Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

877
FXUS63 KLSX 041024
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
524 AM CDT Tue Aug 4 2026

.KEY MESSAGES...

- Showers and thunderstorms are expected along a slow-moving cold
front Wednesday afternoon through Thursday morning.

- A few storms may be strong to severe Wednesday afternoon and
  evening. Damaging winds are the main threat.

&&

.SHORT TERM /THROUGH WEDNESDAY/...
Issued at 239 AM CDT Tue Aug 4 2026

A surface anticyclone has slid eastward into the northeast, with a
ridge axis extending westward into southeast Missouri/southwest
Illinois. GOES-19 nighttime microphysics imagery along with surface
observations show fog across these areas, mainly confined to river
valleys. Similar to yesterday, any fog should quickly evaporate
within an hour or so after sunrise. A mostly sunny sky is expected
for the remainder of the day, with temperatures a few degrees on
average warmer than yesterday.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Issued at 239 AM CDT Tue Aug 4 2026

(Tonight - Thursday)

Increasing mid/high level clouds are expected by this evening as
a midlevel shortwave trough moves into the Upper Midwest. At the
surface, a cold front is expected to gradually progress southeastward
into northeast Missouri and west-central Illinois after 0600 UTC. A
few showers, and possibly a rumble or two of thunder, are expected
overnight in these locations. Due to the aforementioned increase in
cloud cover, lows tonight will be on the milder side, generally in
the mid 60s to low 70s.

Chances of showers and thunderstorms increases by midday on Wednesday
as another midlevel shortwave trough moves across the central Plains.
This trough is expected to be further south than its predecessor, with
the strongest mid/upper level forcing for ascent focused across
northeast Missouri and west-central Illinois. This is also where the
surface boundary should be located at this time, so widespread showers
and thunderstorms are expected. The exact timing of this wave varies
a few hours amongst deterministic guidance, but has sped up compared
to what it looked like yesterday. That means there should be quite a
bit of showers and storms earlier in the day, which also means a bit
less instability to work with. The latest HREF joint probabilities for
at least 1000 J/kg of SBCAPE and 30+ knots of deep-layer shear do peak
in the 50-70% range from central Missouri northeastward through
west-central Illinois however. Multicell clusters are likely to be the
preferred convective mode, though a few transient supercell structures
are also possible. Damaging winds are the main threat, though some
guidance suggests a weak surface low transversing the surface
baroclinic zone. There may be enough low-level shear/helicity near the
track of that low to yield a low threat for a brief, weak tornado.

Instability weakens after sunset, with the severe threat likely ending
shortly thereafter. A west/southwest 20-25+ knot low-level jet is
expected to develop overnight Wednesday night, bringing increasing
moisture convergence to parts of the area. The coverage of showers and
thunderstorms should increase again as a result overnight, first near
the I-70 corridor before activity gradually sinks southward as the
low-level jet veers more to the west/northwest. There is a concern for
some training, with the jet oriented nearly parallel to the surface
boundary in an environment favorable for efficient warm rainfall
processes (deep warm cloud depths, precipitable water values > 90th
percentile of climatology). Locally heavy rainfall is the main concern
late Wednesday night into very early Thursday morning.

There likely will be a lull in activity late Thursday morning into
the afternoon as the low-level jet weakens. The location of the
surface front may be a focused for additional development by the
afternoon as diurnal instability increases. Another midlevel shortwave
trough is forecast to move out of the mid-Missouri Valley Thursday
night. This feature combined with a strengthening low-level jet is
expected to yield another uptick in shower and thunderstorm activity
late Thursday night. The best chances (40-60%) are near and just north
of the expected location of the surface boundary, which is expected
to be across southeast Missouri and southwest Illinois. Guidance suggests
that the front may wash out a bit by Friday afternoon, but wherever
it is may have enough convergence to produce additional showers and
thunderstorms.

(Friday Night - Monday)

Ensemble guidance is in a bit better agreement heading into the
weekend, showing an amplifying mid/upper level ridge across the
Rockies/Plains. We will be on the eastern edge of the higher
heights, but it does look like we are likely to head into at least
a short period of hotter and more humid conditions again starting
Sunday. Highs in the low 90s with increasing humidity will likely
mean 100+ heat index values for much of the area both Sunday and
Monday.

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 522 AM CDT Tue Aug 4 2026

River valley fog may impact KSUS/KCPS at the very beginning of
the period, but should quickly diminish after sunrise.
Thereafter, dry/VFR conditions are expected areawide with light
winds.

&&

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

&&

$$

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