041
FXUS63 KLSX 172259
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
559 PM CDT Sun Aug 17 2025

.KEY MESSAGES...

- Dangerous heat and humidity will continue for at least one more day
  Monday, and potentially into Tuesday for portions of the area.

- Scattered thunderstorms are forecast again on Monday afternoon,
  but more widespread rain chances (30-50%) come with a cold front
  Tuesday afternoon/night.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 229 PM CDT Sun Aug 17 2025

Scattered showers and thunderstorms are expected to continue over
the next several hours into the early evening. Similar to
yesterday, there is very little/no steering flow or deep-layer
shear. Individual showers and thunderstorms should not survive too
long, sending out nearly concentric outflow boundaries. These
outflow boundaries then should generate new convection, with the
west/southwest flank likely being favored. Steep low-level lapse
rates and high DCAPE values (>1000 J/kg) suggest sufficiently tall
thunderstorms will produce microbursts. Most of these should stay
mainly in the 30-50 mph range, but one or two may have enough
longevity to produce some near severe wind gusts. These storms
also should be pretty efficient rainfall producers given
precipitable water values around 1.75" (~90th percentile of
climatology). The overall flash flood threat is low given the
short duration of individual thunderstorms, but if one is able to
survive long enough or there is any training there could be some
extremely localized hydrological problems.

As mentioned above, the favored area for showers and thunderstorms
should continue to move westward early this evening, more into parts
of central and southeast Missouri. This is because renewed
convective development should in theory reinforce the westward
movement of the composite outflow boundary. A loss of some daytime
instability should once again lead to the weakening and eventual
dissipation of any lingering showers and thunderstorms by late
evening. The remainder of the overnight period is likely to be dry,
though there is more of a broad, weak low-level jet with some weak
moisture convergence in northeast Missouri and west-central
Illinois. This suggests there could be an isolated shower or weak
thunderstorm or two overnight, but coverage should be too low to add
the inclusion of any mentionable PoPs.

Another complex of thunderstorms/MCS is likely to be ongoing Monday
morning across the mid-Missouri Valley into the Upper Midwest. This
activity should weaken and eventually dissipate shortly after 1500
UTC as the low-level jet weakens. There may be a remnant outflow
boundary that attempts to move into northeast Missouri and west-
central Illinois, but this is highly uncertain. Isolated to
scattered showers and thunderstorms are expected to develop by early
afternoon in parts of east central and southeast Missouri where
capping is weakest. Chances expand in area as individual concentric
outflows serving as the focus for additional development. The
overall environment should be nearly identical as today (and
yesterday), so gusty winds via microbursts and extremely localized
heavy rainfall are the concerns once again.

Another hot and humid day is on tap on Monday, though there may be a
bit more cloud cover earlier in the day so highs may be a tick or so
cooler for much of the area. In addition, dewpoints should tend to
be a bit lower. We definitely saw some pooling along today`s remnant
outflow boundary and are less likely to see that type of boundary in
our area tomorrow. However, it still will be quite hot and humid
overall with highs mainly in the low to mid 90s and peak heat index
values around 105 degrees for most locations.


Gosselin

&&

.LONG TERM...  (Monday Night through Next Sunday)
Issued at 229 PM CDT Sun Aug 17 2025

(Monday Night - Wednesday)

The concern for Monday night into Tuesday continues to be on MCS
development upstream across the mid-Missouri Valley. Deterministic
guidance are a bit all over the place, but generally have at least
scattered showers and thunderstorms in/near areas northwest of I-70
in Missouri and I-55 in Illinois. Moisture convergence weakens
especially after 1500 UTC as the veering low-level jet abates. This
should lead to the dissipation of showers and thunderstorms across
the area, but even in this scenario several questions remain. Is the
convection organized enough to produce a southward propagating
outflow boundary? How much does lingering cloud cover impact
temperatures? When does renewed convection develop along/near either
the synoptic cold front and/or the remnant outflow boundary? Long
story short, there are a lot of questions regarding temperatures on
Tuesday afternoon. Therefore, did not extend any part of the ongoing
heat advisory through Tuesday. The best chances remain in southeast
Missouri and southwest Illinois with the LREF showing 40-60% chances
for at least a peak heat index of 100 degrees. As alluded to by the
previous forecaster though, these probabilities have decreased about
10-20% from yesterday. There is a scenario where a more-organized
MCS moves into parts of the area Tuesday morning and completely
augments the environment in our whole CWA. While I do not think this
is particularly likely, there has been a tendency for convection to
survive a lot longer than anticipated since July which also gives me
pause in extending any heat headlines into Tuesday.

The timing of the synoptic front also looks slightly sped up
compared to 24-48 hours ago, likely bisecting the area around 0000
UTC Wednesday and clearing out of the area closer to 0600 UTC.

Wednesday will be cooler, though the lower humidity (like many
summer fronts) really lags behind the initial boundary. Temperatures
will be a lot cooler (highs mainly mid to upper 80s), so even with
the lingering humidity, it will feel quite a bit better and put a
definitive end to the heat if convection does not do it on Tuesday.
An elongated midlevel shortwave trough is also forecast to move
through the mid-Mississippi Valley on Wednesday. Some lingering
moisture combined with steepening midlevel lapse rates beneath the
aforementioned trough should lead to scattered showers along with a
few thunderstorms, mainly along/south of Interstate 70.


(Wednesday Night - Friday Night)

Mostly near normal temperatures along with dry weather (and lower
humidity) are forecast then through Saturday with a ridge of surface
high pressure across the region.


(Next Weekend)

Some slight moderation in temperatures is expected ahead of the next
cold front on SaturdaY. Speaking of which, recent trends have shown
a stronger mid/upper level trough moving into the Great Lakes on
Saturday. The air mass behind the associated cold front has also
trended cooler and is starting to look fairly impressive for late
August. The surface high is strong on the NAEFS (~1022 hPa: >90th
percentile of climatology) along with 850-hPa temperature anomalies
on the GEFS/EPS means of 2-6C. The main source of uncertainty is how
far southwest this anomalously cool air will get as the EPS is more
focused into the Great Lakes/eastern seaboard. Meanwhile, the GEFS
plunges well-below normal temperatures through the mid-Mississippi
Valley. Not surprisingly, the spread on the NBM is quite large for
temperatures, depicting IQR spreads of 6 to 10 degrees. So while
confidence in below normal temperatures has increased, the magnitude
and longevity is unknown.

Another sign that this is more of a fall like front is that
precipitation chances look meager along the boundary. The
combination of weak moisture return and the mid/upper level forcing
for ascent staying well to our northeast suggests this may be a
mostly dry, if not completely, dry frontal passage.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 553 PM CDT Sun Aug 17 2025

Any lingering showers and thunderstorms will diminish during the
first few hours of the evening, leaving dry and VFR conditions
through the overnight and morning hours. Afternoon showers and
thunderstorms will return to the region tomorrow afternoon,
similar to today. The best location for thunderstorms will be at
the mid-Missouri and St. Louis metro terminals, though confidence
in where exactly they occur is too low at this point to warrant
more than a PROB30 group. Gusty winds are expected underneath and
near thunderstorms, while MVFR visibilites will be limited to
just underneath thunderstorms.


Delia

&&

.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