Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

006
FXUS63 KLSX 091008
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
508 AM CDT Wed Sep 9 2026

.KEY MESSAGES...

- An arriving cold front and increasing clouds and showers/
  thunderstorms lead to uncertainty in heat index values today,
  but a Heat Advisory has been issued for southeastern MO and
  southwestern IL where confidence is highest in 105+ F values.

- Showers and thunderstorms will be widespread at times this
  evening through Thursday morning. The strongest thunderstorms
  this evening may be capable of gusty to perhaps damaging
  winds. Locally heavy rainfall is possible overnight, but
  confidence is low in flash flooding.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 326 AM CDT Wed Sep 9 2026

A cold front, augmented by ongoing showers and thunderstorms across
IA and northern MO, will advance slowly southward through the CWA
today. Behind the front, showers and weakening thunderstorms are
expected to reach northeastern MO and west-central IL around sunrise
with showers potentially lingering well into the day across that
area, holding temperatures in the upper 70s and low-80s F. Along and
ahead of the front, stalling/slowing near the I-70 corridor around
midday into early afternoon, anomalous 850-hPa temperatures support
another day of high temperatures in the low to mid-90s F. There is
still uncertainty on exact heat index values this afternoon due to
increasing clouds and scattered showers and weak thunderstorms
developing near a pre-frontal surface trough ahead of the front.
However, with dewpoints increasing and pooled near the front, peak
heat index values of 100 to 105 F will not be difficult to achieve,
but HREF probabilities of 105+ F are highest in southeastern MO and
southwestern IL where the hottest temperatures are forecast.
Therefore, a Heat Advisory has been issued for this part of the CWA.

Showers and thunderstorms will increase in coverage at some point
this evening near or just north of the cold front as a modest
nocturnal LLJ strengthens. With the strongest forcing, wind shear,
and instability behind the font; the overall severe threat is
limited with a tendency for any surface-based prefrontal
thunderstorms to be weak and stronger thunderstorms behind the front
being at least slightly elevated. Accounting for this, gusty winds
are the main threat with thunderstorms this evening, but a damaging
wind gust or two cannot be ruled out, especially if a bowing segment
can materialize. Showers and thunderstorms will become widespread at
times, especially late tonight into Thursday morning as large-scale
ascent increases with the arrival of an equatorward right jet
entrance region and upper-level trough. CAMs indicate that
thunderstorms will be most persistent near the I-70 and I-44
corridors. Several ingredients will be present for heavy rainfall
including 2"+ PW (95th+ climatological percentile), deep warm cloud
depths, and deep-layer flow nearly paralleling the slow-moving cold
front. Through Thursday afternoon, HREF and REFS indicate a swath of
2 to highly localized 5" of rainfall somewhere near the I-70 and I-
44 corridors. Considering very dry antecedent soil conditions (3 to
4" 3-hour FFG) and the aforementioned rainfall falling over a
broader period of time, confidence is low that high rainfall rates
will be sustained enough to cause flash flooding. Instead, the the
most likely scenario is for the majority of this rainfall to be
beneficial with perhaps some minor flooding.

Showers and thunderstorms will persist through Thursday morning, but
it is not completely clear when precipitation will end with some
variability in the speed of the upper-level trough. Most CAMs and
model guidance have precipitation ending sometime Thursday afternoon
and evening; however, there are some solutions with a much slower
trough that have showers and a few thunderstorms lingering into
Friday morning across southeastern MO and southwestern IL. With
precipitation, clouds, and the cooler post-frontal airmass; much
cooler high temperatures in the upper 70s to low-80s F are forecast
on Thursday. For Friday, less precipitation and decreasing clouds
will allow high temperatures to be warmer and in the 80s F.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Issued at 326 AM CDT Wed Sep 9 2026

Over the weekend into early next week, global model guidance is in
agreement that quasi-zonal to slightly southwesterly upper-level
flow will prevail over the Mid-Mississippi River Valley with a
series of upper-level troughs grazing the region. This pattern will
cause a front to waver between the Mid-Mississippi River Valley and
Midwest that is reflected by an extended period of varying PoPs
across the CWA. Ensemble model membership with measurable rainfall
is highest Friday night into Saturday and again Monday night into
Tuesday, but there is no single member that has precipitation during
this entire period. So, although there will be opportunities for
showers and thunderstorms, much of this time will be dry vs. wet.
With the CWA residing near or south of the front through this period
as well, above average high temperatures will vary mainly from the
mid-80s to low-90s F. The NBM interquartile range quickly inflates
to 15 F next Tuesday onward, spread from similar above average
temperatures to temperatures closer to average if the front shifts
south of the CWA.

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 507 AM CDT Wed Sep 9 2026

A cold front will continue to slowly work its way southward through
the area today, already having passed KUIN and reaching KCOU, KJEF,
and St. Louis metro terminals later this morning/early afternoon.
Winds will veer toward the northwest with the front. Showers and
weakening thunderstorms are expected to impact KUIN this morning
behind the front with some showers lingering into the afternoon.

Additional showers and thunderstorms will develop near the front
this evening, increasing in coverage and becoming widespread at
times overnight into Thursday morning. KCOU, KJEF, and St. Louis
metro terminals are most favored to see more persistent or frequent
thunderstorms including. The strongest thunderstorms will be capable
of IFR flight conditions and gusty winds, but MVFR flight conditions
have been included for now. Some thunderstorms may continue
overnight, longer than the current TAF indicates. Lower ceilings
will develop and spread into the area Thursday morning as showers
and some thunderstorms continue. Winds outside of thunderstorms will
be predominantly light.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from noon today to 7 PM CDT this evening for
     MOZ074-075-084-085-099.
IL...Heat Advisory from noon today to 7 PM CDT this evening for
     ILZ069-070-074-079-101-102.

&&

$$

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