River Flood Warning 108, NWS St. Louis, MO

Area Forecast Discussion
National Weather Service St Louis MO
634 AM CDT Thu Aug 6 2026

.KEY MESSAGES...

- Showers and thunderstorms are expected to continue through
  this morning, particularly along the I-70 corridor. Localized
  flash flooding is possible.

- Additional rounds of showers and thunderstorms are possible
  through Saturday night before heat and humidity builds back
  into the region this weekend.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 301 AM CDT Thu Aug 6 2026

Widespread showers and thunderstorms have entered central
Missouri on the nose of a southerly 30+ knot low-level jet. RAP
short-term forecasts have this jet veering with time this
morning, which should lead to continued development of new cells
on the western flank. This should increase the threat of
training, though lesser MUCAPE in our area suggests rates may
not be too terribly high. Usually in these cases, the flash
flood threat is highest closer to the genesis region (western
Missouri), where there is also higher MUCAPE per latest SPC
mesoanalysis. All that being said, the flash flood threat in our
area looks a bit lower than it did 12-24 hours ago. The best
chance for isolated flash flooding will be through mid-late
morning in central Missouri near I-70. This is the part of the
CWA that will be closer to the genesis region this morning and
where the REFS/HREF LPMM shows some pockets of 2-3+" of rain
through midday today.

The low-level jet is slow to weaken through the day today, with some
modest moisture convergence near or just east of the Mississippi
River even at 1800 UTC. While much of the convection should be lost
by midday, I do think lingering showers and weak thunderstorms will
continue into early this afternoon. At the very least, widespread
thick cloud cover looks likely to linger through much of the day.
Therefore, have made a couple of changes: 1) extended the flash
flood watch in time until 1800 UTC for far eastern Missouri into
western Illinois, while trimming the far north which will miss the
heavier rains, and 2) cooled high temperatures today on average
nearly 5 degrees from the previous forecast.

Additional showers and thunderstorms are expected to develop late
this afternoon, likely in parts of southwest Missouri near the
surface front, which may be enhanced by differential heating. These
storms may become strong to severe, but should move east/southeast
and stay to our southwest. More showers and storms should develop
late this evening/early overnight as the southwesterly low-level jet
strengthens. The location of the strongest moisture convergence
varies quite a bit, but may be centered over southwest Missouri. Our
best chances (30-50%) are in southeast Missouri and southwest
Illinois. Even if those areas are impacted tonight, the flash flood
threat looks relatively low for a few reasons: 1) weaker low-level
jet forecast tonight, 2) lower PWAT values, and 3) drier antecedent
conditions in SE MO/SW IL and higher FFG values.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Issued at 301 AM CDT Thu Aug 6 2026

(Friday - Friday Night)

The position of the quasi-stationary front Friday morning is
uncertain, but should tend to push northward during the day.
Convergence near this feature though looks very modest, so it is
uncertain if it will be strong enough to help promote any convective
initiation during peak heating. There is an even weaker low-level
jet forecast for Friday night (~15-20 knots), but weak moisture
convergence may interact with the surface boundary to yield at least
scattered showers and thunderstorms along/poleward of the boundary.

(Saturday - Monday Night)

Deterministic models are in good agreement that the front will shift
north of the area as a warm front by Saturday. This should leave at
least most of our area dry, but with increasing heat and humidity
beginning this weekend. Peak heat index values Saturday afternoon
may crest the century mark roughly along/south of I-70, and
potentially over the entire area Sunday and Monday. Magnitude heat
advisory criteria (105+) is also possible, especially Sunday and
Monday. There is a fly in the ointment for Sunday (and maybe even
Monday), at least in our northern counties. With the heart of the
mid/upper level anticyclone centered well to our west, we may be
susceptible to seeing the southern end of an MCS either or both
Sunday/Monday mornings. If this occurs, that could provide enough
clouds and/or an outflow boundary, which could limit the heat
threat.

(Tuesday - Wednesday)

There is still plenty of spread with the strength of a mid/upper
level trough crossing the Great Lakes/southeast Canada early next
week. Clusters of the 500-hPa height pattern show this quite well,
with roughly half of LREF members showing a stronger trough whereas
the other half is weaker. The stronger trough solution would be much
more likely to send some type of backdoor cold front through the
region and help end the hot/humid pattern. The weaker trough
scenario likely would keep the same air mass in place, which likely
would mean a more prolonged period of heat/humidity.

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 627 AM CDT Thu Aug 6 2026

Widespread rain with embedded thunderstorms is expected across
the central Missouri and metro St. Louis terminals. Visibilities
for the most part will be VFR, but IFR to low MVFR conditions
are expected with heavier downpours in thunderstorms. The rain
will end from west to east, at KCOU/KJEF near 15Z and metro St.
Louis closer to 18Z. There may be another chance of
showers/storms this evening, but confidence was too low as to
where and how widespread activity will be to maintain any PROB30
groups. Farther north, a few showers may impact KUIN this
morning, but the concern up there is more low clouds/fog.
Visibilities have improved due to the rain showers earlier and
think the worst of the fog is done. Ceilings should be
predominantly IFR though and slowly lift to VFR by this
afternoon.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch until 1 PM CDT this afternoon for MOZ036-052-
     060>065-073.
     Flood Watch until 10 AM CDT this morning for MOZ026-027-034-
     035-041-042-047>051-059-072.
IL...Flood Watch until 1 PM CDT this afternoon for ILZ058-097>102.

&&

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

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