Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

873
FXUS63 KLSX 091737
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1237 PM 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 WEDNESDAY/...
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 /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1230 PM CDT Wed Sep 9 2026

A slow moving cold front will bring impacts through the period at
all local terminals. Showers and very isolated thunderstorms
with the front will increase in coverage later this afternoon
through this evening. With the rain itself, MVFR visibilities
will prevail, with brief bouts of IFR if sites are directly
impacted by thunderstorms. Additionally, cooling temperatures
and increasing low-level moisture tonight will lower ceilings
through at least tomorrow morning. There is a low to medium
chance that ceilings are lower than what is currently forecast
thanks to the rainfall. While conditions are forecast to
gradually improve tomorrow morning, confidence is low in the
timing of both rain diminishing and ceilings rising.

&&

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

&&

$$

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