Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

243
FXUS63 KLSX 090004
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
704 PM CDT Tue Sep 8 2026

.KEY MESSAGES...

- It will be warm and humid again on Wednesday ahead of a cold
  front. However, current thinking is that clouds and afternoon
  showers and thunderstorms will keep heat index values below
  Heat Advisory criteria

- Thunderstorms on Wednesday afternoon and evening may produce
  gusty winds and hail. Continuing storms overnight night may
  produce locally heavy rainfall.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 257 PM CDT Tue Sep 8 2026

High pressure remains in control of our weather this afternoon.
The high is drifting to the east and a cold front is moving
through the northern Plains into the Upper Midwest. This cold
front will continue to move southeast tonight and most
deterministic guidance shows it in central or southern Iowa by
12Z Wednesday. Short range guidance shows a 25-30kt low level
jet intersecting the 850mb front over Iowa tonight to produce
some modest to strong moisture convergence resulting in
overnight convection. The convection slides south ahead of the
front Wednesday, resulting in at least a chance (30-50%) of
showers and thunderstorms for our area by mid- afternoon.
Current thinking is that scattered convection and clouds will
suppress temperatures enough to keep heat index values below 105
across the area, despite west-southwest flow ahead of the
front. Therefore we are not extending the current Heat Advisory
for central Missouri beyond its expiration time of 8 PM this
evening, nor are we issuing any new advisories at this time.
However, a new Heat Advisory may be needed for parts of the area
on Wednesday if there is less convection and fewer clouds than
expected. The RAP is showing as much as 3000 J/Kg of MLCAPE
Wednesday afternoon generally along and south of I-70. Deep-
layer wind shear is lacking at less than 25kts for most of the
area, though there is a small area where 30-35kts of shear
overlaps with some of the higher instability across parts of
central into east central Missouri as well as south central
Illinois. This jives well with SPC`s day 2 marginal outlook
highlighting the possibility of damaging wind gusts. Don`t think
the severe threat will last too long into the evening if it
materializes at all.

Attention turns to the possibility of heavy rain Wednesday night as
the front sags slowly southward.  Mean precipitable water values are
forecast to be between 2-2.25 inches along and south of I-70
Wednesday night by both the HREF and LREF which is right around the
99th percentile of climatology.  Forecast soundings show warm-cloud
depths in excess of 14kft with tall/skinny CAPE profiles.  However,
the soundings do have some drier air at times, particularly in the
low levels.  There is a low level jet to force convection Wednesday
night, although current guidance indicates it won`t be
particularly strong at 25-30kts.  The HREF 6hr LPMM QPF shows some
bullseyes of 4+ inches mainly south of I-70 by 12Z Thursday morning,
and this along with previously mentioned parameters does increase
confidence in locally heavy rain.  However, the threat for
widespread flooding is balanced by dry ground conditions, so we will
not issue a Flood Watch at this time.  A watch may be necessary
though if subsequent model runs show an increasing likelihood for
heavier rain.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 257 PM CDT Tue Sep 8 2026

The front continues southward on Thursday, and eventually drier air
moves in, ending the precip across southeast Missouri by 00Z Friday.
 Weak high pressure builds into the Upper Midwest and Great Lakes
Thursday night into Friday which will produce dry and relatively
cooler weather for Friday.  However another short wave trough moves
across the Midwest in quasi-zonal flow aloft Friday night into
Saturday.  This pushes another cold front with another round of
convection into Iowa Friday night, and the front stalls over
northern Missouri on Saturday.  The front and any outflow boundaries
that overnight convection produces will provide a focus for
scattered convection Saturday afternoon, though drier low level air
lingering ahead of the front looks like it will keep the atmosphere
from becoming too unstable.  A chance for showers and thunderstorms
continues Sunday into next week as a strong short wave trough moves
from the Intermountain West across the Northern Plains.  This forces
low level cyclogenesis over the Plains and turns the low level flow
over the Mid Mississippi Valley back to the southwest. Temperatures
remain 5-10 degrees above normal in the mid 80s to low 90s through
the end of the forecast period due to the persistent southwest flow.

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 621 PM CDT Tue Sep 8 2026

Dry, VFR conditions are forecast through the evening and much of
the overnight hours across the region. A cold front is slowly
oozing south from Iowa into the Mid-Mississippi Valley,
triggering showers and thunderstorms in its vicinity. That
activity will eventually arrive at KUIN by around sunrise
tomorrow, but will be on a weakening trend and may not have any
lightning associated with it by the time it arrives.
Nevertheless, a PROB30 group for TSRA that was introduced in
previous TAF issuances was kept. Along the front, a
strengthening low-level jet and strong inversion will likely
result in intermittent low-level wind shear at the central
Missouri terminals overnight. Other terminals are less likely
to see this threat as the LLJ is weaker further east from
central Missouri.

As the front makes slow progress southward through the peak of
diurnal heating on Wednesday, additional showers and
thunderstorms will blossom in its vicinity. Most guidance is
pointing to the 23-01Z timeframe for the second round of
thunderstorms, which would threaten reduced visibility and gusty
40-50kt winds. PROB30s were introduced at all terminals except
for KUIN to begin leaning on this potential threat. KUIN may
still be threatened by convection at this time, but most
indications are that the front will be south of the terminal and
any impactful convection will avoid it as a result. Still, if
the front is slower than currently forecast, all TAF sites would
be under a fairly similar threat for thunderstorms.

&&

.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 until 8 PM CDT this evening for MOZ061>065.
     Heat Advisory until 8 PM CDT this evening for MOZ041-047-048.
IL...Air Quality Alert until 8 PM CDT this evening for ILZ100>102.

&&

$$

SHORT TERM...Carney
LONG TERM...Carney
AVIATION...MRB
CLIMATE...Pfahler