Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

780
FXUS63 KLSX 080931
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
431 AM CDT Tue Sep 8 2026

.KEY MESSAGES...

- Heat returns today, with afternoon heat index values around
  105 F in the Heat Advisory for central MO. Humidity will
  increase on Wednesday, but a cold front may limit how hot
  temperatures become and if heat index values are impactful.

- Showers and thunderstorms are forecast Wednesday into
  Thursday, most widespread Wednesday evening into Thursday
  morning. Gusty winds, small hail, and locally heavy rainfall
  are possible.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 327 AM CDT Tue Sep 8 2026

An anomalous upper-level ridge, featuring near record-breaking 500-
hPa heights, will become positioned across the Mid-Mississippi River
Valley today. As a result, low-level flow will become increasingly
southerly with 850-hPa temperatures reaching 21 to 22 C (~95th
climatological percentile), supporting high temperatures mainly in
the low to mid-90s F with considerable insolation. Humidity/
dewpoints will be slow to increase for much of the CWA, remaining in
the upper 50s and 60s F and keeping heat index values close to air
temperatures. The exception will be across central MO, where a
corridor of higher dewpoints nearing 70 F and high temperatures
moreso in the mid/upper 90s F is forecast to yield several hours of
heat index values around 105 F this afternoon. Therefore, a Heat
Advisory has been issued for that area.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 327 AM CDT Tue Sep 8 2026

On Wednesday, a cold front trailing a deepening surface cyclone near
Hudson Bay will reach the Mid-Mississippi River Valley. Model
guidance still varies on how quickly this front advances southward,
but there is a growing consensus that it will reach along or just
south of the I-70 corridor around midday. Showers and thunderstorms
are anticipated to be ongoing mainly behind the front Wednesday
morning before a relative minimum in coverage around midday into
afternoon during a lapse in stronger large-scale forcing/ascent.
South of the front, 850-hPa temperatures will remain anomalously
warm, but the position of the front and extent of cloud cover
decrease confidence in high temperatures, which could reasonably
range from the low to upper 90s F, per the NBM interquartile range.
Confidence is high that dewpoints will increase on Wednesday with
pooled moisture in the vicinity of the front, but it is still
uncertain at this point if 105+ F heat index values materialize.
Although still possibly needed, a Heat Advisory has not been issued
for this reason.

Showers and thunderstorms will increase in coverage again sometime
Wednesday evening, eventually becoming widespread, as large-scale
ascent increases with falling mid-level heights ahead of an
approaching upper-level trough and arrival of an equatorward jet
entrance region. With a capping inversion across the warm sector and
strongest forcing, wind shear, and instability behind the front; it
is uncertain if any severe, surface-based thunderstorms with
damaging winds will develop. Instead, the most organized and
strongest thunderstorms are favored to exist behind the front which
may limit threats to gusty winds and small hail. Anomalous PW
exceeding 2", deep warm cloud depths, and deep-layer flow
paralleling the slow-moving front also supports heavier rainfall
where thunderstorms with efficient, high rainfall rates train.
Through Thursday, the NBM and global ensemble models indicate that
most locations see 0.25 to 1" of rainfall, with a swath of higher
amounts of 1 to 3" somewhere near the I-44 and I-70 corridors. Since
antecedent soil conditions are very dry, 3-hour FFG is high and generally
3 to 4", supporting heavier rainfall being more beneficial than
producing flash flooding.

Showers and a few thunderstorms could linger well into the day
Thursday until an upper-level trough axis arrives and shunts the
front and precipitation more quickly southward. The highest
probabilities of rainfall in ensemble model guidance are along and
south of I-44/I-70 during afternoon, which is where high
temperatures may struggle to reach above 80 F with post-frontal CAA
and clouds as well. To the north, decreasing clouds will promote
slightly warmer, but still much cooler high temperatures in the low-
80s F.

After Thursday, global model guidance is coming into better
agreement on the upper-level wave pattern becoming quasi-zonal
through the weekend and early next week with a front arriving and
wavering across the broader Mid-Mississippi River Valley. This
pattern results in an extended period of at least low PoPs that
begins Friday evening. Ensemble model membership with measurable
rainfall is highest in the Friday night-into-Saturday night
timeframe, but these members also show most of this period into
early next week being dry vs. wet. With this front and waves of
clouds/precipitation, temperatures are not completely certain, but
the NBM interquartile range has been over the last 24 to 48 hours
closing in on above-average high temperatures mainly in the low-80s
to low-90s F.

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 431 AM CDT Tue Sep 8 2026

Dry and VFR flight conditions are favored to continue through
tonight at all terminals with light southeasterly winds veering to
southerly today, backing some again this evening. A cold front begin
moving southward into the region Wednesday morning, accompanied by
winds veering to northwesterly and chances of showers and
thunderstorms. Although KUIN will be impacted first, these impacts
lie just beyond the TAF period.

&&

.CLIMATE...
Issued at 258 AM CDT Sun Sep 6 2026

Record Highs...Record Max Lows (value/year):

September 8:
KSTL: 101/1947...77/1939
KCOU: 103/1947...75/1922
KUIN: 102/1947...79/1922

September 9:
KSTL: 101/1925...79/1925
KCOU:  99/1925...76/1947
KUIN: 100/2013...75/1912

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Air Quality Alert from 10 AM this morning to 8 PM CDT this
     evening for MOZ061>065.
     Heat Advisory from noon today to 8 PM CDT this evening for
     MOZ041-047-048.
IL...Air Quality Alert from 10 AM this morning to 8 PM CDT this
     evening for ILZ100>102.

&&

$$

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