southward across northern Illinois/Indiana late tonight.
These storms should not make it near even our south-central Illinois
counties by early Sunday morning, but an outflow boundary may
propagate southwestward into our area. This boundary may in turn
serve as the focus for additional convective development Sunday
afternoon. Given weaker capping forecast Sunday afternoon compared
to today, I would expect a bit better coverage. Therefore, increased
PoPs about 10-20% compared to the previous forecast. The best
chances (~30%) are in east central and southeast Missouri during the
mid to late afternoon hours. This is where the cap is weakest and
where the remnant outflow boundary from overnight convection is most
likely to reside. The environment should be very similar overall,
with a near total lack of any wind shear basically all but
guaranteeing typical summertime pulse convection. Most storms should
be weak and short lived, but a couple of weaker microbursts may be
possible due to steep low-level lapse rates and high DCAPE values.

In terms of heat and humidity levels, persistence looks like a very
good forecast. Any shower/thunderstorm activity likely won`t have
much of an impact because they won`t be widespread and should not
develop until mid afternoon. This is at a time where high
temperatures likely have already been reached. They may put a bit of
a cap however by eliminating an hour of two of modest potential
warming say between 2-4 PM. This could be the difference though in
isolated locations hitting the century mark or not. I cannot rule
out organized convection moving southward near the Illinois-Indiana
border staying more organized long enough to produce a stronger
composite outflow boundary, but this is not a particularly likely
scenario. However, if this were to occur, highs behind it (say south-
central Illinois and vicinity) could be a few degrees cooler. In the
absence of this happening however, widespread highs in the mid to
upper 90s with peak heat index values generally in the 102-108F
range are expected.


Gosselin

&&

.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 222 PM CDT Sat Aug 16 2025

(Sunday Night - Monday Night)

Any lingering showers and thunderstorms should quickly
weaken/dissipate Sunday evening as instability wanes. Monday looks
eerily similar to the previous two days, with another hot and humid
day on tap. There may be some diurnal showers and thunderstorms once
again given a similar environment but wasn`t quite confident enough
to add in any mentionable PoPs at this time. Regardless, the main
story will be the continuing heat/humidity as highs once again
should climb back into the mid to upper 90s with peak heat index
values of 102-108F.


(Tuesday)

There likely will be one last day of dangerous heat and humidity on
Tuesday ahead of an approaching cold front. There is increasing
confidence that the front will not make it through most of the area
until Tuesday night. However, the bigger concern is any MCS to the
north/northwest Tuesday morning would come south/southeast into our
area and potentially mitigate the heat. The pattern supports this
possible MCS, with the mid/upper level ridge shifting into the
intermountain west. The LREF has trended a bit upward on
probabilities for >100F heat index values on Tuesday, now showing 60-
80% chances roughly along/south of Interstate 70. While an extension
of the heat advisory is more likely than not at some point in the
future, I was not confident enough with this forecast package given
the overall pattern and at least some potential for an MCS. Even
widespread debris clouds and a strong composite outflow boundary
passing through Tuesday morning may be enough to preclude the
necessity for the extension of heat headlines.


(Tuesday Night - Next Saturday)

Models have continued to come into better agreement with the cold
frontal passage, coming through most if not all of the area Tuesday
night. This front will bring the best chance of showers and
thunderstorms (30-50%) to the region, but more importantly, bring a
definitive end to the heat and humidity.

Behind the front, highs in the 80s are expected on Wednesday. Like a
typical summer front, the relative humidity drop lags several hours
further behind however. The combination of lingering moisture and
cooler temperatures collocated with a midlevel trough passing
overhead may lead to some diurnal showers and possibly a few
thunderstorms Wednesday afternoon. Dry weather is then favored
thereafter as a broad surface ridge of high pressure entrenches
itself over the Upper Midwest.

Thursday should be the coolest day of the week as 850-hPa
temperatures on the EPS/GEFS mean drop back to near +15C. There
remains a strong signal for deep low-level northeasterly flow. This
should allow for cooler and less humid air to move across the Great
Lakes into the mid-Mississippi Valley. Temperatures near to slightly
below normal are favored, with lows in the low to mid 60s and highs
in the low to mid 80s.

Clusters of the 500-hPa height pattern show more spread with the
next mid/upper level trough moving toward the US-Canadian border
heading into next weekend. There are timing, strength, and track
differences with this feature which all leads to low confidence.
There should be another cold frontal passage sometime either
Saturday or next Sunday, but exactly when and how much it cools down
behind it are highly uncertain. Ahead of this front, temperatures
should warm back up back into the upper 80s to near 90 degrees by
Saturday.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1223 PM CDT Sat Aug 16 2025

Isolated showers and weak thunderstorms are possible this
afternoon into early this evening, mainly in central/east
central/southeast Missouri. The coverage looked too low to add any
VC or PROB30 wording however in the metro St. Louis or central
Missouri terminals. Outside of any shower or thunderstorm directly
impacting a site, look for dry/VFR conditions with light
southerly winds.

Gosselin

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT Monday for Audrain MO-Boone MO-
     Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Madison MO-
     Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike
     MO-Ralls MO-Reynolds MO-Saint Charles MO-Saint Francois MO-
     Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-
     Shelby MO-Warren MO-Washington MO.

IL...Heat Advisory until 8 PM CDT Monday 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