Area Forecast Discussion

250
FXUS63 KLSX 062020 CCA
AFDLSX

Area Forecast Discussion...CORRECTED
National Weather Service St Louis MO
320 PM CDT Thu Aug 6 2026

.KEY MESSAGES...

- Showers and thunderstorms are expected at times through Friday
  night, but coverage is uncertain along with their ability to
  produce additional heavy rainfall/flash flooding.

- There is a potential for a few rounds of showers and
  thunderstorms north of I-70 this weekend into early next week
  with multiple days heat and humidity in locations that are not
  impacted.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 318 PM CDT Thu Aug 6 2026

At the nose of a slow-to-weaken LLJ, numerous showers and
thunderstorms have persisted into the afternoon along/south of I-70
with flash and nuisance flooding even lingering through midday
across central MO. However, coverage and intensity will tend to
decrease through the remainder of the afternoon as the LLJ and
associated forcing diminishes. An extensive cold pool and the
lingering clouds/precip will severely mute daytime heating with
temperatures remaining mainly in the 70s F.

With the effective front/boundary shunted far to the south of the
CWA across northern AR into southwestern MO confidence is low in
additional daytime shower and thunderstorm development in CWA until
another nocturnal LLJ develops and extends over the front/boundary
after sunset. At least isolated showers and thunderstorms are
anticipated to develop tonight somewhere across southeastern/east-
central MO into south-central/southwestern IL, but the impact of
additional thunderstorms this afternoon across southwestern/southern
MO, weak mid-level subsidence, and drying decreases confidence in
organization/upscale growth of thunderstorms into an MCS locally.
Additionally, PW will be lowering to approximately 1.7", decreasing
the concern for high rainfall rates. That being said, the latest
HREF LPMM still contains very localized amounts of 2 to 4", leading
to a concern of locally heavy rainfall. However, it is uncertain if
there will be an overlap with this morning`s heavy rainfall, which
would seemingly be required for any flash flooding.

CAMs are in much better agreement that MCS development will take
place across southeastern KS overnight into Friday morning, which
has implications for a potential remnant MCV that tracks eastward
through MO on Friday. This feature would be the main (conditional)
catalyst for the development of showers and thunderstorm Friday
afternoon and evening, but some guidance suggests that it may be
competing with large-scale subsidence just behind the main upper-
level trough axis. Accordingly, CAMs vary on how much development
actually occurs, but have a loose preference for portions of the CWA
along/south of I-70. Similar uncertainties carry into Friday night
with another develop nocturnal LLJ; however, the general consensus
is for this LLJ to be relatively weak moreso focus to the east of
the CWA.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Issued at 318 PM CDT Thu Aug 6 2026

Through the weekend, the eastern branch of an upper-level ridge will
begin to edge into the Mid-Mississippi River Valley, favoring a
northward shift of the weak baroclinic zone, that has been the
general focus of showers and thunderstorms, northward. However,
there has been some variability in the strength of the ridge and a
longwave trough centered across southeastern Canada into the Great
Lakes and the balance of these features will determine how much of
the CWA sees increasing heat and humidity Saturday onward and is
conversely susceptible to the southern flanks of episodic
thunderstorm clusters or MCSs. As such, global ensemble 6-hour
probabilities of measurable rainfall range from 20 to 50 percent
north of I-70, which is where confidence is relatively lower in the
persistence of heat and humidity. Elsewhere to the south, multiple
days of afternoon heat index values of 100 to 105 F are favored
through Tuesday, with support for the greatest footprint and
magnitude on Monday.

During mid to late next week, there is greater agreement in global
model guidance that height falls over the Great Lakes associated
with an upper-level trough rounding the longwave trough will force
the baroclinic zone and more favorable corridor for showers and
thunderstorms southward into the CWA. This process will bring an end
to heat and humidity in lieu of cooler temperatures and showers and
thunderstorms sometime around Wednesday onward.

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1242 PM CDT Thu Aug 6 2026

Lingering showers and thunderstorms across central/east-central MO
into southwestern IL will likely continue to gradually decrease in
coverage and intensity through afternoon with flight conditions
improving to VFR through 19 or 20z across most of the area. The
subsequent shower and thunderstorm forecast through the remainder of
the TAF period is very uncertain, but there are low chances tonight
and again on Friday. However, confidence is too low in timing,
coverage, and even overall occurrence for explicit inclusion at this
point. Some fog and lower ceilings will expand into northeastern
MO/west-central IL late tonight/Friday morning, impacting KUIN with
at MVFR flight conditions. Elsewhere to the south, ceilings may be
more sporadic. Outside of thunderstorms, winds will remain
light.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.

&&

$$

SHORT TERM...Pfahler
LONG TERM...Pfahler
AVIATION...Pfahler

NWS LSX Office Area Forecast Discussion