579
FXUS63 KLSX 051043
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
543 AM CDT Wed Aug 5 2026

.KEY MESSAGES...

- Thunderstorms are expected this afternoon through Thursday morning.
  A flash flood watch is in effect where localized amounts of
  3-5+" are possible.

- There is a lower threat for a few strong to severe thunderstorms
  this afternoon/early evening capable of producing damaging
  winds.

- Additional rounds of showers and thunderstorms are possible through
  Friday night before heat and humidity returns this weekend.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 251 AM CDT Wed Aug 5 2026

Regional radar imagery shows the remnants of a small MCV moving
east across west-central Illinois early this morning. Further
upstream, more widespread elevated convection exits across
eastern Nebraska and western Iowa along and poleward of a slow-
moving cold front. This boundary is expected to ooze southward
during the day today, and weak low-level convergence in the
vicinity of the boundary and a subtle midlevel shortwave trough
should provide enough large-scale ascent for scattered to
widespread showers and thunderstorms to enter northeast Missouri
and west-central Illinois near or just after midday. The
earlier arrival of showers and storms (or at least thicker
mid/high level clouds) casts doubts on instability and severe
potential across these areas later today. In fact, the 0Z HREF
has the highest probabilities for at least 1000 J/kg of SBCAPE
and 30+ knots of deep-layer shear at around 50% in central
Missouri late this afternoon. Essentially, there is likely to be
a spatial gap between the higher shear (farther north) and the
higher instability (farther south). That being said, a few
strong to severe storms cannot be ruled out from late this
afternoon through early/mid evening with damaging winds the
main threat.

The focus then shifts to the threat for heavy rainfall and flash
flooding beginning late this evening as a 30+ knot low-level jet
strengthens. The v component (north-south) is quite strong, above
the 90th percentile of climatology.  This jet also veers more
westerly and becomes more parallel to the surface cold front, which
suggests training is possible. CAMs all have at least some training
late tonight into early Thursday morning, but exactly where is
uncertain. My best guess is near to just north of the I-70 corridor,
but it will depend on exactly where the front is located tonight.
The possibility of training thunderstorms in an environment
characterized by deep warm cloud depths and anomalously high
precipitable water values (~2.0", >95th percentile) suggest high
rainfall totals and an increasing potential for flash flooding.
The LPMM of the HREF has pockets of 2-4", with the REFS in the
3-5+" range. Given the pattern and the environment described
above, I would not be surprised to see a narrow west/northwest
to east/southeast axis where 3-5+" occurs, with locally higher
totals not out of the realm of possibility. Given this increased
threat, have issued a flash flood watch from 10 PM tonight
through 10 AM Thursday morning. The area was extended spatially a
bit farther both to the north and south due to the uncertainty
in the exact location of the front tonight.

Low-level moisture convergence weakens and shifts southeastward on
Thursday morning. Mid/upper level heights also rise behind a
departing midlevel shortwave trough. This should mean an end of the
flash flood threat and a lot of dry time from late morning into the
early afternoon hours. Isolated showers/thunderstorms may redevelop
Thursday afternoon in the vicinity of the front, but this could
be south of the area as morning convection may produce outflow
and move the effective front more equatorward.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Issued at 251 AM CDT Wed Aug 5 2026

(Thursday Night - Friday Night)

There looks to be two additional rounds of widespread showers and
thunderstorms, one Thursday night and another Friday night. For
Thursday night, the focus should shift southward, more toward
southeast Missouri and southwest Illinois. This is where most
guidance favors the stronger low-level moisture convergence.
Training may be a concern again with a veering low-level jet more
parallel to the quasi-stationary front. A couple of factors though
suggest the flash flood threat probably will be lower: 1)
precipitable water values are not as high, closer to 1.8" (near or
just below 90th percentile), and 2) locations favored for storms
Thursday night are farther south than tonight.

The approach of a midlevel shortwave trough Friday evening and an
increasing low-level jet should mean another round of nocturnal
convection Friday night. This round probably will shift back north,
and may occur in areas that get significant rainfall tonight into
Thursday morning. If this is indeed the case, that could mean a
renewed flash flood threat. However, the jet itself looks weaker
than either tonight or Thursday night and the rainfall does not look
as efficient either with lower warm cloud depths and precipitable
water values.


(Saturday - Monday Night)

Guidance is in pretty good agreement that the quasi-stationary front
will move northward on Saturday as a warm front. After this front
moves through, the chances for any showers and storms ceases. The
pattern quickly though shifts to heat and humidity as both
temperatures and dewpoints increase from southwest to northeast.
Saturday may be a bit of a transition day, with highs
approaching 90 degrees and peak heat index values likely around
the century mark. Sunday and Monday look hot and humid areawide
with highs in the low to mid 90s and widespread 100-105 heat
index values. Some areas may see 105+F heat index values, which
is magnitude heat advisory criteria.

(Tuesday - Wednesday)

Ensemble guidance suggests a midlevel trough will dig into the Great
Lakes early/mid next week, with the ridge shifting more toward the
desert southwest. There is quite a bit of spread however with the
trough across the Great Lakes. Roughly half of members from the LREF
are weaker with this feature, with a surface cold front possibly not
making it into our area. The other half has a stronger trough, with
a cold frontal passage across the area sometime Tuesday/Tuesday
night. This latter scenario would end the heat/humidity,
whereas the first scenario likely would continue the 100-105+
heat index values each afternoon.

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 537 AM CDT Wed Aug 5 2026

Scattered showers and possibly a rumble of thunder should impact
KCOU/KJEF/KUIN over the next few hours. Visibilities may briefly
drop to 3-5SM, with an outside chance of IFR conditions in a
heavier downpour. KUIN has the best chance of showers/storms
along the front later this afternoon, with central Missouri and
metro St. Louis terminals likely to experience at least a few
hours of thunderstorms overnight. Gusty winds and IFR
visibilities are possible with any storms that directly impact a
terminal. The front is forecast to push south of KUIN late
tonight, with low MVFR ceilings attempting to push into the
site.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch from this evening through Thursday morning for
     MOZ018-019-026-027-034>036-041-042-047>052-059>065-072-
     073.
IL...Flood Watch from this evening through Thursday morning for
     ILZ095>098.

&&

$$

SHORT TERM...Gosselin
LONG TERM...Gosselin
AVIATION...Gosselin