602
FXUS63 KLSX 052347
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
647 PM CDT Wed Aug 5 2026

.KEY MESSAGES...

- Thunderstorms with heavy rainfall are expected this evening
  through Thursday morning. A Flood Watch is in effect where
  there is increasing potential for a corridor of rainfall
  amounts of 3 to 5+" to lead to area of flash flooding.

- The threat of severe weather is lowering this afternoon/evening,
  but an isolated instance of damaging winds cannot be ruled
  out.

- Additional rounds of showers and thunderstorms are forecast
  for portions of the area through early Saturday before heat
  and humidity builds through the weekend.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 254 PM CDT Wed Aug 5 2026

Showers, weak thunderstorms, and associated cloud cover have
lingered into the afternoon across central, northeastern MO and west-
central IL leading to the greatest daytime heating being displaced
from the greatest moisture, which is casts increasing doubt in the
severe thunderstorm potential later this afternoon/evening. There is
an attempt at airmass recovery across western MO amidst a few breaks
in cloud cover, but this process has been slow to occur and
translate into the CWA. Therefore, it is uncertain if surface-based
instability (convective inhibition) will increase (decrease) enough
to achieve surface-based thunderstorm initiation near a cold front
that will be reaching northeastern MO and west-central IL by this
evening which would limit the threat of a thunderstorm being capable
of damaging winds or a brief tornado.

Elevated thunderstorms will increase in coverage after sunset this
evening as a seasonably strong 30-kt LLJ develops and transports
increasingly anomalous moisture (PW exceeding 2", 95th
climatological percentile) along with the arrival of an equatorward
upper-level jet streak entrance region. There is strong support for
upscale growth of thunderstorms to modulate/reinforce the front that
interfaces the LLJ in a pattern that is very favorable for training
of thunderstorms, resulting in corridors of high sustained rainfall
rates owing to the anomalous PW and deep warm-cloud depths. The HREF
and REFS LPMM continue to depict corridors of rainfall totals of 3
to 5+" through Thursday morning which would be sufficient for flash
flooding; however, there are exceptionally large spatial differences
(as much as 100-150 mi) due to the large, sensitive component of
preconditioning from initial thunderstorms this evening in this
setup. The Flood Watch is broadly across northeastern, central, east-
central MO and west-central IL to encompass this uncertainty and has
been extend across the IL counties of the St. Louis metro. That
being said, only a small subset of this watch area will in reality
see flash flooding.

A gradual decrease in the coverage and intensity of thunderstorms is
forecast after sunrise Thursday morning as the LLJ and attendant
forcing also gradually abates. How quick this process occurs will
have implications on cloud cover, temperatures, and, more
importantly, instability for shower and thunderstorm redevelopment
during the afternoon. Currently, it appears the effective
front/boundary will likely be shifted in to southeastern
MO/southwestern IL, which would serve as the catalyst for any
daytime thunderstorm development, if it occurs at all.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
Issued at 254 PM CDT Wed Aug 5 2026

Confidence is greater in showers and thunderstorms forming Thursday
night as another LLJ develops but the location is not completely
clear, although conceptually to the south and east of tonight`s
round would be most favored (i.e., southeastern MO/southwestern IL).
Although there could be some training of thunderstorms, the overall
pattern does not look as favorable for heavy rainfall/flash flooding
with lower PW, weaker LLJ, and weaker boundary/front.

Yet another round of showers and thunderstorms is expected sometime
between Friday afternoon and early Saturday morning as an upper-
level shortwave trough or MCV propagates through the Mid-Mississippi
River Valley and then another LLJ develops. If this round impacts
areas that receive heavy rainfall tonight/Thursday morning, there
would be at least a low threat of flash flooding, but the pattern
once again does not appear as optimal for flash flooding overall.

Saturday through Monday, global model guidance indicate that gradual
height rises associated with an upper-level ridge impinging on the
Mid-Mississippi River Valley from the west will help heat and
humidity build into the CWA. With high temperatures warming into the
90s F across most of the area by Sunday and dewpoints also rising
into the 70s F, multiple days of peak heat index values reaching
around 100 to 105 F are possible. However, with time, there is some
concern that a potential weakening of the ridge and longwave
troughing across the Great Lakes could facilitate southward
advancement of a front near or into the CWA along with accompanying
showers, thunderstorms, and cloud cover.

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 642 PM CDT Wed Aug 5 2026

Thunderstorms remain on track to impact all local terminals to
varying degrees this evening, overnight, and tomorrow morning.
Storms are expected to impact COU, JEF, and UIN first, with some
weaker activity near the beginning of the period, followed by
more substantial storms and lower visibilities due to heavy rain
overnight, along with occasional gusty winds. These storms are
also expected to spread east into the St. Louis area later in
the night and tomorrow morning, and will likely impact STL, SUS,
and CPS as well. A lull in activity is likely during the day
tomorrow, but low clouds and category reductions to MVFR levels
may continue for several hours afterwards before lifting. At the
end of the period, redeveloping thunderstorms are possible, but
for now only impacts the end of the 30 hour STL TAF period.

BRC

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch from 10 PM CDT this evening through Thursday
     morning for MOZ018-019-026-027-034>036-041-042-047>052-
     059>065-072-073.
IL...Flood Watch from 10 PM CDT this evening through Thursday
     morning for ILZ058-095>102.

&&

$$

SHORT TERM...Pfahler
LONG TERM...Pfahler
AVIATION...19